Merge branch 'master' into 1547-use-mqttclientdefaultcertificatevalidationhandlerhandle-by-default

.github/workflows/ReleaseNotes.md

@@ -1,7 +1 @@
-* [Core] MQTT Packets being sent over web socket transport are now setting the web socket frame boundaries correctly (#1499).
-* [Core] Add support for attaching and detaching events from different threads.
-* [Core] Fixed a deadlock in _AsyncLock_ implementation (#1520).
-* [Client] Keep alive mechanism now uses the configured timeout value from the options (thanks to @Stannieman, #1495).
-* [Client] The _PingAsync_ will fallback to the timeout specified in the client options when the cancellation token cannot be cancelled.
-* [Server] A DISCONNECT packet is no longer sent to MQTT clients < 5.0.0 (thanks to @logicaloud, #1506).
-* [Server] Improved "take over" process handling.
+

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ name: CI
 on: [push, pull_request]
 
 env:
-  VERSION: "4.1.1.${{github.run_number}}"
+  VERSION: "4.1.3.${{github.run_number}}"
 
 jobs:
   build:

MQTTnet.sln.DotSettings

@@ -242,5 +242,6 @@ See the LICENSE file in the project root for more information.</s:String>
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ESettingsUpgrade_002EMigrateThisQualifierSettings/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=PINGREQ/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=PINGRESP/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/UserDictionary/Words/=PUBCOMP/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=Tnet/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=unsub/@EntryIndexedValue">True</s:Boolean></wpf:ResourceDictionary>

Samples/Client/Client_Connection_Samples.cs

@@ -119,7 +119,7 @@ public static class Client_Connection_Samples
                         // The used public broker sometimes has invalid certificates. This sample accepts all
                         // certificates. This should not be used in live environments.
                         o.CertificateValidationHandler = _ => true;
-                        
+
                         // The default value is determined by the OS. Set manually to force version.
                         o.SslProtocol = SslProtocols.Tls12;
                     })
@@ -134,15 +134,17 @@ public static class Client_Connection_Samples
         }
     }
 
-    public static async Task Connect_Client_Using_WebSockets()
+    public static async Task Connect_Client_Using_WebSocket4Net()
     {
         /*
          * This sample creates a simple MQTT client and connects to a public broker using a WebSocket connection.
+         * Instead of the .NET implementation of WebSockets the implementaion from WebSocket4Net is used. It provides more
+         * encryption algorithms and supports more platforms.
          * 
          * This is a modified version of the sample _Connect_Client_! See other sample for more details.
          */
 
-        var mqttFactory = new MqttFactory();
+        var mqttFactory = new MqttFactory().UseWebSocket4Net();
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
@@ -155,18 +157,16 @@ public static class Client_Connection_Samples
             response.DumpToConsole();
         }
     }
-    
-    public static async Task Connect_Client_Using_WebSocket4Net()
+
+    public static async Task Connect_Client_Using_WebSockets()
     {
         /*
          * This sample creates a simple MQTT client and connects to a public broker using a WebSocket connection.
-         * Instead of the .NET implementation of WebSockets the implementaion from WebSocket4Net is used. It provides more
-         * encryption algorithms and supports more platforms.
          * 
          * This is a modified version of the sample _Connect_Client_! See other sample for more details.
          */
 
-        var mqttFactory = new MqttFactory().UseWebSocket4Net();
+        var mqttFactory = new MqttFactory();
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
@@ -206,7 +206,7 @@ public static class Client_Connection_Samples
             using (var timeout = new CancellationTokenSource(5000))
             {
                 var response = await mqttClient.ConnectAsync(mqttClientOptions, timeout.Token);
-                
+
                 Console.WriteLine("The MQTT client is connected.");
 
                 response.DumpToConsole();
@@ -214,6 +214,49 @@ public static class Client_Connection_Samples
         }
     }
 
+    public static async Task Disconnect_Clean()
+    {
+        /*
+         * This sample disconnects from the server with sending a DISCONNECT packet.
+         * This way of disconnecting is treated as a clean disconnect which will not
+         * trigger sending the last will etc.
+         */
+
+        var mqttFactory = new MqttFactory();
+
+        using (var mqttClient = mqttFactory.CreateMqttClient())
+        {
+            var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
+
+            await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
+
+            // Calling _DisconnectAsync_ will send a DISCONNECT packet before closing the connection.
+            // Using a reason code requires MQTT version 5.0.0!
+            await mqttClient.DisconnectAsync(MqttClientDisconnectReason.ImplementationSpecificError);
+        }
+    }
+
+    public static async Task Disconnect_Non_Clean()
+    {
+        /*
+         * This sample disconnects from the server without sending a DISCONNECT packet.
+         * This way of disconnecting is treated as a non clean disconnect which will
+         * trigger sending the last will etc.
+         */
+
+        var mqttFactory = new MqttFactory();
+
+        var mqttClient = mqttFactory.CreateMqttClient();
+
+        var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
+
+        await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
+
+        // Calling _Dispose_ or use of a _using_ statement will close the transport connection
+        // without sending a DISCONNECT packet to the server.
+        mqttClient.Dispose();
+    }
+
     public static async Task Inspect_Certificate_Validation_Errors()
     {
         /*
@@ -248,7 +291,7 @@ public static class Client_Connection_Samples
             }
         }
     }
-    
+
     public static async Task Ping_Server()
     {
         /*

Samples/Client/Client_Subscribe_Samples.cs

@@ -8,6 +8,7 @@
 
 using MQTTnet.Client;
 using MQTTnet.Packets;
+using MQTTnet.Protocol;
 using MQTTnet.Samples.Helpers;
 
 namespace MQTTnet.Samples.Client;
@@ -24,9 +25,7 @@ public static class Client_Subscribe_Samples
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
-            var mqttClientOptions = new MqttClientOptionsBuilder()
-                .WithTcpServer("broker.hivemq.com")
-                .Build();
+            var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
 
             // Setup message handling before connecting so that queued messages
             // are also handled properly. When there is no event handler attached all
@@ -42,7 +41,11 @@ public static class Client_Subscribe_Samples
             await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
 
             var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
-                .WithTopicFilter(f => { f.WithTopic("mqttnet/samples/topic/2"); })
+                .WithTopicFilter(
+                    f =>
+                    {
+                        f.WithTopic("mqttnet/samples/topic/2");
+                    })
                 .Build();
 
             await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);
@@ -54,35 +57,6 @@ public static class Client_Subscribe_Samples
         }
     }
 
-    public static async Task Subscribe_Topic()
-    {
-        /*
-         * This sample subscribes to a topic.
-         */
-
-        var mqttFactory = new MqttFactory();
-
-        using (var mqttClient = mqttFactory.CreateMqttClient())
-        {
-            var mqttClientOptions = new MqttClientOptionsBuilder()
-                .WithTcpServer("broker.hivemq.com")
-                .Build();
-
-            await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
-
-            var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
-                .WithTopicFilter(f => { f.WithTopic("mqttnet/samples/topic/1"); })
-                .Build();
-
-            var response = await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);
-
-            Console.WriteLine("MQTT client subscribed to topic.");
-
-            // The response contains additional data sent by the server after subscribing.
-            response.DumpToConsole();
-        }
-    }
-    
     public static async Task Send_Responses()
     {
         /*
@@ -100,22 +74,98 @@ public static class Client_Subscribe_Samples
                 // Now respond to the broker with a reason code other than success.
                 args.ReasonCode = MqttApplicationMessageReceivedReasonCode.ImplementationSpecificError;
                 args.ResponseReasonString = "That did not work!";
-                
+
                 // User properties require MQTT v5!
                 args.ResponseUserProperties.Add(new MqttUserProperty("My", "Data"));
-                
+
                 // Now the broker will resend the message again.
                 return Task.CompletedTask;
             };
-            
-            var mqttClientOptions = new MqttClientOptionsBuilder()
-                .WithTcpServer("broker.hivemq.com")
-                .Build();
+
+            var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
 
             await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
 
             var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
-                .WithTopicFilter(f => { f.WithTopic("mqttnet/samples/topic/1"); })
+                .WithTopicFilter(
+                    f =>
+                    {
+                        f.WithTopic("mqttnet/samples/topic/1");
+                    })
+                .Build();
+
+            var response = await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);
+
+            Console.WriteLine("MQTT client subscribed to topic.");
+
+            // The response contains additional data sent by the server after subscribing.
+            response.DumpToConsole();
+        }
+    }
+
+    public static async Task Subscribe_Multiple_Topics()
+    {
+        /*
+         * This sample subscribes to several topics in a single request.
+         */
+
+        var mqttFactory = new MqttFactory();
+
+        using (var mqttClient = mqttFactory.CreateMqttClient())
+        {
+            var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
+
+            await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
+
+            // Create the subscribe options including several topics with different options.
+            // It is also possible to all of these topics using a dedicated call of _SubscribeAsync_ per topic.
+            var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
+                .WithTopicFilter(
+                    f =>
+                    {
+                        f.WithTopic("mqttnet/samples/topic/1");
+                    })
+                .WithTopicFilter(
+                    f =>
+                    {
+                        f.WithTopic("mqttnet/samples/topic/2").WithNoLocal();
+                    })
+                .WithTopicFilter(
+                    f =>
+                    {
+                        f.WithTopic("mqttnet/samples/topic/3").WithRetainHandling(MqttRetainHandling.SendAtSubscribe);
+                    })
+                .Build();
+
+            var response = await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);
+
+            Console.WriteLine("MQTT client subscribed to topics.");
+
+            // The response contains additional data sent by the server after subscribing.
+            response.DumpToConsole();
+        }
+    }
+
+    public static async Task Subscribe_Topic()
+    {
+        /*
+         * This sample subscribes to a topic.
+         */
+
+        var mqttFactory = new MqttFactory();
+
+        using (var mqttClient = mqttFactory.CreateMqttClient())
+        {
+            var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
+
+            await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
+
+            var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
+                .WithTopicFilter(
+                    f =>
+                    {
+                        f.WithTopic("mqttnet/samples/topic/1");
+                    })
                 .Build();
 
             var response = await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);

Samples/Server/Server_Intercepting_Samples.cs

@@ -0,0 +1,47 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// ReSharper disable UnusedType.Global
+// ReSharper disable UnusedMember.Global
+// ReSharper disable InconsistentNaming
+// ReSharper disable EmptyConstructor
+// ReSharper disable MemberCanBeMadeStatic.Local
+
+using MQTTnet.Internal;
+using MQTTnet.Server;
+
+namespace MQTTnet.Samples.Server;
+
+public static class Server_Intercepting_Samples
+{
+    public static async Task Intercept_Application_Messages()
+    {
+        /*
+         * This sample starts a simple MQTT server which manipulate all processed application messages.
+         * Please see _Server_Simple_Samples_ for more details on how to start a server.
+         */
+
+        var mqttFactory = new MqttFactory();
+        var mqttServerOptions = new MqttServerOptionsBuilder().WithDefaultEndpoint().Build();
+
+        using (var mqttServer = mqttFactory.CreateMqttServer(mqttServerOptions))
+        {
+            mqttServer.InterceptingPublishAsync += args =>
+            {
+                // Here we only change the topic of the received application message.
+                // but also changing the payload etc. is required. Changing the QoS after
+                // transmitting is not supported and makes no sense at all.
+                args.ApplicationMessage.Topic += "/manipulated";
+
+                return CompletedTask.Instance;
+            };
+
+            await mqttServer.StartAsync();
+
+            Console.WriteLine("Press Enter to exit.");
+            Console.ReadLine();
+            await mqttServer.StopAsync();
+        }
+    }
+}

Source/MQTTnet.AspNetCore.Tests/Mockups/ConnectionContextMockup.cs

@@ -1,26 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-// See the LICENSE file in the project root for more information.
-
-using System.Collections.Generic;
-using System.IO.Pipelines;
-using Microsoft.AspNetCore.Connections;
-using Microsoft.AspNetCore.Http.Features;
-
-namespace MQTTnet.AspNetCore.Tests.Mockups
-{
-    public class ConnectionContextMockup : ConnectionContext
-    {
-        public override string ConnectionId { get; set; }
-
-        public override IFeatureCollection Features { get; }
-
-        public override IDictionary<object, object> Items { get; set; }
-        public override IDuplexPipe Transport { get; set; }
-
-        public ConnectionContextMockup()
-        {
-            //Transport = new DefaultConnectionContext
-        }
-    }
-}

Source/MQTTnet.AspNetCore.Tests/Mockups/ConnectionHandlerMockup.cs

@@ -2,20 +2,24 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using Microsoft.AspNetCore.Connections;
-using MQTTnet.Adapter;
-using MQTTnet.Formatter;
-using MQTTnet.Server;
 using System;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Connections;
+using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
+using MQTTnet.Formatter;
+using MQTTnet.Server;
 
 namespace MQTTnet.AspNetCore.Tests.Mockups
 {
-    public class ConnectionHandlerMockup: IMqttServerAdapter
+    public sealed class ConnectionHandlerMockup : IMqttServerAdapter
     {
-        public TaskCompletionSource<MqttConnectionContext> Context { get; } = new TaskCompletionSource<MqttConnectionContext>();
         public Func<IMqttChannelAdapter, Task> ClientHandler { get; set; }
+        public TaskCompletionSource<MqttConnectionContext> Context { get; } = new TaskCompletionSource<MqttConnectionContext>();
+
+        public void Dispose()
+        {
+        }
 
         public async Task OnConnectedAsync(ConnectionContext connection)
         {
@@ -42,9 +46,5 @@ namespace MQTTnet.AspNetCore.Tests.Mockups
         {
             return Task.CompletedTask;
         }
-
-        public void Dispose()
-        {
-        }
     }
-}
+}

Source/MQTTnet.AspNetCore.Tests/Mockups/DuplexPipeMockup.cs

@@ -6,7 +6,7 @@ using System.IO.Pipelines;
 
 namespace MQTTnet.AspNetCore.Tests.Mockups
 {
-    public class DuplexPipeMockup : IDuplexPipe
+    public sealed class DuplexPipeMockup : IDuplexPipe
     {
         public DuplexPipeMockup()
         {
@@ -20,9 +20,8 @@ namespace MQTTnet.AspNetCore.Tests.Mockups
 
         PipeWriter IDuplexPipe.Output => Send.Writer;
 
+        public Pipe Receive { get; } 
 
-        public Pipe Receive { get; set; } 
-
-        public Pipe Send { get; set; }
+        public Pipe Send { get; }
     }
 }

Source/MQTTnet.AspNetCore.Tests/Mockups/LimitedMemoryPool.cs

@@ -6,7 +6,7 @@ using System.Buffers;
 
 namespace MQTTnet.AspNetCore.Tests.Mockups
 {
-    public class LimitedMemoryPool : MemoryPool<byte>
+    public sealed class LimitedMemoryPool : MemoryPool<byte>
     {
         protected override void Dispose(bool disposing)
         {

Source/MQTTnet.AspNetCore.Tests/Mockups/MemoryOwner.cs

@@ -7,9 +7,9 @@ using System.Buffers;
 
 namespace MQTTnet.AspNetCore.Tests.Mockups
 {
-    public class MemoryOwner : IMemoryOwner<byte>
+    public sealed class MemoryOwner : IMemoryOwner<byte>
     {
-        private readonly byte[] _raw;
+        readonly byte[] _raw;
 
         public MemoryOwner(int size)
         {
@@ -27,11 +27,11 @@ namespace MQTTnet.AspNetCore.Tests.Mockups
             Memory = _raw;
         }
 
+        public Memory<byte> Memory { get; }
+
         public void Dispose()
         {
             ArrayPool<byte>.Shared.Return(_raw);
         }
-
-        public Memory<byte> Memory { get; }
     }
 }

Source/MQTTnet.AspNetCore.Tests/MqttConnectionContextTest.cs

@@ -2,22 +2,21 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using Microsoft.AspNetCore.Builder;
-using Microsoft.AspNetCore.Connections;
-using Microsoft.AspNetCore.Hosting;
-using Microsoft.Extensions.DependencyInjection;
-using Microsoft.VisualStudio.TestTools.UnitTesting;
-
 using System;
 using System.Net;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Connections;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Adapter;
 using MQTTnet.AspNetCore.Tests.Mockups;
+using MQTTnet.Client;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
 using MQTTnet.Packets;
-using MQTTnet.Client;
 using MQTTnet.Protocol;
 using MQTTnet.Tests.Extensions;
 
@@ -26,20 +25,6 @@ namespace MQTTnet.AspNetCore.Tests
     [TestClass]
     public class MqttConnectionContextTest
     {
-        [TestMethod]
-        public async Task TestReceivePacketAsyncThrowsWhenReaderCompleted()
-        {
-            var serializer = new MqttPacketFormatterAdapter(MqttProtocolVersion.V311, new MqttBufferWriter(4096, 65535));
-            var pipe = new DuplexPipeMockup();
-            var connection = new DefaultConnectionContext();
-            connection.Transport = pipe;
-            var ctx = new MqttConnectionContext(serializer, connection);
-
-            pipe.Receive.Writer.Complete();
-
-            await Assert.ThrowsExceptionAsync<MqttCommunicationException>(() => ctx.ReceivePacketAsync(CancellationToken.None));
-        }
-
         [TestMethod]
         public async Task TestCorruptedConnectPacket()
         {
@@ -49,13 +34,43 @@ namespace MQTTnet.AspNetCore.Tests
             var connection = new DefaultConnectionContext();
             connection.Transport = pipe;
             var ctx = new MqttConnectionContext(serializer, connection);
-            
+
             await pipe.Receive.Writer.WriteAsync(writer.AddMqttHeader(MqttControlPacketType.Connect, new byte[0]));
 
             await Assert.ThrowsExceptionAsync<MqttProtocolViolationException>(() => ctx.ReceivePacketAsync(CancellationToken.None));
 
             // the first exception should complete the pipes so if someone tries to use the connection after that it should throw immidiatly
-            await Assert.ThrowsExceptionAsync<InvalidOperationException>(() =>  ctx.ReceivePacketAsync(CancellationToken.None));
+            await Assert.ThrowsExceptionAsync<InvalidOperationException>(() => ctx.ReceivePacketAsync(CancellationToken.None));
+        }
+
+        [TestMethod]
+        public async Task TestEndpoint()
+        {
+            var mockup = new ConnectionHandlerMockup();
+
+
+            using (var host = new WebHostBuilder().UseKestrel(kestrel => kestrel.ListenLocalhost(1883, listener => listener.Use((ctx, next) => mockup.OnConnectedAsync(ctx))))
+                       .UseStartup<Startup>()
+                       .ConfigureServices(
+                           (hostContext, services) =>
+                           {
+                               services.AddHostedMqttServer(o => o.WithoutDefaultEndpoint());
+                               services.AddSingleton<IMqttServerAdapter>(mockup);
+                           })
+                       .Build())
+
+            using (var client = new MqttFactory().CreateMqttClient())
+            {
+                host.Start();
+                await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("localhost").Build(), CancellationToken.None);
+
+                var ctx = await mockup.Context.Task;
+#if NETCOREAPP3_1
+                var ep = IPEndPoint.Parse(ctx.Endpoint);
+                Assert.IsNotNull(ep);
+#endif
+                Assert.IsNotNull(ctx);
+            }
         }
 
         // COMMENTED OUT DUE TO DEAD LOCK? OR VERY VERY SLOW PERFORMANCE ON LOCAL DEV MACHINE. TEST WAS STILL RUNNING AFTER SEVERAL MINUTES!
@@ -78,7 +93,7 @@ namespace MQTTnet.AspNetCore.Tests
 
         //    await Task.WhenAll(tasks).ConfigureAwait(false);
         //}
-        
+
         [TestMethod]
         public async Task TestLargePacket()
         {
@@ -94,43 +109,25 @@ namespace MQTTnet.AspNetCore.Tests
             Assert.IsTrue(readResult.Buffer.Length > 20000);
         }
 
-        private class Startup 
+        [TestMethod]
+        public async Task TestReceivePacketAsyncThrowsWhenReaderCompleted()
+        {
+            var serializer = new MqttPacketFormatterAdapter(MqttProtocolVersion.V311, new MqttBufferWriter(4096, 65535));
+            var pipe = new DuplexPipeMockup();
+            var connection = new DefaultConnectionContext();
+            connection.Transport = pipe;
+            var ctx = new MqttConnectionContext(serializer, connection);
+
+            pipe.Receive.Writer.Complete();
+
+            await Assert.ThrowsExceptionAsync<MqttCommunicationException>(() => ctx.ReceivePacketAsync(CancellationToken.None));
+        }
+
+        class Startup
         {
             public void Configure(IApplicationBuilder app)
-            { 
+            {
             }
         }
-
-        [TestMethod]
-        public async Task TestEndpoint()
-        {
-            var mockup = new ConnectionHandlerMockup();
-
-
-            using (var host = new WebHostBuilder()
-                .UseKestrel(kestrel => kestrel.ListenLocalhost(1883, listener => listener.Use((ctx, next) => mockup.OnConnectedAsync(ctx))))
-                .UseStartup<Startup>()
-                .ConfigureServices((hostContext, services) =>
-                {
-                    services.AddHostedMqttServer(o => o.WithoutDefaultEndpoint());
-                    services.AddSingleton<IMqttServerAdapter>(mockup);
-                })
-                .Build())
-                
-            using (var client = new MqttFactory().CreateMqttClient())
-            {
-                host.Start();
-                await client.ConnectAsync(new MqttClientOptionsBuilder()
-                    .WithTcpServer("localhost")
-                    .Build(), CancellationToken.None);
-
-                var ctx = await mockup.Context.Task;
-#if NETCOREAPP3_1
-                var ep = IPEndPoint.Parse(ctx.Endpoint);
-                Assert.IsNotNull(ep);
-#endif
-                Assert.IsNotNull(ctx);
-            }               
-        }
     }
-}
+}

Source/MQTTnet.AspNetCore.Tests/ReaderExtensionsTest.cs

@@ -12,7 +12,7 @@ using MQTTnet.Packets;
 namespace MQTTnet.AspNetCore.Tests
 {
     [TestClass]
-    public class ReaderExtensionsTest
+    public sealed class ReaderExtensionsTest
     {
         [TestMethod]
         public void TestTryDeserialize()

Source/MQTTnet.AspnetCore/MqttConnectionContext.cs

@@ -171,7 +171,7 @@ namespace MQTTnet.AspNetCore
         {
             var formatter = PacketFormatterAdapter;
             
-            using (await _writerLock.WaitAsync(cancellationToken).ConfigureAwait(false))
+            using (await _writerLock.EnterAsync(cancellationToken).ConfigureAwait(false))
             {
                 var buffer = formatter.Encode(packet);
                 var msg = buffer.Join().AsMemory();

Source/MQTTnet.Benchmarks/AsyncLockBenchmark.cs

@@ -29,7 +29,7 @@ namespace MQTTnet.Benchmarks
                 tasks[i] = Task.Run(
                     async () =>
                     {
-                        using (await asyncLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+                        using (await asyncLock.EnterAsync().ConfigureAwait(false))
                         {
                             var localI = globalI;
                             await Task.Delay(5).ConfigureAwait(false); // Increase the chance for wrong data.
@@ -56,7 +56,7 @@ namespace MQTTnet.Benchmarks
             {
                 for (var i = 0; i < 100000; i++)
                 {
-                    using (await asyncLock.WaitAsync(cancellationToken.Token).ConfigureAwait(false))
+                    using (await asyncLock.EnterAsync(cancellationToken.Token).ConfigureAwait(false))
                     {
                     }
                 }

Source/MQTTnet.Extensions.ManagedClient/IManagedMqttClient.cs

@@ -43,7 +43,7 @@ namespace MQTTnet.Extensions.ManagedClient
         
         Task StartAsync(ManagedMqttClientOptions options);
         
-        Task StopAsync();
+        Task StopAsync(bool cleanDisconnect = true);
         
         Task SubscribeAsync(ICollection<MqttTopicFilter> topicFilters);
         

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs

@@ -62,6 +62,7 @@ namespace MQTTnet.Extensions.ManagedClient
         CancellationTokenSource _publishingCancellationToken;
 
         ManagedMqttClientStorageManager _storageManager;
+        bool _isCleanDisconnect;
 
         public ManagedMqttClient(IMqttClient mqttClient, IMqttNetLogger logger)
         {
@@ -160,14 +161,14 @@ namespace MQTTnet.Extensions.ManagedClient
                 throw new InvalidOperationException("call StartAsync before publishing messages");
             }
 
-            MqttTopicValidator.ThrowIfInvalid(applicationMessage.ApplicationMessage.Topic);
+            MqttTopicValidator.ThrowIfInvalid(applicationMessage.ApplicationMessage);
 
             ManagedMqttApplicationMessage removedMessage = null;
             ApplicationMessageSkippedEventArgs applicationMessageSkippedEventArgs = null;
 
             try
             {
-                using (await _messageQueueLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+                using (await _messageQueueLock.EnterAsync().ConfigureAwait(false))
                 {
                     if (_messageQueue.Count >= Options.MaxPendingMessages)
                     {
@@ -255,10 +256,12 @@ namespace MQTTnet.Extensions.ManagedClient
             _logger.Info("Started");
         }
 
-        public async Task StopAsync()
+        public async Task StopAsync(bool cleanDisconnect = true)
         {
             ThrowIfDisposed();
 
+            _isCleanDisconnect = cleanDisconnect;
+            
             StopPublishing();
             StopMaintainingConnection();
 
@@ -296,7 +299,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
             _subscriptionsQueuedSignal.Release();
 
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public Task UnsubscribeAsync(ICollection<string> topics)
@@ -319,7 +322,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
             _subscriptionsQueuedSignal.Release();
 
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         protected override void Dispose(bool disposing)
@@ -382,9 +385,12 @@ namespace MQTTnet.Extensions.ManagedClient
                 {
                     try
                     {
-                        using (var disconnectTimeout = new CancellationTokenSource(Options.ClientOptions.Timeout))
+                        if (_isCleanDisconnect)
                         {
-                            await InternalClient.DisconnectAsync(new MqttClientDisconnectOptions(), disconnectTimeout.Token).ConfigureAwait(false);
+                            using (var disconnectTimeout = new CancellationTokenSource(Options.ClientOptions.Timeout))
+                            {
+                                await InternalClient.DisconnectAsync(new MqttClientDisconnectOptions(), disconnectTimeout.Token).ConfigureAwait(false);
+                            }
                         }
                     }
                     catch (OperationCanceledException)
@@ -693,7 +699,7 @@ namespace MQTTnet.Extensions.ManagedClient
             {
                 await InternalClient.PublishAsync(message.ApplicationMessage).ConfigureAwait(false);
 
-                using (await _messageQueueLock.WaitAsync(CancellationToken.None).ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
+                using (await _messageQueueLock.EnterAsync().ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
                 {
                     // While publishing this message, this.PublishAsync could have booted this
                     // message off the queue to make room for another (when using a cap
@@ -724,7 +730,7 @@ namespace MQTTnet.Extensions.ManagedClient
                     //contradict the expected behavior of QoS 1 and 2, that's also true
                     //for the usage of a message queue cap, so it's still consistent
                     //with prior behavior in that way.
-                    using (await _messageQueueLock.WaitAsync(CancellationToken.None).ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
+                    using (await _messageQueueLock.EnterAsync().ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
                     {
                         _messageQueue.RemoveFirst(i => i.Id.Equals(message.Id));
 

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClientStorageManager.cs

@@ -34,7 +34,7 @@ namespace MQTTnet.Extensions.ManagedClient
         {
             if (applicationMessage == null) throw new ArgumentNullException(nameof(applicationMessage));
 
-            using (await _messagesLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+            using (await _messagesLock.EnterAsync().ConfigureAwait(false))
             {
                 _messages.Add(applicationMessage);
                 await SaveAsync().ConfigureAwait(false);
@@ -45,7 +45,7 @@ namespace MQTTnet.Extensions.ManagedClient
         {
             if (applicationMessage == null) throw new ArgumentNullException(nameof(applicationMessage));
 
-            using (await _messagesLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+            using (await _messagesLock.EnterAsync().ConfigureAwait(false))
             {
                 var index = _messages.IndexOf(applicationMessage);
                 if (index == -1)

Source/MQTTnet.Extensions.Rpc/MqttRpcClient.cs

@@ -9,7 +9,7 @@ using System;
 using System.Collections.Concurrent;
 using System.Threading;
 using System.Threading.Tasks;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Extensions.Rpc
 {
@@ -116,7 +116,7 @@ namespace MQTTnet.Extensions.Rpc
         {
             if (!_waitingCalls.TryRemove(eventArgs.ApplicationMessage.Topic, out var awaitable))
             {
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             }
 
 #if NET452
@@ -128,7 +128,7 @@ namespace MQTTnet.Extensions.Rpc
             // Set this message to handled to that other code can avoid execution etc.
             eventArgs.IsHandled = true;
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         public void Dispose()

Source/MQTTnet.Extensions.WebSocket4Net/WebSocket4NetMqttChannel.cs

@@ -14,6 +14,7 @@ using System.Threading.Tasks;
 using MQTTnet.Channel;
 using MQTTnet.Client;
 using MQTTnet.Exceptions;
+using MQTTnet.Internal;
 using SuperSocket.ClientEngine;
 using WebSocket4Net;
 
@@ -124,7 +125,7 @@ namespace MQTTnet.Extensions.WebSocket4Net
                 _webSocket.Close();
             }
             
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
@@ -157,7 +158,7 @@ namespace MQTTnet.Extensions.WebSocket4Net
         public Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken)
         {
             _webSocket.Send(buffer.Array, buffer.Offset, buffer.Count);
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public void Dispose()

Source/MQTTnet.TestApp/AsyncLockTest.cs

@@ -18,7 +18,7 @@ namespace MQTTnet.TestApp
             {
                 for (var i = 0; i < 100000; i++)
                 {
-                    using (await asyncLock.WaitAsync(cancellationToken.Token).ConfigureAwait(false))
+                    using (await asyncLock.EnterAsync(cancellationToken.Token).ConfigureAwait(false))
                     {
                     }
                 } 

Source/MQTTnet.TestApp/ClientTest.cs

@@ -7,7 +7,7 @@ using System;
 using System.Text;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.TestApp
@@ -40,7 +40,7 @@ namespace MQTTnet.TestApp
                     Console.WriteLine($"+ Retain = {e.ApplicationMessage.Retain}");
                     Console.WriteLine();
                     
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 client.ConnectedAsync += async e =>

Source/MQTTnet.TestApp/ManagedClientTest.cs

@@ -9,7 +9,7 @@ using Newtonsoft.Json;
 using System.Collections.Generic;
 using MQTTnet.Client;
 using MQTTnet.Extensions.ManagedClient;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.TestApp
@@ -42,7 +42,7 @@ namespace MQTTnet.TestApp
                 managedClient.ApplicationMessageReceivedAsync += e =>
                 {
                     Console.WriteLine(">> RECEIVED: " + e.ApplicationMessage.Topic);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await managedClient.StartAsync(options);
@@ -85,7 +85,7 @@ namespace MQTTnet.TestApp
             public Task SaveQueuedMessagesAsync(IList<ManagedMqttApplicationMessage> messages)
             {
                 File.WriteAllText(Filename, JsonConvert.SerializeObject(messages));
-                return Task.FromResult(0);
+                return CompletedTask.Instance;
             }
 
             public Task<IList<ManagedMqttApplicationMessage>> LoadQueuedMessagesAsync()

Source/MQTTnet.TestApp/MessageThroughputTest.cs

@@ -5,6 +5,7 @@ using System;
 using System.Threading;
 using System.Threading.Tasks;
 using System.Linq;
+using MQTTnet.Internal;
 
 namespace MQTTnet.TestApp
 {
@@ -124,7 +125,7 @@ namespace MQTTnet.TestApp
             {
                 _cancellationTokenSource.Cancel();
             }
-            return Implementations.PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         public async Task Cleanup()

Source/MQTTnet.TestApp/PublicBrokerTest.cs

@@ -8,7 +8,7 @@ using System.Security.Authentication;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.TestApp
@@ -144,7 +144,7 @@ namespace MQTTnet.TestApp
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.ConnectAsync(options);

Source/MQTTnet.TestApp/ServerTest.cs

@@ -4,13 +4,11 @@
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics;
 using System.IO;
 using System.Text;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
-using MQTTnet.Extensions.ManagedClient;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 using Newtonsoft.Json;
@@ -69,13 +67,13 @@ namespace MQTTnet.TestApp
                     }
 
                     File.WriteAllText(Filename, JsonConvert.SerializeObject(e.StoredRetainedMessages));
-                    return Task.FromResult(0);
+                    return CompletedTask.Instance;
                 };
                 
                 mqttServer.RetainedMessagesClearedAsync += e =>
                 {
                     File.Delete(Filename);
-                    return Task.FromResult(0);
+                    return CompletedTask.Instance;
                 };
                 
                 mqttServer.LoadingRetainedMessageAsync += e =>
@@ -93,7 +91,7 @@ namespace MQTTnet.TestApp
 
                     e.LoadedRetainedMessages = retainedMessages;
 
-                    return Task.FromResult(0);
+                    return CompletedTask.Instance;
                 };
 
                 mqttServer.InterceptingPublishAsync += e =>
@@ -111,7 +109,7 @@ namespace MQTTnet.TestApp
                         e.CloseConnection = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 mqttServer.ValidatingConnectionAsync += e =>
@@ -124,7 +122,7 @@ namespace MQTTnet.TestApp
                         }
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 mqttServer.InterceptingSubscriptionAsync += e =>
@@ -140,7 +138,7 @@ namespace MQTTnet.TestApp
                         e.CloseConnection = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 mqttServer.InterceptingPublishAsync += e =>
@@ -149,7 +147,7 @@ namespace MQTTnet.TestApp
                         $"'{e.ClientId}' reported '{e.ApplicationMessage.Topic}' > '{Encoding.UTF8.GetString(e.ApplicationMessage.Payload ?? new byte[0])}'",
                         ConsoleColor.Magenta);
                     
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 //options.ApplicationMessageInterceptor = c =>
@@ -177,7 +175,7 @@ namespace MQTTnet.TestApp
                 mqttServer.ClientConnectedAsync += e =>
                 {
                     Console.Write("Client disconnected event fired.");
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await mqttServer.StartAsync();

Source/MQTTnet.Tests/Client/LowLevelMqttClient_Tests.cs

@@ -64,7 +64,7 @@ namespace MQTTnet.Tests.Client
 
             await client.ConnectAsync(options, CancellationToken.None).ConfigureAwait(false);
         }
-        
+
         [TestMethod]
         [ExpectedException(typeof(MqttCommunicationException))]
         public async Task Connect_To_Wrong_Host()
@@ -80,6 +80,8 @@ namespace MQTTnet.Tests.Client
         {
             using (var testEnvironment = CreateTestEnvironment())
             {
+                testEnvironment.IgnoreServerLogErrors = true;
+
                 testEnvironment.ServerPort = 8364;
                 var server = await testEnvironment.StartServer();
 
@@ -116,6 +118,8 @@ namespace MQTTnet.Tests.Client
         {
             using (var testEnvironment = CreateTestEnvironment())
             {
+                testEnvironment.IgnoreServerLogErrors = true;
+
                 var server = await testEnvironment.StartServer();
 
                 using (var lowLevelClient = testEnvironment.CreateLowLevelClient())
@@ -123,7 +127,7 @@ namespace MQTTnet.Tests.Client
                     Assert.IsFalse(lowLevelClient.IsConnected);
 
                     var clientOptions = new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort).WithTimeout(TimeSpan.FromSeconds(1)).Build();
-                    
+
                     await lowLevelClient.ConnectAsync(clientOptions, CancellationToken.None);
 
                     Assert.IsTrue(lowLevelClient.IsConnected);
@@ -148,7 +152,7 @@ namespace MQTTnet.Tests.Client
                     catch
                     {
                     }
-           
+
                     Assert.IsFalse(lowLevelClient.IsConnected);
                 }
             }

Source/MQTTnet.Tests/Client/ManagedMqttClient_Tests.cs

@@ -11,18 +11,76 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Diagnostics;
 using MQTTnet.Extensions.ManagedClient;
-using MQTTnet.Implementations;
+using MQTTnet.Formatter;
 using MQTTnet.Internal;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 using MQTTnet.Tests.Mockups;
-using MqttClient = MQTTnet.Client.MqttClient;
 
 namespace MQTTnet.Tests.Client
 {
     [TestClass]
     public sealed class ManagedMqttClient_Tests : BaseTestClass
     {
+        [TestMethod]
+        public async Task Receive_While_Not_Cleanly_Disconnected()
+        {
+            using (var testEnvironment = CreateTestEnvironment(MqttProtocolVersion.V500))
+            {
+                await testEnvironment.StartServer(o => o.WithPersistentSessions());
+
+                var senderClient = await testEnvironment.ConnectClient();
+
+                // Prepare managed client.
+                var managedClient = testEnvironment.Factory.CreateManagedMqttClient();
+                await managedClient.SubscribeAsync("#");
+                var receivedMessages = testEnvironment.CreateApplicationMessageHandler(managedClient);
+                
+                var managedClientOptions = new ManagedMqttClientOptions
+                {
+                    ClientOptions = testEnvironment.Factory.CreateClientOptionsBuilder()
+                        .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                        .WithClientId(nameof(Receive_While_Not_Cleanly_Disconnected) + "_managed")
+                        .WithCleanSession(false)
+                        .Build()
+                };
+
+                await managedClient.StartAsync(managedClientOptions);
+                await LongTestDelay();
+                
+                // Send test data.
+                await senderClient.PublishStringAsync("topic1");
+                await LongTestDelay();
+                
+                receivedMessages.AssertReceivedCountEquals(1);
+
+                // Stop the managed client but keep session at server (not clean disconnect required).
+                await managedClient.StopAsync(false);
+                await LongTestDelay();
+                
+                // Send new messages in the meantime.
+                await senderClient.PublishStringAsync("topic2", qualityOfServiceLevel: MqttQualityOfServiceLevel.ExactlyOnce);
+                await LongTestDelay();
+                
+                // Start the managed client, it should receive the new message.
+                await managedClient.StartAsync(managedClientOptions);
+                await LongTestDelay();
+                
+                receivedMessages.AssertReceivedCountEquals(2);
+                
+                // Stop and start again, no new message should be received.
+                for (var i = 0; i < 3; i++)
+                {
+                    await managedClient.StopAsync(false);
+                    await LongTestDelay();
+                    await managedClient.StartAsync(managedClientOptions);
+                    await LongTestDelay();
+                }
+
+                receivedMessages.AssertReceivedCountEquals(2);
+            }
+        }
+
         [TestMethod]
         public async Task Connect_To_Invalid_Server()
         {
@@ -35,11 +93,10 @@ namespace MQTTnet.Tests.Client
                 var managedClient = testEnvironment.Factory.CreateManagedMqttClient(mqttClient);
 
                 ConnectingFailedEventArgs connectingFailedEventArgs = null;
-
                 managedClient.ConnectingFailedAsync += args =>
                 {
                     connectingFailedEventArgs = args;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await managedClient.StartAsync(
@@ -117,7 +174,7 @@ namespace MQTTnet.Tests.Client
                 receivingClient.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await receivingClient.SubscribeAsync("My/last/will");
@@ -281,7 +338,7 @@ namespace MQTTnet.Tests.Client
                     });
 
                 // Wait a bit for the retained message to be available
-                await Task.Delay(1000);
+                await LongTestDelay();
 
                 await sendingClient.DisconnectAsync();
 
@@ -290,28 +347,29 @@ namespace MQTTnet.Tests.Client
                 var clientOptions = new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort);
 
                 var receivedManagedMessages = new List<MqttApplicationMessage>();
-                var managedClient = new ManagedMqttClient(testEnvironment.CreateClient(), new MqttNetEventLogger());
+                
+                var managedClient = testEnvironment.Factory.CreateManagedMqttClient(testEnvironment.CreateClient());
                 managedClient.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedManagedMessages.Add(e.ApplicationMessage);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await managedClient.SubscribeAsync("topic");
 
                 await managedClient.StartAsync(new ManagedMqttClientOptionsBuilder().WithClientOptions(clientOptions).WithAutoReconnectDelay(TimeSpan.FromSeconds(1)).Build());
 
-                await Task.Delay(1000);
+                await LongTestDelay();
 
                 Assert.AreEqual(1, receivedManagedMessages.Count);
 
                 await managedClient.StopAsync();
 
-                await Task.Delay(500);
+                await LongTestDelay();
 
                 await managedClient.StartAsync(new ManagedMqttClientOptionsBuilder().WithClientOptions(clientOptions).WithAutoReconnectDelay(TimeSpan.FromSeconds(1)).Build());
 
-                await Task.Delay(1000);
+                await LongTestDelay();
 
                 // After reconnect and then some delay, the retained message must not be received,
                 // showing that the subscriptions were cleared
@@ -319,7 +377,9 @@ namespace MQTTnet.Tests.Client
 
                 // Make sure that it gets received after subscribing again.
                 await managedClient.SubscribeAsync("topic");
-                await Task.Delay(500);
+                
+                await LongTestDelay();
+                
                 Assert.AreEqual(2, receivedManagedMessages.Count);
             }
         }
@@ -428,7 +488,7 @@ namespace MQTTnet.Tests.Client
             managedClient.ConnectedAsync += e =>
             {
                 connected.TrySetResult(true);
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             };
 
             return connected.Task;
@@ -457,7 +517,7 @@ namespace MQTTnet.Tests.Client
                     result.TrySetResult(receivedMessages);
                 }
 
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             };
 
             return result.Task;
@@ -486,7 +546,7 @@ namespace MQTTnet.Tests.Client
         public Task SaveQueuedMessagesAsync(IList<ManagedMqttApplicationMessage> messages)
         {
             _messages = messages;
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
     }
 }

Source/MQTTnet.Tests/Client/MqttClient_Tests.cs

@@ -14,7 +14,7 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Tests.Mockups;
@@ -58,7 +58,7 @@ namespace MQTTnet.Tests.Client
                     }
 
                     _ = InvokeInternal();
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var publishes = Task.WhenAll(publisher.PublishStringAsync("a", null, qos), publisher.PublishStringAsync("b", null, qos));
@@ -109,7 +109,7 @@ namespace MQTTnet.Tests.Client
                     client.DisconnectedAsync += args =>
                     {
                         disconnectHandlerCalled = true;
-                        return PlatformAbstractionLayer.CompletedTask;
+                        return CompletedTask.Instance;
                     };
 
                     await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("1.2.3.4").Build());
@@ -123,7 +123,7 @@ namespace MQTTnet.Tests.Client
                     //Assert.IsInstanceOfType(exception.InnerException, typeof(SocketException));
                 }
 
-                await Task.Delay(100); // disconnected handler is called async
+                await LongTestDelay(); // disconnected handler is called async
                 Assert.IsTrue(disconnectHandlerCalled);
             }
         }
@@ -138,7 +138,7 @@ namespace MQTTnet.Tests.Client
                 client.DisconnectedAsync += e =>
                 {
                     ex = e.Exception;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 try
@@ -170,7 +170,7 @@ namespace MQTTnet.Tests.Client
                 server.InterceptingPublishAsync += c =>
                 {
                     i++;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.SendAsync(
@@ -211,7 +211,7 @@ namespace MQTTnet.Tests.Client
                 client.DisconnectedAsync += e =>
                 {
                     handlerFired = true;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await server.StopAsync();
@@ -255,7 +255,7 @@ namespace MQTTnet.Tests.Client
                 receiveClient.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedPayload = e.ApplicationMessage.ConvertPayloadToString();
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await receiveClient.SubscribeAsync("x");
@@ -313,7 +313,7 @@ namespace MQTTnet.Tests.Client
                 receiver.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await sender.PublishAsync(message.Build(), CancellationToken.None);
@@ -456,7 +456,7 @@ namespace MQTTnet.Tests.Client
                         receivedValues.Add(value);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 }
 
                 client1.ApplicationMessageReceivedAsync += Handler1;
@@ -534,7 +534,7 @@ namespace MQTTnet.Tests.Client
                 client2.ApplicationMessageReceivedAsync += e =>
                 {
                     client2TopicResults.Add(Encoding.UTF8.GetString(e.ApplicationMessage.Payload));
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client2.SubscribeAsync(client2Topic);
@@ -569,7 +569,7 @@ namespace MQTTnet.Tests.Client
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client1.SubscribeAsync("a");
@@ -728,7 +728,7 @@ namespace MQTTnet.Tests.Client
                         replies.Add(eventArgs.ApplicationMessage.Topic);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 }
 
                 client2.ApplicationMessageReceivedAsync += Handler2;
@@ -766,7 +766,7 @@ namespace MQTTnet.Tests.Client
                         replyReceived = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 client2.ApplicationMessageReceivedAsync += async e =>
@@ -817,7 +817,7 @@ namespace MQTTnet.Tests.Client
                         replyReceived = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.PublishStringAsync("request", null, MqttQualityOfServiceLevel.AtLeastOnce);
@@ -840,7 +840,7 @@ namespace MQTTnet.Tests.Client
                 client.DisconnectedAsync += e =>
                 {
                     Assert.IsTrue(e.ClientWasConnected);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.DisconnectAsync();
@@ -860,7 +860,7 @@ namespace MQTTnet.Tests.Client
                 client.DisconnectedAsync += e =>
                 {
                     Assert.IsTrue(e.ClientWasConnected);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await server.StopAsync();
@@ -897,7 +897,7 @@ namespace MQTTnet.Tests.Client
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("localhost", testEnvironment.ServerPort).Build());
@@ -922,14 +922,14 @@ namespace MQTTnet.Tests.Client
                 client1.DisconnectedAsync += c =>
                 {
                     disconnectedFired = true;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var messageReceived = false;
                 client1.ApplicationMessageReceivedAsync += e =>
                 {
                     messageReceived = true;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("topic1").WithExactlyOnceQoS().Build());

Source/MQTTnet.Tests/Diagnostics/PacketInspection_Tests.cs

@@ -7,7 +7,7 @@ using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Diagnostics
 {
@@ -33,7 +33,7 @@ namespace MQTTnet.Tests.Diagnostics
                     mqttClient.InspectPackage += eventArgs =>
                     {
                         packets.Add(eventArgs.Direction + ":" + Convert.ToBase64String(eventArgs.Buffer));
-                        return PlatformAbstractionLayer.CompletedTask;
+                        return CompletedTask.Instance;
                     };
                 
                     await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);

Source/MQTTnet.Tests/Factory/MqttFactory_Tests.cs

@@ -8,7 +8,7 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Diagnostics;
 using MQTTnet.Extensions.ManagedClient;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Server;
 
 namespace MQTTnet.Tests.Factory
@@ -56,7 +56,7 @@ namespace MQTTnet.Tests.Factory
                 managedClient.ConnectingFailedAsync += e =>
                 {
                     tcs.TrySetResult(null);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 await Task.WhenAny(Task.Delay(managedClient.Options.ClientOptions.Timeout), tcs.Task);

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V3_Binary_Tests.cs

@@ -13,13 +13,12 @@ using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
-using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Tests.Extensions;
 using MQTTnet.Tests.Mockups;
 
-namespace MQTTnet.Tests
+namespace MQTTnet.Tests.Formatter
 {
     [TestClass]
     public sealed class MqttPacketSerialization_V3_Binary_Tests

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V3_Tests.cs

@@ -10,7 +10,7 @@ using MQTTnet.Formatter;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
-namespace MQTTnet.Tests
+namespace MQTTnet.Tests.Formatter
 {
     [TestClass]
     public sealed class MqttPacketSerialization_V3_Tests

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V5_Tests.cs

@@ -9,7 +9,7 @@ using MQTTnet.Formatter;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
-namespace MQTTnet.Tests
+namespace MQTTnet.Tests.Formatter
 {
     [TestClass]
     public sealed class MqttPacketSerialization_V5_Tests

Source/MQTTnet.Tests/Internal/AsyncEvent_Tests.cs

@@ -6,7 +6,6 @@ using System;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Internal
@@ -16,6 +15,28 @@ namespace MQTTnet.Tests.Internal
     {
         int _testEventAsyncCount;
 
+        [TestMethod]
+        public async Task Attach_and_Detach_Sync_And_Async_EventHandler()
+        {
+            var testClass = new TestClass();
+
+            testClass.TestEventAsync += OnTestEventAsync;
+            testClass.TestEvent += OnTestEvent;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+            Assert.AreEqual(2, _testEventAsyncCount);
+
+            testClass.TestEvent -= OnTestEvent;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+            Assert.AreEqual(3, _testEventAsyncCount);
+
+            testClass.TestEventAsync -= OnTestEventAsync;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+            Assert.AreEqual(3, _testEventAsyncCount);
+        }
+
         [TestMethod]
         public async Task Attach_EventHandler()
         {
@@ -28,6 +49,19 @@ namespace MQTTnet.Tests.Internal
             Assert.AreEqual(1, _testEventAsyncCount);
         }
 
+        [TestMethod]
+        public async Task Attach_Sync_And_Async_EventHandler()
+        {
+            var testClass = new TestClass();
+
+            testClass.TestEventAsync += OnTestEventAsync;
+            testClass.TestEvent += OnTestEvent;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+
+            Assert.AreEqual(2, _testEventAsyncCount);
+        }
+
         [TestMethod]
         public async Task Detach_EventHandler()
         {
@@ -68,16 +102,27 @@ namespace MQTTnet.Tests.Internal
             Assert.AreEqual(false, testClass.HasTestHandlers);
         }
 
+        void OnTestEvent(EventArgs arg)
+        {
+            Interlocked.Increment(ref _testEventAsyncCount);
+        }
+
         Task OnTestEventAsync(EventArgs arg)
         {
             Interlocked.Increment(ref _testEventAsyncCount);
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         sealed class TestClass
         {
             readonly AsyncEvent<EventArgs> _testEvent = new AsyncEvent<EventArgs>();
 
+            public event Action<EventArgs> TestEvent
+            {
+                add => _testEvent.AddHandler(value);
+                remove => _testEvent.RemoveHandler(value);
+            }
+
             public event Func<EventArgs, Task> TestEventAsync
             {
                 add => _testEvent.AddHandler(value);

Source/MQTTnet.Tests/Internal/AsyncLock_Tests.cs

@@ -12,26 +12,130 @@ using MQTTnet.Internal;
 namespace MQTTnet.Tests.Internal
 {
     [TestClass]
-    public class AsyncLock_Tests
+    public sealed class AsyncLock_Tests
     {
         [TestMethod]
-        public void Lock_10_Parallel_Tasks()
+        public async Task Cancellation_Of_Awaiter()
         {
-            const int ThreadsCount = 10;
-
-            var threads = new Task[ThreadsCount];
             var @lock = new AsyncLock();
+
+            // This call will not yet "release" the lock due to missing _using_.
+            var releaser = await @lock.EnterAsync().ConfigureAwait(false);
+
+            var counter = 0;
+
+            Debug.WriteLine("Prepared locked lock.");
+
+            _ = Task.Run(
+                async () =>
+                {
+                    // SHOULD GET TIMEOUT!
+                    using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(1)))
+                    {
+                        using (await @lock.EnterAsync(timeout.Token))
+                        {
+                            Debug.WriteLine("Task 1 incremented");
+                            counter++;
+                        }
+                    }
+                });
+
+            _ = Task.Run(
+                async () =>
+                {
+                    // SHOULD GET TIMEOUT!
+                    using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(2)))
+                    {
+                        using (await @lock.EnterAsync(timeout.Token))
+                        {
+                            Debug.WriteLine("Task 2 incremented");
+                            counter++;
+                        }
+                    }
+                });
+
+            _ = Task.Run(
+                async () =>
+                {
+                    // SHOULD GET TIMEOUT!
+                    using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
+                    {
+                        using (await @lock.EnterAsync(timeout.Token))
+                        {
+                            Debug.WriteLine("Task 3 incremented");
+                            counter++;
+                        }
+                    }
+                });
+
+            _ = Task.Run(
+                async () =>
+                {
+                    using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(4)))
+                    {
+                        using (await @lock.EnterAsync(timeout.Token))
+                        {
+                            Debug.WriteLine("Task 4 incremented");
+                            counter++;
+                        }
+                    }
+                });
+
+            _ = Task.Run(
+                async () =>
+                {
+                    using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5)))
+                    {
+                        using (await @lock.EnterAsync(timeout.Token))
+                        {
+                            Debug.WriteLine("Task 5 incremented");
+                            counter++;
+                        }
+                    }
+                });
+
+            _ = Task.Run(
+                async () =>
+                {
+                    using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(6)))
+                    {
+                        using (await @lock.EnterAsync(timeout.Token))
+                        {
+                            Debug.WriteLine("Task 6 incremented");
+                            counter++;
+                        }
+                    }
+                });
+
+            Debug.WriteLine("Delay before release...");
+            await Task.Delay(TimeSpan.FromSeconds(3.1));
+            releaser.Dispose();
+
+            Debug.WriteLine("Wait for all tasks...");
+            await Task.Delay(TimeSpan.FromSeconds(6.1));
+
+            Assert.AreEqual(3, counter);
+        }
+
+        [TestMethod]
+        public void Lock_Parallel_Tasks()
+        {
+            const int taskCount = 50;
+            
+            var @lock = new AsyncLock();
+            
+            var tasks = new Task[taskCount];
             var globalI = 0;
-            for (var i = 0; i < ThreadsCount; i++)
+            for (var i = 0; i < taskCount; i++)
             {
 #pragma warning disable CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
-                threads[i] = Task.Run(
+                tasks[i] = Task.Run(
                     async () =>
                     {
-                        using (await @lock.WaitAsync(CancellationToken.None))
+                        using (await @lock.EnterAsync())
                         {
                             var localI = globalI;
-                            await Task.Delay(10); // Increase the chance for wrong data.
+                            await Task.Delay(5); // Increase the chance for wrong data.
                             localI++;
                             globalI = localI;
                         }
@@ -39,8 +143,8 @@ namespace MQTTnet.Tests.Internal
 #pragma warning restore CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
             }
 
-            Task.WaitAll(threads);
-            Assert.AreEqual(ThreadsCount, globalI);
+            Task.WaitAll(tasks);
+            Assert.AreEqual(taskCount, globalI);
         }
 
         [TestMethod]
@@ -57,7 +161,7 @@ namespace MQTTnet.Tests.Internal
                 threads[i] = Task.Run(
                         async () =>
                         {
-                            using (await @lock.WaitAsync(CancellationToken.None))
+                            using (await @lock.EnterAsync())
                             {
                                 var localI = globalI;
                                 await Task.Delay(10); // Increase the chance for wrong data.
@@ -96,7 +200,7 @@ namespace MQTTnet.Tests.Internal
             var @lock = new AsyncLock();
             for (var i = 0; i < 100; i++)
             {
-                using (await @lock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+                using (await @lock.EnterAsync().ConfigureAwait(false))
                 {
                     sum++;
                 }
@@ -112,11 +216,11 @@ namespace MQTTnet.Tests.Internal
             var @lock = new AsyncLock();
 
             // This call will never "release" the lock due to missing _using_.
-            await @lock.WaitAsync(CancellationToken.None).ConfigureAwait(false);
+            await @lock.EnterAsync().ConfigureAwait(false);
 
             using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
             {
-                await @lock.WaitAsync(cts.Token).ConfigureAwait(false);
+                await @lock.EnterAsync(cts.Token).ConfigureAwait(false);
             }
         }
 
@@ -125,15 +229,15 @@ namespace MQTTnet.Tests.Internal
         {
             var asyncLock = new AsyncLock();
 
-            var releaser = await asyncLock.WaitAsync(CancellationToken.None).ConfigureAwait(false);
+            var releaser = await asyncLock.EnterAsync().ConfigureAwait(false);
 
             try
             {
                 using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
                 {
-                    await asyncLock.WaitAsync(timeout.Token).ConfigureAwait(false);
+                    await asyncLock.EnterAsync(timeout.Token).ConfigureAwait(false);
                 }
-                
+
                 Assert.Fail("Exception should be thrown!");
             }
             catch (OperationCanceledException)
@@ -142,10 +246,46 @@ namespace MQTTnet.Tests.Internal
 
             releaser.Dispose();
 
-            using (await asyncLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+            using (await asyncLock.EnterAsync(CancellationToken.None).ConfigureAwait(false))
             {
                 // When the method finished, the thread got access.
             }
         }
+
+        [TestMethod]
+        public async Task Use_After_Cancellation()
+        {
+            var @lock = new AsyncLock();
+
+            // This call will not yet "release" the lock due to missing _using_.
+            var releaser = await @lock.EnterAsync().ConfigureAwait(false);
+
+            try
+            {
+                using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
+                {
+                    await @lock.EnterAsync(cts.Token).ConfigureAwait(false);
+                }
+            }
+            catch (OperationCanceledException)
+            {
+                // Expected exception!
+            }
+
+            releaser.Dispose();
+
+            // Regular usage after cancellation.
+            using (await @lock.EnterAsync().ConfigureAwait(false))
+            {
+            }
+
+            using (await @lock.EnterAsync().ConfigureAwait(false))
+            {
+            }
+
+            using (await @lock.EnterAsync().ConfigureAwait(false))
+            {
+            }
+        }
     }
 }

Source/MQTTnet.Tests/Internal/AsyncSignal_Tests.cs

@@ -0,0 +1,128 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Diagnostics;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Internal;
+
+namespace MQTTnet.Tests.Internal
+{
+    [TestClass]
+    public sealed class AsyncSignal_Tests
+    {
+        [TestMethod]
+        [ExpectedException(typeof(TaskCanceledException))]
+        public async Task Cancel_If_No_Signal()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(2)))
+            {
+                await asyncSignal.WaitAsync(timeout.Token);
+
+                Assert.Fail("There is no signal. So we must fail here!");
+            }
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(ObjectDisposedException))]
+        public async Task Dispose_Properly()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            // The timeout will not be reached but another task will kill the async signal.
+            using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(999)))
+            {
+                _ = Task.Run(
+                    async () =>
+                    {
+                        await Task.Delay(2000, CancellationToken.None);
+                        asyncSignal.Dispose();
+                    },
+                    CancellationToken.None);
+
+                await asyncSignal.WaitAsync(timeout.Token);
+
+                Assert.Fail("There is no signal. So we must fail here!");
+            }
+        }
+
+        [TestMethod]
+        public async Task Reset_Signal()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            // WaitAsync should fail because no signal is available.
+            for (var i = 0; i < 10; i++)
+            {
+                try
+                {
+                    using (var timeout = new CancellationTokenSource(TimeSpan.FromMilliseconds(100)))
+                    {
+                        await asyncSignal.WaitAsync(timeout.Token);
+                    }
+                    
+                    Assert.Fail("This must fail because the signal is not yet set.");
+                }
+                catch (OperationCanceledException)
+                {
+                }
+                
+                asyncSignal.Set();
+                
+                // WaitAsync should return directly because the signal is available.
+                await asyncSignal.WaitAsync();
+            }
+        }
+
+        [TestMethod]
+        public async Task Signal()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            // The timeout will not be reached but another task will kill the async signal.
+            using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
+            {
+                var stopwatch = Stopwatch.StartNew();
+
+                _ = Task.Run(
+                    async () =>
+                    {
+                        await Task.Delay(1000, CancellationToken.None);
+                        asyncSignal.Set();
+                    },
+                    CancellationToken.None);
+
+                await asyncSignal.WaitAsync(timeout.Token);
+
+                stopwatch.Stop();
+
+                Assert.IsTrue(stopwatch.ElapsedMilliseconds > 900);
+            }
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(InvalidOperationException))]
+        public async Task Fail_For_Two_Waiters()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            // This thread will wait properly because it is the first waiter.
+            _ = Task.Run(
+                async () =>
+                {
+                    await asyncSignal.WaitAsync();
+                },
+                CancellationToken.None);
+
+            await Task.Delay(1000);
+            
+            // Now the current thread must fail because there is already a waiter.
+            await asyncSignal.WaitAsync();
+        }
+    }
+}

Source/MQTTnet.Tests/Internal/MqttPacketBusItem_Tests.cs

@@ -0,0 +1,43 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Internal;
+using MQTTnet.Packets;
+
+namespace MQTTnet.Tests.Internal
+{
+    [TestClass]
+    public sealed class MqttPacketBusItem_Tests
+    {
+        [TestMethod]
+        public void Fire_Completed_Event()
+        {
+            var eventFired = false;
+
+            var item = new MqttPacketBusItem(new MqttPublishPacket());
+            item.Completed += (_, __) =>
+            {
+                eventFired = true;
+            };
+
+            item.Complete();
+
+            Assert.IsTrue(eventFired);
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(TaskCanceledException))]
+        public async Task Wait_Packet_Bus_Item_After_Already_Canceled()
+        {
+            var item = new MqttPacketBusItem(new MqttPublishPacket());
+
+            // Finish the item before the actual 
+            item.Cancel();
+
+            await item.WaitAsync();
+        }
+    }
+}

Source/MQTTnet.Tests/Internal/MqttPacketBus_Tests.cs

@@ -13,69 +13,26 @@ namespace MQTTnet.Tests.Internal
 {
     [TestClass]
     public sealed class MqttPacketBus_Tests
-    {
-        [TestMethod]
-        [ExpectedException(typeof(OperationCanceledException))]
-        public async Task Wait_With_Empty_Bus()
-        {
-            var bus = new MqttPacketBus();
-
-            using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(1)))
-            {
-                await bus.DequeueItemAsync(timeout.Token);
-            }
-        }
-        
-        [TestMethod]
-        public async Task Fill_From_Different_Task()
-        {
-            const int MessageCount = 500;
-
-            var delayRandom = new Random();
-            
-            var bus = new MqttPacketBus();
-
-            _ = Task.Run(
-                () =>
-                {
-                    for (int i = 0; i < MessageCount; i++)
-                    {
-                        bus.EnqueueItem(new MqttPacketBusItem(MqttPingReqPacket.Instance), MqttPacketBusPartition.Health);
-                        
-                        Thread.Sleep(delayRandom.Next(0, 10));
-                    }
-                });
-            
-            for (var i = 0; i < MessageCount; i++)
-            {
-                using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(10)))
-                {
-                    await bus.DequeueItemAsync(timeout.Token);
-                }    
-            }
-            
-            Assert.AreEqual(0, bus.TotalItemsCount);
-        }
-        
+    {       
         [TestMethod]
         public void Alternate_Priorities()
         {
             var bus = new MqttPacketBus();
-            
+
             bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
             bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
             bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            
+
             bus.EnqueueItem(new MqttPacketBusItem(new MqttSubAckPacket()), MqttPacketBusPartition.Control);
             bus.EnqueueItem(new MqttPacketBusItem(new MqttSubAckPacket()), MqttPacketBusPartition.Control);
             bus.EnqueueItem(new MqttPacketBusItem(new MqttSubAckPacket()), MqttPacketBusPartition.Control);
-            
+
             bus.EnqueueItem(new MqttPacketBusItem(new MqttPingRespPacket()), MqttPacketBusPartition.Health);
             bus.EnqueueItem(new MqttPacketBusItem(new MqttPingRespPacket()), MqttPacketBusPartition.Health);
             bus.EnqueueItem(new MqttPacketBusItem(new MqttPingRespPacket()), MqttPacketBusPartition.Health);
 
             Assert.AreEqual(9, bus.TotalItemsCount);
-            
+
             Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPublishPacket));
             Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttSubAckPacket));
             Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPingRespPacket));
@@ -85,33 +42,10 @@ namespace MQTTnet.Tests.Internal
             Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPublishPacket));
             Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttSubAckPacket));
             Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPingRespPacket));
-            
+
             Assert.AreEqual(0, bus.TotalItemsCount);
         }
-        
-        [TestMethod]
-        public void Export_Packets_Without_Dequeue()
-        {
-            var bus = new MqttPacketBus();
 
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            
-            Assert.AreEqual(3, bus.TotalItemsCount);
-
-            var exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Control);
-            Assert.AreEqual(0, exportedPackets.Count);
-            
-            exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Health);
-            Assert.AreEqual(0, exportedPackets.Count);
-            
-            exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Data);
-            Assert.AreEqual(3, exportedPackets.Count);
-            
-            Assert.AreEqual(3, bus.TotalItemsCount);
-        }
-        
         [TestMethod]
         public void Await_Single_Packet()
         {
@@ -121,31 +55,97 @@ namespace MQTTnet.Tests.Internal
 
             var item1 = new MqttPacketBusItem(new MqttPublishPacket());
             var item2 = new MqttPacketBusItem(new MqttPublishPacket());
-            
+
             var item3 = new MqttPacketBusItem(new MqttPublishPacket());
-            item3.Delivered += (_, __) =>
+            item3.Completed += (_, __) =>
             {
                 delivered = true;
             };
-            
+
             bus.EnqueueItem(item1, MqttPacketBusPartition.Data);
             bus.EnqueueItem(item2, MqttPacketBusPartition.Data);
             bus.EnqueueItem(item3, MqttPacketBusPartition.Data);
-            
+
             Assert.IsFalse(delivered);
 
-            bus.DequeueItemAsync(CancellationToken.None).Result.MarkAsDelivered();
-            
+            bus.DequeueItemAsync(CancellationToken.None).Result.Complete();
+
             Assert.IsFalse(delivered);
-            
-            bus.DequeueItemAsync(CancellationToken.None).Result.MarkAsDelivered();
-            
+
+            bus.DequeueItemAsync(CancellationToken.None).Result.Complete();
+
             Assert.IsFalse(delivered);
-            
-            bus.DequeueItemAsync(CancellationToken.None).Result.MarkAsDelivered();
-            
+
+            bus.DequeueItemAsync(CancellationToken.None).Result.Complete();
+
             // The third packet has the event attached.
             Assert.IsTrue(delivered);
         }
+
+        [TestMethod]
+        public void Export_Packets_Without_Dequeue()
+        {
+            var bus = new MqttPacketBus();
+
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
+
+            Assert.AreEqual(3, bus.TotalItemsCount);
+
+            var exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Control);
+            Assert.AreEqual(0, exportedPackets.Count);
+
+            exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Health);
+            Assert.AreEqual(0, exportedPackets.Count);
+
+            exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Data);
+            Assert.AreEqual(3, exportedPackets.Count);
+
+            Assert.AreEqual(3, bus.TotalItemsCount);
+        }
+
+        [TestMethod]
+        public async Task Fill_From_Different_Task()
+        {
+            const int MessageCount = 500;
+
+            var delayRandom = new Random();
+
+            var bus = new MqttPacketBus();
+
+            _ = Task.Run(
+                () =>
+                {
+                    for (var i = 0; i < MessageCount; i++)
+                    {
+                        bus.EnqueueItem(new MqttPacketBusItem(MqttPingReqPacket.Instance), MqttPacketBusPartition.Health);
+
+                        Thread.Sleep(delayRandom.Next(0, 10));
+                    }
+                });
+
+            for (var i = 0; i < MessageCount; i++)
+            {
+                using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(10)))
+                {
+                    await bus.DequeueItemAsync(timeout.Token);
+                }
+            }
+
+            Assert.AreEqual(0, bus.TotalItemsCount);
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(TaskCanceledException))]
+        public async Task Wait_With_Empty_Bus()
+        {
+            var bus = new MqttPacketBus();
+
+            using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(1)))
+            {
+                await bus.DequeueItemAsync(timeout.Token);
+            }
+        }
     }
 }

Source/MQTTnet.Tests/MQTTnet.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFrameworks>net452;net461;netcoreapp3.1;net6.0</TargetFrameworks>
+        <TargetFrameworks>net452;net461;net48;netcoreapp3.1;net6.0</TargetFrameworks>
         <IsPackable>false</IsPackable>
         <LangVersion>7.3</LangVersion>
         <EnableNETAnalyzers>false</EnableNETAnalyzers>

Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs

@@ -8,10 +8,9 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
-using MQTTnet.Tests.Mockups;
 
 namespace MQTTnet.Tests.MQTTv5
 {
@@ -43,7 +42,7 @@ namespace MQTTnet.Tests.MQTTv5
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.PublishAsync(new MqttApplicationMessageBuilder()
@@ -155,7 +154,7 @@ namespace MQTTnet.Tests.MQTTv5
                 var result = await client.PublishStringAsync("a", "b", MqttQualityOfServiceLevel.AtLeastOnce);
                 await client.DisconnectAsync();
 
-                Assert.AreEqual(MqttClientPublishReasonCode.Success, result.ReasonCode);
+                Assert.AreEqual(MqttClientPublishReasonCode.NoMatchingSubscribers, result.ReasonCode);
             }
         }
 
@@ -170,7 +169,7 @@ namespace MQTTnet.Tests.MQTTv5
                 var result = await client.PublishStringAsync("a", "b", MqttQualityOfServiceLevel.ExactlyOnce);
                 await client.DisconnectAsync();
 
-                Assert.AreEqual(MqttClientPublishReasonCode.Success, result.ReasonCode);
+                Assert.AreEqual(MqttClientPublishReasonCode.NoMatchingSubscribers, result.ReasonCode);
             }
         }
 
@@ -245,7 +244,7 @@ namespace MQTTnet.Tests.MQTTv5
                 receiver.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var sender = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500));

Source/MQTTnet.Tests/MQTTv5/Server_Tests.cs

@@ -8,7 +8,7 @@ using MQTTnet.Formatter;
 using MQTTnet.Tests.Mockups;
 using System.Threading;
 using System.Threading.Tasks;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests.MQTTv5
@@ -31,7 +31,7 @@ namespace MQTTnet.Tests.MQTTv5
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
@@ -171,7 +171,6 @@ namespace MQTTnet.Tests.MQTTv5
                 // Session should be present
 
                 Assert.IsTrue(!result3.IsSessionPresent, "Session is present when it should not");
-
             }
         }
 
@@ -180,7 +179,7 @@ namespace MQTTnet.Tests.MQTTv5
         {
             using (var testEnvironment = CreateTestEnvironment())
             {
-                var disconnectReason = MQTTnet.Client.MqttClientDisconnectReason.UnspecifiedError;
+                var disconnectReason = MqttClientDisconnectReason.UnspecifiedError;
 
                 string testClientId = null;
 
@@ -189,15 +188,15 @@ namespace MQTTnet.Tests.MQTTv5
                 testEnvironment.Server.ClientConnectedAsync += e =>
                 {
                     testClientId = e.ClientId;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
-                var client = await testEnvironment.ConnectClient(new MQTTnet.Client.MqttClientOptionsBuilder().WithProtocolVersion(MQTTnet.Formatter.MqttProtocolVersion.V500));
+                var client = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500));
 
                 client.DisconnectedAsync += e =>
                 {
                     disconnectReason = e.Reason;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await LongTestDelay();
@@ -208,17 +207,17 @@ namespace MQTTnet.Tests.MQTTv5
 
                 // Have the server disconnect the client with AdministrativeAction reason
 
-                await testEnvironment.Server.DisconnectClientAsync(testClientId, Protocol.MqttDisconnectReasonCode.AdministrativeAction);
+                await testEnvironment.Server.DisconnectClientAsync(testClientId, MqttDisconnectReasonCode.AdministrativeAction);
 
                 await LongTestDelay();
 
                 // The reason should be returned to the client in the DISCONNECT packet
 
-                Assert.AreEqual(MQTTnet.Client.MqttClientDisconnectReason.AdministrativeAction, disconnectReason);
+                Assert.AreEqual(MqttClientDisconnectReason.AdministrativeAction, disconnectReason);
             }
         }
 
-        MqttClientOptions CreateClientOptions(TestEnvironment testEnvironment, string clientId, bool cleanSession, uint sessionExpiryInterval)
+        static MqttClientOptions CreateClientOptions(TestEnvironment testEnvironment, string clientId, bool cleanSession, uint sessionExpiryInterval)
         {
             return testEnvironment.Factory.CreateClientOptionsBuilder()
                 .WithProtocolVersion(MqttProtocolVersion.V500)

Source/MQTTnet.Tests/Mockups/MemoryMqttChannel.cs

@@ -8,6 +8,7 @@ using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Channel;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Mockups
 {
@@ -33,12 +34,12 @@ namespace MQTTnet.Tests.Mockups
 
         public Task ConnectAsync(CancellationToken cancellationToken)
         {
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public Task DisconnectAsync(CancellationToken cancellationToken)
         {
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)

Source/MQTTnet.Tests/Mockups/TestApplicationMessageReceivedHandler.cs

@@ -2,12 +2,14 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Text;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Mockups
 {
@@ -17,7 +19,12 @@ namespace MQTTnet.Tests.Mockups
 
         public TestApplicationMessageReceivedHandler(IMqttClient mqttClient)
         {
-            mqttClient.ApplicationMessageReceivedAsync += MqttClientOnApplicationMessageReceivedAsync;
+            if (mqttClient == null)
+            {
+                throw new ArgumentNullException(nameof(mqttClient));
+            }
+
+            mqttClient.ApplicationMessageReceivedAsync += OnApplicationMessageReceivedAsync;
         }
 
         public List<MqttApplicationMessageReceivedEventArgs> ReceivedEventArgs
@@ -39,14 +46,32 @@ namespace MQTTnet.Tests.Mockups
             }
         }
 
-        Task MqttClientOnApplicationMessageReceivedAsync(MqttApplicationMessageReceivedEventArgs eventArgs)
+        public string GeneratePayloadSequence()
+        {
+            var sequence = new StringBuilder();
+
+            lock (_receivedEventArgs)
+            {
+                foreach (var receivedEventArg in _receivedEventArgs)
+                {
+                    var payload = receivedEventArg.ApplicationMessage.ConvertPayloadToString();
+
+                    // An empty payload is not part of the sequence!
+                    sequence.Append(payload);
+                }
+            }
+
+            return sequence.ToString();
+        }
+
+        Task OnApplicationMessageReceivedAsync(MqttApplicationMessageReceivedEventArgs eventArgs)
         {
             lock (_receivedEventArgs)
             {
                 _receivedEventArgs.Add(eventArgs);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
     }
 }

Source/MQTTnet.Tests/Mockups/TestEnvironment.cs

@@ -11,9 +11,10 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Diagnostics;
+using MQTTnet.Extensions.ManagedClient;
 using MQTTnet.Extensions.Rpc;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.LowLevelClient;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
@@ -109,7 +110,7 @@ namespace MQTTnet.Tests.Mockups
             configureOptions.Invoke(optionsBuilder);
 
             var options = optionsBuilder.Build();
-            
+
             var client = CreateClient();
 
             if (timeout == TimeSpan.Zero)
@@ -137,7 +138,7 @@ namespace MQTTnet.Tests.Mockups
             options = options.WithTcpServer("127.0.0.1", ServerPort);
 
             var client = CreateClient();
-            
+
             if (timeout == TimeSpan.Zero)
             {
                 await client.ConnectAsync(options.Build()).ConfigureAwait(false);
@@ -161,7 +162,7 @@ namespace MQTTnet.Tests.Mockups
             }
 
             var client = CreateClient();
-            
+
             if (timeout == TimeSpan.Zero)
             {
                 await client.ConnectAsync(options).ConfigureAwait(false);
@@ -199,6 +200,16 @@ namespace MQTTnet.Tests.Mockups
             return new TestApplicationMessageReceivedHandler(mqttClient);
         }
 
+        public TestApplicationMessageReceivedHandler CreateApplicationMessageHandler(IManagedMqttClient managedClient)
+        {
+            if (managedClient == null)
+            {
+                throw new ArgumentNullException(nameof(managedClient));
+            }
+
+            return new TestApplicationMessageReceivedHandler(managedClient.InternalClient);
+        }
+
         public IMqttClient CreateClient()
         {
             lock (_clients)
@@ -220,7 +231,7 @@ namespace MQTTnet.Tests.Mockups
                         }
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 return client;
@@ -261,7 +272,7 @@ namespace MQTTnet.Tests.Mockups
                     }
                 }
 
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             };
 
             return Server;
@@ -325,7 +336,7 @@ namespace MQTTnet.Tests.Mockups
             var options = optionsBuilder.Build();
             var server = CreateServer(options);
             await server.StartAsync();
-            
+
             // The OS has chosen the port to we have to properly expose it to the tests.
             ServerPort = options.DefaultEndpointOptions.Port;
             return server;
@@ -344,7 +355,7 @@ namespace MQTTnet.Tests.Mockups
             var options = optionsBuilder.Build();
             var server = CreateServer(options);
             await server.StartAsync();
-            
+
             // The OS has chosen the port to we have to properly expose it to the tests.
             ServerPort = options.DefaultEndpointOptions.Port;
             return server;

Source/MQTTnet.Tests/RPC_Tests.cs

@@ -13,6 +13,7 @@ using MQTTnet.Extensions.Rpc;
 using MQTTnet.Protocol;
 using MQTTnet.Formatter;
 using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Tests
 {
@@ -189,7 +190,7 @@ namespace MQTTnet.Tests
                 responseSender.ApplicationMessageReceivedAsync += async e =>
                 {
                     Assert.IsNull(e.ApplicationMessage.ResponseTopic);
-                    await PlatformAbstractionLayer.CompletedTask;
+                    await CompletedTask.Instance;
                 };
 
                 var requestSender = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500));

Source/MQTTnet.Tests/RoundtripTime_Tests.cs

@@ -8,7 +8,7 @@ using System.Diagnostics;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Tests.Mockups;
 
 namespace MQTTnet.Tests
@@ -33,7 +33,7 @@ namespace MQTTnet.Tests
                 receiverClient.ApplicationMessageReceivedAsync += e => 
                 {
                     response?.TrySetResult(e.ApplicationMessage.ConvertPayloadToString());
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await receiverClient.SubscribeAsync("#");

Source/MQTTnet.Tests/Server/Assigned_Client_ID_Tests.cs

@@ -7,7 +7,7 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests.Server
@@ -47,20 +47,20 @@ namespace MQTTnet.Tests.Server
                         e.ReasonCode = MqttConnectReasonCode.Success;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 testEnvironment.Server.ClientConnectedAsync += args =>
                 {
                     serverConnectedClientId = args.ClientId;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 testEnvironment.Server.ClientDisconnectedAsync += args =>
                 {
                     serverDisconnectedClientId = args.ClientId;
                     disconnectedMre.Set();
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 // Arrange client
@@ -68,7 +68,7 @@ namespace MQTTnet.Tests.Server
                 client.ConnectedAsync += args =>
                 {
                     clientAssignedClientId = args.ConnectResult.AssignedClientIdentifier;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 // Act

Source/MQTTnet.Tests/Server/Events_Tests.cs

@@ -7,7 +7,7 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 
@@ -27,7 +27,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientConnectedAsync += e =>
                 {
                     eventArgs = e;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await testEnvironment.ConnectClient(o => o.WithCredentials("TheUser"));
@@ -54,7 +54,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientDisconnectedAsync += e =>
                 {
                     eventArgs = e;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient(o => o.WithCredentials("TheUser"));
@@ -81,7 +81,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientSubscribedTopicAsync += e =>
                 {
                     eventArgs = e;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -108,7 +108,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientUnsubscribedTopicAsync += e =>
                 {
                     eventArgs = e;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
         
                 var client = await testEnvironment.ConnectClient();
@@ -134,7 +134,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     eventArgs = e;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
         
                 var client = await testEnvironment.ConnectClient();
@@ -161,7 +161,7 @@ namespace MQTTnet.Tests.Server
                 server.StartedAsync += e =>
                 {
                     eventArgs = e;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 await server.StartAsync();
@@ -183,7 +183,7 @@ namespace MQTTnet.Tests.Server
                 server.StoppedAsync += e =>
                 {
                     eventArgs = e;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await server.StopAsync();

Source/MQTTnet.Tests/Server/General.cs

@@ -11,8 +11,7 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Adapter;
 using MQTTnet.Client;
-using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
@@ -99,7 +98,7 @@ namespace MQTTnet.Tests.Server
                 server.ValidatingConnectionAsync += e =>
                 {
                     e.ReasonCode = MqttConnectReasonCode.NotAuthorized;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var connectingFailedException = await Assert.ThrowsExceptionAsync<MqttConnectingFailedException>(() => testEnvironment.ConnectClient());
@@ -122,7 +121,7 @@ namespace MQTTnet.Tests.Server
                         e.Response.ReasonCode = MqttSubscribeReasonCode.UnspecifiedError;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 // Prepare some retained messages.
@@ -146,7 +145,7 @@ namespace MQTTnet.Tests.Server
                         buffer.Append(e.ApplicationMessage.Topic);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client2.SubscribeAsync("y");
@@ -172,13 +171,13 @@ namespace MQTTnet.Tests.Server
                 server.ClientConnectedAsync += e =>
                 {
                     Interlocked.Increment(ref clientConnectedCalled);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.ClientDisconnectedAsync += e =>
                 {
                     Interlocked.Increment(ref clientDisconnectedCalled);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var c1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder());
@@ -225,7 +224,7 @@ namespace MQTTnet.Tests.Server
                                         // Clear retained message.
                                         await client.PublishAsync(
                                             new MqttApplicationMessageBuilder().WithTopic("r" + i2)
-                                                .WithPayload(PlatformAbstractionLayer.EmptyByteArray)
+                                                .WithPayload(EmptyBuffer.Array)
                                                 .WithRetainFlag()
                                                 .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce)
                                                 .Build());
@@ -276,7 +275,7 @@ namespace MQTTnet.Tests.Server
                 {
                     e.ApplicationMessage = new MqttApplicationMessage { Topic = "new_topic" };
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 string receivedTopic = null;
@@ -286,7 +285,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += a =>
                 {
                     receivedTopic = a.ApplicationMessage.Topic;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("original_topic").Build());
@@ -305,7 +304,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     e.ApplicationMessage.Payload = Encoding.ASCII.GetBytes("extended");
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var c1 = await testEnvironment.ConnectClient();
@@ -316,7 +315,7 @@ namespace MQTTnet.Tests.Server
                 c2.ApplicationMessageReceivedAsync += e =>
                 {
                     isIntercepted = string.Compare("extended", Encoding.UTF8.GetString(e.ApplicationMessage.Payload), StringComparison.Ordinal) == 0;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("test").Build());
@@ -339,7 +338,7 @@ namespace MQTTnet.Tests.Server
                 server.ApplicationMessageNotConsumedAsync += e =>
                 {
                     undeliverd = e.ApplicationMessage.Topic;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -376,7 +375,7 @@ namespace MQTTnet.Tests.Server
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await Task.Delay(500);
@@ -400,7 +399,7 @@ namespace MQTTnet.Tests.Server
                 s.RetainedMessageChangedAsync += e =>
                 {
                     savedRetainedMessages = e.StoredRetainedMessages;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var c1 = await testEnvironment.ConnectClient();
@@ -440,7 +439,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedTopic = e.ApplicationMessage.Topic;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync("#");
@@ -487,7 +486,7 @@ namespace MQTTnet.Tests.Server
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var message = new MqttApplicationMessageBuilder().WithTopic("a").WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce).Build();
@@ -521,13 +520,13 @@ namespace MQTTnet.Tests.Server
                 c2.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 c3.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c2.SubscribeAsync(new MqttTopicFilter { Topic = "a", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce }).ConfigureAwait(false);
@@ -581,7 +580,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("c");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.ClientDisconnectedAsync += e =>
@@ -591,7 +590,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("d");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var clientOptionsBuilder = new MqttClientOptionsBuilder().WithClientId(Guid.NewGuid().ToString());
@@ -621,7 +620,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("r");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c2.SubscribeAsync("topic");
@@ -669,7 +668,7 @@ namespace MQTTnet.Tests.Server
                 server.ValidatingConnectionAsync += e =>
                 {
                     ConnectionValidationHandler(e);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var events = new List<string>();
@@ -681,7 +680,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("c");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.ClientDisconnectedAsync += e =>
@@ -691,7 +690,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("d");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var clientOptions = new MqttClientOptionsBuilder().WithClientId("same_id");
@@ -706,7 +705,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("x");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 c1.ApplicationMessageReceivedAsync += e =>
@@ -716,7 +715,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("r");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 c1.SubscribeAsync("topic").Wait();
@@ -785,7 +784,7 @@ namespace MQTTnet.Tests.Server
                 client1.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedBody = e.ApplicationMessage.Payload;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client1.SubscribeAsync("string");
@@ -822,7 +821,7 @@ namespace MQTTnet.Tests.Server
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await Task.Delay(500);
@@ -850,7 +849,7 @@ namespace MQTTnet.Tests.Server
                 c1.DisconnectedAsync += e =>
                 {
                     disconnectCalled++;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await Task.Delay(100);
@@ -950,7 +949,7 @@ namespace MQTTnet.Tests.Server
                 testEnvironment.Server.ClientConnectedAsync += e =>
                 {
                     testClientId = e.ClientId;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = testEnvironment.CreateClient();
@@ -959,12 +958,12 @@ namespace MQTTnet.Tests.Server
                 {
                     if (e.Buffer.Length > 0)
                     {
-                        if (e.Buffer[0] == (byte)Protocol.MqttControlPacketType.Disconnect << 4)
+                        if (e.Buffer[0] == (byte)MqttControlPacketType.Disconnect << 4)
                         {
                             disconnectPacketReceived = true;
                         }
                     }
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.ConnectAsync(

Source/MQTTnet.Tests/Server/Load_Tests.cs

@@ -3,7 +3,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
@@ -26,7 +26,7 @@ namespace MQTTnet.Tests.Server
                     receiverClient.ApplicationMessageReceivedAsync += e =>
                     {
                         Interlocked.Increment(ref receivedMessages);
-                        return PlatformAbstractionLayer.CompletedTask;
+                        return CompletedTask.Instance;
                     };
 
                     await receiverClient.SubscribeAsync("t/+");
@@ -55,7 +55,7 @@ namespace MQTTnet.Tests.Server
                             });
                     }
 
-                    SpinWait.SpinUntil(() => receivedMessages == 100000, TimeSpan.FromSeconds(60));
+                    SpinWait.SpinUntil(() => receivedMessages == 100000, TimeSpan.FromSeconds(120));
 
                     Assert.AreEqual(100000, receivedMessages);
                 }
@@ -74,7 +74,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessages);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 for (var i = 0; i < 100; i++)
@@ -137,7 +137,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessages);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 for (var i = 0; i < 100; i++)

Source/MQTTnet.Tests/Server/MqttSubscriptionsManager_Tests.cs

@@ -130,9 +130,7 @@ namespace MQTTnet.Tests.Server
 
         CheckSubscriptionsResult CheckSubscriptions(string topic, MqttQualityOfServiceLevel applicationMessageQoSLevel, string senderClientId)
         {
-            ulong topicHashMask; // not needed
-            bool hasWildcard; // not needed
-            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out topicHashMask, out hasWildcard);
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out _, out _);
             return _subscriptionsManager.CheckSubscriptions(topic, topicHash, applicationMessageQoSLevel, senderClientId);
         }
     }

Source/MQTTnet.Tests/Server/Publish_Tests.cs

@@ -0,0 +1,58 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Formatter;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Tests.Server
+{
+    [TestClass]
+    public sealed class Publish_Tests : BaseTestClass
+    {
+        [TestMethod]
+        public async Task Return_Success_When_Subscribed()
+        {
+            using (var testEnvironment = CreateTestEnvironment(MqttProtocolVersion.V500))
+            {
+                await testEnvironment.StartServer();
+
+                var sender = await testEnvironment.ConnectClient();
+                var receiver = await testEnvironment.ConnectClient();
+
+                await receiver.SubscribeAsync("A");
+
+                // AtLeastOnce is required to get an ACK packet from the server.
+                var publishResult = await sender.PublishStringAsync("A", "Payload", MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                Assert.AreEqual(MqttClientPublishReasonCode.Success, publishResult.ReasonCode);
+                
+                Assert.AreEqual(true, publishResult.IsSuccess);
+            }
+        }
+        
+        [TestMethod]
+        public async Task Return_NoMatchingSubscribers_When_Not_Subscribed()
+        {
+            using (var testEnvironment = CreateTestEnvironment(MqttProtocolVersion.V500))
+            {
+                await testEnvironment.StartServer();
+
+                var sender = await testEnvironment.ConnectClient();
+                var receiver = await testEnvironment.ConnectClient();
+             
+                await receiver.SubscribeAsync("A");
+
+                // AtLeastOnce is required to get an ACK packet from the server.
+                var publishResult = await sender.PublishStringAsync("B", "Payload", MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                Assert.AreEqual(MqttClientPublishReasonCode.NoMatchingSubscribers, publishResult.ReasonCode);
+                
+                Assert.AreEqual(true, publishResult.IsSuccess);
+            }
+        }
+    }
+}

Source/MQTTnet.Tests/Server/Publishing_Tests.cs

@@ -0,0 +1,31 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Tests.Server
+{
+    [TestClass]
+    public sealed class Publishing_Tests : BaseTestClass
+    {
+        [TestMethod]
+        [ExpectedException(typeof(MqttClientDisconnectedException))]
+        public async Task Disconnect_While_Publishing()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                // The client will be disconnect directly after subscribing!
+                server.InterceptingPublishAsync += ev => server.DisconnectClientAsync(ev.ClientId, MqttDisconnectReasonCode.NormalDisconnection);
+
+                var client = await testEnvironment.ConnectClient();
+                await client.PublishStringAsync("test", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+            }
+        }
+    }
+}

Source/MQTTnet.Tests/Server/QoS_Tests.cs

@@ -6,7 +6,7 @@ using System.Collections.Generic;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 
@@ -15,6 +15,57 @@ namespace MQTTnet.Tests.Server
     [TestClass]
     public sealed class QoS_Tests : BaseTestClass
     {
+        [TestMethod]
+        public async Task Preserve_Message_Order_For_Queued_Messages()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer(o => o.WithPersistentSessions());
+
+                // Create a session which will contain the messages.
+                var dummyClient = await testEnvironment.ConnectClient(o => o.WithClientId("A").WithCleanSession(false));
+                await dummyClient.SubscribeAsync("#", MqttQualityOfServiceLevel.AtLeastOnce);
+                dummyClient.Dispose();
+                
+                // Now inject messages which are appended to the queue of the client.
+                await server.InjectApplicationMessage("T", "0", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "2", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                await server.InjectApplicationMessage("T", "1", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "4", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                await server.InjectApplicationMessage("T", "3", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "6", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                await server.InjectApplicationMessage("T", "5", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "8", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                await server.InjectApplicationMessage("T", "7", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "9", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await LongTestDelay();
+                
+                // Create a new client for the existing message.
+                var client = await testEnvironment.ConnectClient(o => o.WithClientId("A").WithCleanSession(false));
+                var messages = testEnvironment.CreateApplicationMessageHandler(client);
+                
+                await LongTestDelay();
+                
+                var payloadSequence = messages.GeneratePayloadSequence();
+                Assert.AreEqual("0214365879", payloadSequence);
+
+                // Disconnect and reconnect to make sure that the server will not send the messages twice.
+                await client.DisconnectAsync();
+                await LongTestDelay();
+                await client.ReconnectAsync();
+                await LongTestDelay();
+                
+                payloadSequence = messages.GeneratePayloadSequence();
+                Assert.AreEqual("0214365879", payloadSequence);
+            }
+        }
+        
         [TestMethod]
         public async Task Fire_Event_On_Client_Acknowledges_QoS_0()
         {
@@ -26,7 +77,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientAcknowledgedPublishPacketAsync += args =>
                 {
                     eventArgs = args;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client1 = await testEnvironment.ConnectClient();
@@ -53,7 +104,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientAcknowledgedPublishPacketAsync += args =>
                 {
                     eventArgs = args;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client1 = await testEnvironment.ConnectClient();
@@ -88,7 +139,7 @@ namespace MQTTnet.Tests.Server
                         eventArgs.Add(args);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client1 = await testEnvironment.ConnectClient();
@@ -99,25 +150,16 @@ namespace MQTTnet.Tests.Server
 
                 await LongTestDelay();
 
-                Assert.AreEqual(2, eventArgs.Count);
+                Assert.AreEqual(1, eventArgs.Count);
 
                 var firstEvent = eventArgs[0];
 
                 Assert.IsNotNull(firstEvent);
                 Assert.IsNotNull(firstEvent.PublishPacket);
                 Assert.IsNotNull(firstEvent.AcknowledgePacket);
-                Assert.IsFalse(firstEvent.IsCompleted);
+                Assert.IsTrue(firstEvent.IsCompleted);
 
                 Assert.AreEqual("A", firstEvent.PublishPacket.Topic);
-
-                var secondEvent = eventArgs[1];
-
-                Assert.IsNotNull(secondEvent);
-                Assert.IsNotNull(secondEvent.PublishPacket);
-                Assert.IsNotNull(secondEvent.AcknowledgePacket);
-                Assert.IsTrue(secondEvent.IsCompleted);
-
-                Assert.AreEqual("A", secondEvent.PublishPacket.Topic);
             }
         }
     }

Source/MQTTnet.Tests/Server/Retained_Messages_Tests.cs

@@ -65,6 +65,66 @@ namespace MQTTnet.Tests.Server
             }
         }
 
+        [TestMethod]
+        public async Task Downgrade_QoS_Level()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                await testEnvironment.StartServer();
+
+                var c1 = await testEnvironment.ConnectClient();
+
+                // Add the retained message with QoS 2!
+                await c1.PublishAsync(
+                    new MqttApplicationMessageBuilder().WithTopic("retained")
+                        .WithPayload(new byte[3])
+                        .WithRetainFlag()
+                        .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.ExactlyOnce)
+                        .Build());
+
+                // The second client uses QoS 1 so a downgrade is required.
+                var c2 = await testEnvironment.ConnectClient();
+                var messageHandler = testEnvironment.CreateApplicationMessageHandler(c2);
+                await c2.SubscribeAsync(new MqttTopicFilter { Topic = "retained", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce });
+
+                await LongTestDelay();
+
+                messageHandler.AssertReceivedCountEquals(1);
+
+                Assert.AreEqual(MqttQualityOfServiceLevel.AtLeastOnce, messageHandler.ReceivedEventArgs.First().ApplicationMessage.QualityOfServiceLevel);
+            }
+        }
+
+        [TestMethod]
+        public async Task No_Upgrade_QoS_Level()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                await testEnvironment.StartServer();
+
+                var c1 = await testEnvironment.ConnectClient();
+
+                // Add the retained message with QoS 1!
+                await c1.PublishAsync(
+                    new MqttApplicationMessageBuilder().WithTopic("retained")
+                        .WithPayload(new byte[3])
+                        .WithRetainFlag()
+                        .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce)
+                        .Build());
+
+                // The second client uses QoS 2 so an upgrade is expected but according to the MQTT spec this is not supported!
+                var c2 = await testEnvironment.ConnectClient();
+                var messageHandler = testEnvironment.CreateApplicationMessageHandler(c2);
+                await c2.SubscribeAsync(new MqttTopicFilter { Topic = "retained", QualityOfServiceLevel = MqttQualityOfServiceLevel.ExactlyOnce });
+
+                await LongTestDelay();
+
+                messageHandler.AssertReceivedCountEquals(1);
+
+                Assert.AreEqual(MqttQualityOfServiceLevel.AtLeastOnce, messageHandler.ReceivedEventArgs.First().ApplicationMessage.QualityOfServiceLevel);
+            }
+        }
+
         [TestMethod]
         public async Task Receive_No_Retained_Message_After_Subscribe()
         {
@@ -124,7 +184,7 @@ namespace MQTTnet.Tests.Server
                         .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce)
                         .WithRetainFlag()
                         .Build());
-                
+
                 await c1.DisconnectAsync();
 
                 // Subscribe using a new client.

Source/MQTTnet.Tests/Server/Security_Tests.cs

@@ -8,7 +8,7 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Adapter;
 using MQTTnet.Client;
 using MQTTnet.Exceptions;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests.Server
@@ -33,7 +33,7 @@ namespace MQTTnet.Tests.Server
                         publishedApplicationMessages.Add(eventArgs.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 testEnvironment.Server.ValidatingConnectionAsync += eventArgs =>
@@ -47,7 +47,7 @@ namespace MQTTnet.Tests.Server
                         eventArgs.ReasonCode = MqttConnectReasonCode.NotAuthorized;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 using (var validClient = testEnvironment.CreateClient())
@@ -157,7 +157,7 @@ namespace MQTTnet.Tests.Server
                         e.ReasonCode = MqttConnectReasonCode.BadUserNameOrPassword;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = testEnvironment.CreateClient();

Source/MQTTnet.Tests/Server/Server_Reference_Tests.cs

@@ -8,6 +8,7 @@ using MQTTnet.Adapter;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
 using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests.Server
@@ -28,7 +29,7 @@ namespace MQTTnet.Tests.Server
                 {
                     e.ReasonCode = MqttConnectReasonCode.ServerMoved;
                     e.ServerReference = "new_server";
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 try

Source/MQTTnet.Tests/Server/Session_Tests.cs

@@ -10,9 +10,8 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
-using MQTTnet.Server;
 using MQTTnet.Tests.Mockups;
 
 namespace MQTTnet.Tests.Server
@@ -29,7 +28,7 @@ namespace MQTTnet.Tests.Server
 
                 await testEnvironment.StartServer(o => o.WithPersistentSessions());
 
-                const string ClientId = "Client1";
+                const string clientId = "Client1";
 
                 // Create client with clean session and long session expiry interval
 
@@ -38,7 +37,7 @@ namespace MQTTnet.Tests.Server
                         .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
                         .WithSessionExpiryInterval(9999) // not relevant for v311 but testing impact
                         .WithCleanSession() // start and end with clean session
-                        .WithClientId(ClientId)
+                        .WithClientId(clientId)
                         .Build());
 
                 // Disconnect; empty session should be removed from server
@@ -57,7 +56,7 @@ namespace MQTTnet.Tests.Server
                     .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
                     .WithSessionExpiryInterval(9999) // not relevant for v311 but testing impact
                     .WithCleanSession(false) // see if there is a session
-                    .WithClientId(ClientId)
+                    .WithClientId(clientId)
                     .Build();
 
 
@@ -83,7 +82,7 @@ namespace MQTTnet.Tests.Server
                 server.SessionDeletedAsync += e =>
                 {
                     deletedEventFired = true;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -111,7 +110,7 @@ namespace MQTTnet.Tests.Server
                     e.SessionItems["can_subscribe_x"] = true;
                     e.SessionItems["default_payload"] = "Hello World";
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await testEnvironment.ConnectClient();
@@ -123,6 +122,8 @@ namespace MQTTnet.Tests.Server
             }
         }
 
+        #if !NET452
+        
         [TestMethod]
         [DataRow(MqttProtocolVersion.V311)]
         [DataRow(MqttProtocolVersion.V500)]
@@ -134,7 +135,7 @@ namespace MQTTnet.Tests.Server
 
                 await testEnvironment.StartServer();
 
-                var options = new MqttClientOptionsBuilder().WithClientId("1").WithTimeout(TimeSpan.FromSeconds(1)).WithProtocolVersion(protocolVersion).WithKeepAlivePeriod(TimeSpan.FromSeconds(5));
+                var options = new MqttClientOptionsBuilder().WithClientId("1").WithTimeout(TimeSpan.FromSeconds(1)).WithProtocolVersion(protocolVersion);
 
                 var hasReceive = false;
 
@@ -149,23 +150,24 @@ namespace MQTTnet.Tests.Server
                 // Try to connect 50 clients at the same time.
                 var clients = await Task.WhenAll(Enumerable.Range(0, 50).Select(i => ConnectAndSubscribe(testEnvironment, options, OnReceive)));
 
+                await Task.Delay(TimeSpan.FromSeconds(10));
+                
+                var connectedClients = clients.Where(c => c?.TryPingAsync().GetAwaiter().GetResult() == true).ToList();
+
                 await LongTestDelay();
                 
-                var connectedClients = clients.Where(c => c != null).Where(c => c.TryPingAsync().GetAwaiter().GetResult()).ToList();
-
                 Assert.AreEqual(1, connectedClients.Count);
 
-                await Task.Delay(5000);
-
                 var option2 = new MqttClientOptionsBuilder().WithClientId("2").WithKeepAlivePeriod(TimeSpan.FromSeconds(10));
                 var sendClient = await testEnvironment.ConnectClient(option2);
                 await sendClient.PublishStringAsync("aaa", "1");
 
-                await Task.Delay(3000);
+                await Task.Delay(TimeSpan.FromSeconds(5));
 
                 Assert.AreEqual(true, hasReceive);
             }
         }
+#endif
         
         [DataTestMethod]
         [DataRow(MqttQualityOfServiceLevel.ExactlyOnce)]
@@ -186,7 +188,7 @@ namespace MQTTnet.Tests.Server
                     c.AutoAcknowledge = false;
                     ++count;
                     Console.WriteLine("process");
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await subscriber.SubscribeAsync("#", qos);
@@ -223,7 +225,7 @@ namespace MQTTnet.Tests.Server
                 client1.DisconnectedAsync += c =>
                 {
                     disconnectReason = c.Reason;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client2 = await testEnvironment.ConnectClient(options);
@@ -249,7 +251,7 @@ namespace MQTTnet.Tests.Server
                     e.SessionItems["can_subscribe_x"] = true;
                     e.SessionItems["default_payload"] = "Hello World";
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.InterceptingSubscriptionAsync += e =>
@@ -262,13 +264,13 @@ namespace MQTTnet.Tests.Server
                         }
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.InterceptingPublishAsync += e =>
                 {
                     e.ApplicationMessage.Payload = Encoding.UTF8.GetBytes(e.SessionItems["default_payload"] as string ?? string.Empty);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 string receivedPayload = null;
@@ -277,7 +279,7 @@ namespace MQTTnet.Tests.Server
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedPayload = e.ApplicationMessage.ConvertPayloadToString();
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var subscribeResult = await client.SubscribeAsync("x");
@@ -327,7 +329,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
@@ -335,10 +337,10 @@ namespace MQTTnet.Tests.Server
                 // C2 has the last will defined.
                 var clientOptions = new MqttClientOptionsBuilder().WithWillTopic("My/last/will").WithClientId("WillOwner");
 
-                var c2 = await testEnvironment.ConnectClient(clientOptions);
+                await testEnvironment.ConnectClient(clientOptions);
 
                 // C3 will do the connection takeover.
-                var c3 = await testEnvironment.ConnectClient(clientOptions);
+                await testEnvironment.ConnectClient(clientOptions);
 
                 await Task.Delay(1000);
 
@@ -350,34 +352,22 @@ namespace MQTTnet.Tests.Server
         {
             try
             {
-                var sendClient = await testEnvironment.ConnectClient(options);
+                var client = await testEnvironment.ConnectClient(options).ConfigureAwait(false);
 
-                sendClient.ApplicationMessageReceivedAsync += e =>
+                client.ApplicationMessageReceivedAsync += e =>
                 {
                     onReceive();
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
-
-                using (var subscribeTimeout = new CancellationTokenSource(TimeSpan.FromSeconds(5)))
+                
+                using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5)))
                 {
-                    await sendClient.SubscribeAsync("aaa", MqttQualityOfServiceLevel.AtMostOnce, subscribeTimeout.Token).ConfigureAwait(false);
+                    await client.SubscribeAsync("aaa", MqttQualityOfServiceLevel.AtMostOnce, timeout.Token).ConfigureAwait(false);
                 }
 
-                return sendClient;
+                return client;
             }
-            catch
-            {
-                return null;
-            }
-        }
-
-        static async Task<IMqttClient> TryConnect(TestEnvironment testEnvironment, MqttClientOptionsBuilder options)
-        {
-            try
-            {
-                return await testEnvironment.ConnectClient(options);
-            }
-            catch
+            catch (Exception)
             {
                 return null;
             }

Source/MQTTnet.Tests/Server/Subscribe_Tests.cs

@@ -9,7 +9,7 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
@@ -33,7 +33,7 @@ namespace MQTTnet.Tests.Server
                         e.Response.ReasonCode = MqttSubscribeReasonCode.TopicFilterInvalid;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -46,6 +46,22 @@ namespace MQTTnet.Tests.Server
             }
         }
 
+        [TestMethod]
+        [ExpectedException(typeof(MqttClientDisconnectedException))]
+        public async Task Disconnect_While_Subscribing()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                // The client will be disconnect directly after subscribing!
+                server.ClientSubscribedTopicAsync += ev => server.DisconnectClientAsync(ev.ClientId, MqttDisconnectReasonCode.NormalDisconnection);
+
+                var client = await testEnvironment.ConnectClient();
+                await client.SubscribeAsync("#");
+            }
+        }
+        
         [TestMethod]
         public async Task Enqueue_Message_After_Subscription()
         {
@@ -56,7 +72,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientSubscribedTopicAsync += e =>
                 {
                     server.InjectApplicationMessage(new InjectedMqttApplicationMessage(new MqttApplicationMessageBuilder().WithTopic("test_topic").Build()));
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -81,7 +97,7 @@ namespace MQTTnet.Tests.Server
                 {
                     // Set the topic to "a" regards what the client wants to subscribe.
                     e.TopicFilter.Topic = "a";
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var topicAReceived = false;
@@ -99,7 +115,7 @@ namespace MQTTnet.Tests.Server
                         topicBReceived = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.SubscribeAsync("b");
@@ -146,7 +162,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 for (var i = 0; i < 500; i++)
@@ -189,7 +205,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var optionsBuilder = new MqttClientSubscribeOptionsBuilder();
@@ -229,7 +245,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttClientSubscribeOptionsBuilder().WithTopicFilter("a").Build());
@@ -267,7 +283,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttClientSubscribeOptionsBuilder().WithTopicFilter("a").WithTopicFilter("b").WithTopicFilter("c").Build());
@@ -301,7 +317,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var c2 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("c2"));
@@ -316,7 +332,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientSubscribedTopicAsync += e =>
                 {
                     subscribeEventCalled = e.TopicFilter.Topic == "a" && e.ClientId == c1.Options.ClientId;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilter { Topic = "a", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce });
@@ -331,7 +347,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientUnsubscribedTopicAsync += e =>
                 {
                     unsubscribeEventCalled = e.TopicFilter == "a" && e.ClientId == c1.Options.ClientId;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.UnsubscribeAsync("a");

Source/MQTTnet.Tests/Server/Subscription_TopicHash_Tests.cs

@@ -1,11 +1,11 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Text;
-using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
-using MQTTnet.Client;
+using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 using MQTTnet.Tests.Mockups;
@@ -13,49 +13,301 @@ using MQTTnet.Tests.Mockups;
 namespace MQTTnet.Tests.Server
 {
     [TestClass]
-    public class Subscription_TopicHash_Tests
+    public sealed class SubscriptionTopicHashTests
     {
-        enum TopicHashSelector {  NoWildcard, SingleWildcard, MultiWildcard };
+        MqttSession _clientSession;
+
+        /// <summary>
+        ///     Even fairly regularly named topics as generated by the topic generator should result in shallow hash buckets
+        /// </summary>
+        /// <returns></returns>
+        [TestMethod]
+        public void Check_Hash_Bucket_Depth()
+        {
+            const int numPublishers = 5000;
+            const int numTopicsPerPublisher = 10;
+
+            TopicGenerator.Generate(
+                numPublishers,
+                numTopicsPerPublisher,
+                out var topicsByPublisher,
+                out _,
+                out _);
+
+            // There will be many 'similar' topics ending with, i.e. "sensor100", "sensor101", ...
+            // Hash bucket depths should remain low.
+            var bucketDepths = new Dictionary<ulong, int>();
+            var maxBucketDepth = 0;
+            ulong maxBucketDepthHash = 0;
+
+            var topicsByHash = new Dictionary<ulong, List<string>>();
+
+            foreach (var t in topicsByPublisher)
+            {
+                var topics = t.Value;
+                foreach (var topic in topics)
+                {
+                    MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var hashMask, out var hasWildcard);
+
+                    bucketDepths.TryGetValue(topicHash, out var currentValue);
+                    ++currentValue;
+                    bucketDepths[topicHash] = currentValue;
+
+                    if (currentValue > maxBucketDepth)
+                    {
+                        maxBucketDepth = currentValue;
+                        maxBucketDepthHash = topicHash;
+                    }
+
+                    if (!topicsByHash.TryGetValue(topicHash, out var topicList))
+                    {
+                        topicList = new List<string>();
+                        topicsByHash.Add(topicHash, topicList);
+                    }
+
+                    topicList.Add(topic);
+                }
+            }
+
+            var maxDepthTopics = topicsByHash[maxBucketDepthHash];
+
+            Console.Write("Max bucket depth is " + maxBucketDepth);
+
+            // for the test case the bucket depth should be less than 100
+            Assert.IsTrue(maxBucketDepth < 100, "Unexpected high topic hash bucket depth");
+        }
 
-        MQTTnet.Server.MqttSession _clientSession;
 
         [TestMethod]
-        public void Match_Hash_Test_SingleWildCard()
+        public void Check_Selected_Topic_Hashes()
         {
-            var l0 = "pub0";
-            var l1 = "topic1";
-            var l2 = "+";
-            var l3 = "prop1";
-            var topic = string.Format("{0}/{1}/{2}/{3}", l0, l1, l2, l3);
+            CheckTopicHash("client1/building1/level1/sensor1", 0x655D4AF100000000, 0xFFFFFFFFFFFFFFFF);
+            CheckTopicHash("client1/building1/+/sensor1", 0x655D00F100000000, 0xFFFF00FFFFFFFFFF);
+            CheckTopicHash("client1/+/level1/+", 0x65004A0000000000, 0xFF00FF00FFFFFFFF);
+            CheckTopicHash("client1/building1/level1/#", 0x655D4A0000000000, 0xFFFFFF0000000000);
+            CheckTopicHash("client1/+/level1/#", 0x65004A0000000000, 0xFF00FF0000000000);
+        }
+
+        /// <summary>
+        ///     Test long topic name with last level being #
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_DetectMultiLevelWildcard()
+        {
+            var topic = "asdfasdf/asdfasdf/asdfasdf/asdfasdf/asdfas/dfaf/assfdgsdfgdf/#";
+
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
+
+            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
+        }
 
 
-            UInt64 topicHash;
-            UInt64 topicHashMask;
-            bool topicHasWildcard;
-            MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out topicHashMask, out topicHasWildcard);
+        /// <summary>
+        ///     Test long topic name with last level being +
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_DetectSingleLevelWildcard()
+        {
+            var topic = "asdfasdf/asdfasdf/asdfasdf/asdfasdf/asdfas/dfaf/assfdgsdfgdf/+";
+
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
+
+            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
+        }
+
+        /// <summary>
+        ///     Test long topic name with last level being #
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_MultiWildcard()
+        {
+            var sb = new StringBuilder();
+            const int numLevels = 8;
+            var levelNames = new string[numLevels];
+            for (var i = 0; i < numLevels; ++i)
+            {
+                if (i > 0)
+                {
+                    sb.Append("/");
+                }
+
+                string levelName;
+                if (i == numLevels - 1)
+                {
+                    // last one is #
+                    levelName = "#";
+                }
+                else
+                {
+                    levelName = "level" + i;
+                }
+
+                levelNames[i] = levelName;
+                sb.Append(levelName);
+            }
+
+            var topic = sb.ToString();
+
+            // UInt64 is limited to 8 levels
+
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
 
             Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
 
             var hashBytes = GetBytes(topicHash);
-            Assert.AreNotEqual(hashBytes[0], 0, "checksum 0 mismatch");
-            Assert.AreNotEqual(hashBytes[1], 0, "checksum 1 mismatch");
-            Assert.AreEqual(hashBytes[2], 0, "checksum 2 mismatch");
-            Assert.AreNotEqual(hashBytes[3], 0, "checksum 3 mismatch");
-            Assert.AreEqual(hashBytes[4], 0, "checksum 4 mismatch");
-            Assert.AreEqual(hashBytes[5], 0, "checksum 5 mismatch");
-            Assert.AreEqual(hashBytes[6], 0, "checksum 6 mismatch");
-            Assert.AreEqual(hashBytes[7], 0, "checksum 7 mismatch");
+            // all bytes should contain checksum
+            var count = 0;
+            foreach (var h in hashBytes)
+            {
+                if (count < 7)
+                {
+                    Assert.AreNotEqual(h, 0, "checksum mismatch");
+                }
+                else
+                {
+                    Assert.AreEqual(h, 0, "checksum mismatch");
+                }
 
-            // The mask should have zeroes where the wildcard and ff at the end
+                ++count;
+            }
+
+            // The mask should have ff except for last level
             var hashMaskBytes = GetBytes(topicHashMask);
-            Assert.AreEqual(hashMaskBytes[0], 0xff, "mask 0 mismatch");
-            Assert.AreEqual(hashMaskBytes[1], 0xff, "mask 1 mismatch");
-            Assert.AreEqual(hashMaskBytes[2], 0, "mask 2 mismatch");
-            Assert.AreEqual(hashMaskBytes[3], 0xff, "mask 3 mismatch");
-            Assert.AreEqual(hashMaskBytes[4], 0xff, "mask 4 mismatch");
-            Assert.AreEqual(hashMaskBytes[5], 0xff, "mask 5 mismatch");
-            Assert.AreEqual(hashMaskBytes[6], 0xff, "mask 6 mismatch");
-            Assert.AreEqual(hashMaskBytes[7], 0xff, "mask 7 mismatch");
+            count = 0;
+            foreach (var h in hashMaskBytes)
+            {
+                if (count < 7)
+                {
+                    Assert.AreEqual(h, 0xff, "mask mismatch");
+                }
+                else
+                {
+                    Assert.AreEqual(h, 0, "last mask mismatch");
+                }
+
+                ++count;
+            }
+        }
+
+        /// <summary>
+        ///     Test long topic name exceeding 8 levels
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_NoWildCard()
+        {
+            var sb = new StringBuilder();
+            const int numLevels = 9;
+            var levelNames = new string[numLevels];
+            for (var i = 0; i < numLevels; ++i)
+            {
+                if (i > 0)
+                {
+                    sb.Append("/");
+                }
+
+                var levelName = "level" + i;
+                levelNames[i] = levelName;
+                sb.Append(levelName);
+            }
+
+            var topic = sb.ToString();
+
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
+
+            Assert.IsFalse(topicHasWildcard, "Wildcard detected when not present");
+
+
+            var hashBytes = GetBytes(topicHash);
+            // all bytes should contain checksum
+            var count = 0;
+            foreach (var h in hashBytes)
+            {
+                Assert.AreNotEqual(h, 0, "checksum mismatch");
+                ++count;
+            }
+
+            // The mask should have ff 
+            var hashMaskBytes = GetBytes(topicHashMask);
+            count = 0;
+            foreach (var h in hashMaskBytes)
+            {
+                Assert.AreEqual(h, 0xff, "mask mismatch");
+                ++count;
+            }
+        }
+
+        /// <summary>
+        ///     Test long topic name with last level being +
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_SingleWildCard()
+        {
+            var sb = new StringBuilder();
+            const int numLevels = 8;
+            var levelNames = new string[numLevels];
+            for (var i = 0; i < numLevels; ++i)
+            {
+                if (i > 0)
+                {
+                    sb.Append("/");
+                }
+
+                string levelName;
+                if (i == numLevels - 1)
+                {
+                    // last one is +
+                    levelName = "+";
+                }
+                else
+                {
+                    levelName = "level" + i;
+                }
+
+                levelNames[i] = levelName;
+                sb.Append(levelName);
+            }
+
+            var topic = sb.ToString();
+
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
+
+            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
+
+
+            var hashBytes = GetBytes(topicHash);
+            // all bytes should contain checksum
+            var count = 0;
+            foreach (var h in hashBytes)
+            {
+                if (count < 7)
+                {
+                    Assert.AreNotEqual(h, 0, "checksum mismatch");
+                }
+                else
+                {
+                    // wildcard position
+                    Assert.AreEqual(h, 0, "checksum mismatch");
+                }
+
+                ++count;
+            }
+
+            // The mask should have ff 
+            var hashMaskBytes = GetBytes(topicHashMask);
+            count = 0;
+            foreach (var h in hashMaskBytes)
+            {
+                if (count < 7)
+                {
+                    Assert.AreEqual(h, 0xff, "mask mismatch");
+                }
+                else
+                {
+                    Assert.AreEqual(h, 0, "last mask mismatch");
+                }
+
+                ++count;
+            }
         }
 
         [TestMethod]
@@ -65,13 +317,9 @@ namespace MQTTnet.Tests.Server
             var l1 = "topic1";
             var l2 = "sub1";
             var l3 = "#";
-            var topic = string.Format("{0}/{1}/{2}/{3}", l0, l1, l2, l3);
+            var topic = $"{l0}/{l1}/{l2}/{l3}";
 
-            UInt64 topicHash;
-            UInt64 topicHashMask;
-            bool topicHasWildcard;
-
-            MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out topicHashMask, out topicHasWildcard);
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
 
             Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
 
@@ -104,13 +352,9 @@ namespace MQTTnet.Tests.Server
             var l1 = "topic1";
             var l2 = "sub1";
             var l3 = "prop1";
-            var topic = string.Format("{0}/{1}/{2}/{3}", l0, l1, l2, l3);
+            var topic = $"{l0}/{l1}/{l2}/{l3}";
 
-            UInt64 topicHash;
-            UInt64 topicHashMask;
-            bool topicHasWildcard;
-
-            MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out topicHashMask, out topicHasWildcard);
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
 
             Assert.IsFalse(topicHasWildcard, "Wildcard detected when not wildcard present");
 
@@ -137,225 +381,13 @@ namespace MQTTnet.Tests.Server
             Assert.AreEqual(hashMaskBytes[7], 0xff, "mask 7 mismatch");
         }
 
-        /// <summary>
-        /// Test long topic name with last level being #
-        /// </summary>
         [TestMethod]
-        public void Match_Hash_Test_LongTopic_MultiWildcard()
+        public async Task Match_Hash_Test_Search_MultiWildcard()
         {
-            var sb = new StringBuilder();
-            const int NumLevels = 8;
-            var levelNames = new string[NumLevels];
-            for (var i = 0; i < NumLevels; ++i)
-            {
-                if (i > 0)
-                    sb.Append("/");
-                string levelName;
-                if (i == NumLevels - 1)
-                {
-                    // last one is #
-                    levelName = "#";
-                }
-                else
-                {
-                   levelName = "level" + i;
-                }
-                levelNames[i] = levelName;
-                sb.Append(levelName);
-            }
-            var topic = sb.ToString();
-            
-            // UInt64 is limited to 8 levels
-
-            UInt64 topicHash;
-            UInt64 topicHashMask;
-            bool topicHasWildcard;
-
-            MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out topicHashMask, out topicHasWildcard);
-
-            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
-
-            var hashBytes = GetBytes(topicHash);
-            // all bytes should contain checksum
-            int count = 0;
-            foreach (var h in hashBytes)
-            {
-                if (count < 7)
-                {
-                    Assert.AreNotEqual(h, 0, "checksum mismatch");
-                }
-                else
-                {
-                    Assert.AreEqual(h, 0, "checksum mismatch");
-                }
-                ++count;
-            }
-            // The mask should have ff except for last level
-            var hashMaskBytes = GetBytes(topicHashMask);
-            count = 0;
-            foreach (var h in hashMaskBytes)
-            {
-                if (count < 7)
-                {
-                    Assert.AreEqual(h, 0xff, "mask mismatch");
-                }
-                else
-                {
-                    Assert.AreEqual(h, 0, "last mask mismatch");
-                }
-                ++count;
-            }
-        }
-
-        /// <summary>
-        /// Test long topic name with last level being +
-        /// </summary>
-        [TestMethod]
-        public void Match_Hash_Test_LongTopic_SingleWildCard()
-        {
-            var sb = new StringBuilder();
-            const int NumLevels = 8;
-            var levelNames = new string[NumLevels];
-            for (var i = 0; i < NumLevels; ++i)
-            {
-                if (i > 0)
-                    sb.Append("/");
-                string levelName;
-                if (i == NumLevels - 1)
-                {
-                    // last one is +
-                    levelName = "+";
-                }
-                else
-                {
-                    levelName = "level" + i;
-                }
-                levelNames[i] = levelName;
-                sb.Append(levelName);
-            }
-            var topic = sb.ToString();
-
-            UInt64 topicHash;
-            UInt64 topicHashMask;
-            bool topicHasWildcard;
-
-            MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out topicHashMask, out topicHasWildcard);
-
-            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
-
-
-            var hashBytes = GetBytes(topicHash);
-            // all bytes should contain checksum
-            int count = 0;
-            foreach (var h in hashBytes)
-            {
-                if (count < 7)
-                {
-                    Assert.AreNotEqual(h, 0, "checksum mismatch");
-                }
-                else
-                {
-                    // wildcard position
-                    Assert.AreEqual(h, 0, "checksum mismatch");
-                }
-                ++count;
-            }
-            // The mask should have ff 
-            var hashMaskBytes = GetBytes(topicHashMask);
-            count = 0;
-            foreach (var h in hashMaskBytes)
-            {
-                if (count < 7)
-                {
-                    Assert.AreEqual(h, 0xff, "mask mismatch");
-                }
-                else
-                {
-                    Assert.AreEqual(h, 0, "last mask mismatch");
-                }
-                ++count;
-            }
-        }
-
-        /// <summary>
-        /// Test long topic name exceeding 8 levels
-        /// </summary>
-        [TestMethod]
-        public void Match_Hash_Test_LongTopic_NoWildCard()
-        {
-            var sb = new StringBuilder();
-            const int NumLevels = 9;
-            var levelNames = new string[NumLevels];
-            for(var i=0; i < NumLevels; ++i)
-            {
-                if (i > 0)
-                    sb.Append("/");
-                var levelName = "level" + i;
-                levelNames[i] = levelName;
-                sb.Append(levelName);
-            }
-            var topic = sb.ToString();
-
-            UInt64 topicHash;
-            UInt64 topicHashMask;
-            bool topicHasWildcard;
-
-            MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out topicHashMask, out topicHasWildcard);
-
-            Assert.IsFalse(topicHasWildcard, "Wildcard detected when not present");
-
-
-            var hashBytes = GetBytes(topicHash);
-            // all bytes should contain checksum
-            int count = 0;
-            foreach(var h in hashBytes)
-            {
-                Assert.AreNotEqual(h, 0, "checksum mismatch");
-                ++count;
-            }
-            // The mask should have ff 
-            var hashMaskBytes = GetBytes(topicHashMask);
-            count = 0;
-            foreach (var h in hashMaskBytes)
-            {
-                Assert.AreEqual(h, 0xff, "mask mismatch");
-                ++count;
-            }
-        }
-
-
-        /// <summary>
-        /// Test long topic name with last level being +
-        /// </summary>
-        [TestMethod]
-        public void Match_Hash_Test_LongTopic_DetectSingleLevelWildcard()
-        {
-            var topic = "asdfasdf/asdfasdf/asdfasdf/asdfasdf/asdfas/dfaf/assfdgsdfgdf/+";
-
-            UInt64 topicHash;
-            UInt64 topicHashMask;
-            bool topicHasWildcard;
-
-            MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out topicHashMask, out topicHasWildcard);
-
-            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
-        }
-
-        /// <summary>
-        /// Test long topic name with last level being #
-        /// </summary>
-        [TestMethod]
-        public void Match_Hash_Test_LongTopic_DetectMultiLevelWildcard()
-        {
-            var topic = "asdfasdf/asdfasdf/asdfasdf/asdfasdf/asdfas/dfaf/assfdgsdfgdf/#";
-
-            UInt64 topicHash;
-            UInt64 topicHashMask;
-            bool topicHasWildcard;
-
-            MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out topicHashMask, out topicHasWildcard);
-
-            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
+            var topics = await PrepareTopicHashSubscriptions(TopicHashSelector.MultiWildcard);
+            var matchCount = CheckTopicSubscriptions(_clientSession, topics, "Multi Wildcard");
+            // Should match all topics
+            Assert.AreEqual(topics.Count, matchCount, "Topics not matched");
         }
 
         [TestMethod]
@@ -407,134 +439,125 @@ namespace MQTTnet.Tests.Server
         }
 
         [TestMethod]
-        public async Task Match_Hash_Test_Search_MultiWildcard()
+        public void Match_Hash_Test_SingleWildCard()
         {
-            var topics = await PrepareTopicHashSubscriptions(TopicHashSelector.MultiWildcard);
-            var matchCount = CheckTopicSubscriptions(_clientSession, topics, "Multi Wildcard");
-            // Should match all topics
-            Assert.AreEqual(topics.Count, matchCount, "Topics not matched");
-        }
+            var l0 = "pub0";
+            var l1 = "topic1";
+            var l2 = "+";
+            var l3 = "prop1";
+            var topic = $"{l0}/{l1}/{l2}/{l3}";
 
-        /// <summary>
-        /// Even fairly regularly named topics as generated by the topic generator should result in shallow hash buckets
-        /// </summary>
-        /// <returns></returns>
-        [TestMethod]
-        public void Check_Hash_Bucket_Depth()
-        {
-            Dictionary<string, List<string>> topicsByPublisher;
-            Dictionary<string, List<string>> singleWildcardTopicsByPublisher;
-            Dictionary<string, List<string>> multiWildcardTopicsByPublisher;
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
 
-            const int NumPublishers = 5000;
-            const int NumTopicsPerPublisher = 10;
+            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
 
-            TopicGenerator.Generate(NumPublishers, NumTopicsPerPublisher, out topicsByPublisher, out singleWildcardTopicsByPublisher, out multiWildcardTopicsByPublisher);
+            var hashBytes = GetBytes(topicHash);
+            Assert.AreNotEqual(hashBytes[0], 0, "checksum 0 mismatch");
+            Assert.AreNotEqual(hashBytes[1], 0, "checksum 1 mismatch");
+            Assert.AreEqual(hashBytes[2], 0, "checksum 2 mismatch");
+            Assert.AreNotEqual(hashBytes[3], 0, "checksum 3 mismatch");
+            Assert.AreEqual(hashBytes[4], 0, "checksum 4 mismatch");
+            Assert.AreEqual(hashBytes[5], 0, "checksum 5 mismatch");
+            Assert.AreEqual(hashBytes[6], 0, "checksum 6 mismatch");
+            Assert.AreEqual(hashBytes[7], 0, "checksum 7 mismatch");
 
-            // There will be many 'similar' topics ending with, i.e. "sensor100", "sensor101", ...
-            // Hash bucket depths should remain low.
-            var bucketDepths = new Dictionary<UInt64, int>();
-            int maxBucketDepth = 0;
-            UInt64 maxBucketDepthHash = 0;
-
-            var topicsByHash = new Dictionary<UInt64, List<string>>();
-
-            foreach (var t in topicsByPublisher)
-            {
-                var topics = t.Value;
-                foreach (var topic in topics)
-                {
-                    UInt64 topicHash;
-                    UInt64 hashMask;
-                    bool hasWildcard;
-                    MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out hashMask, out hasWildcard);
-
-                    bucketDepths.TryGetValue(topicHash, out var currentValue);
-                    ++currentValue;
-                    bucketDepths[topicHash] = currentValue;
-
-                    if (currentValue > maxBucketDepth)
-                    {
-                        maxBucketDepth = currentValue;
-                        maxBucketDepthHash = topicHash;
-                    }
-
-                    if (!topicsByHash.TryGetValue(topicHash, out var topicList))
-                    {
-                        topicList = new List<string>();
-                        topicsByHash.Add(topicHash, topicList);
-                    }
-                    topicList.Add(topic);
-                }
-            }
-
-            var maxDepthTopics = topicsByHash[maxBucketDepthHash];
-
-            Console.Write("Max bucket depth is " + maxBucketDepth);
-
-            // for the test case the bucket depth should be less than 100
-            Assert.IsTrue(maxBucketDepth < 100, "Unexpected high topic hash bucket depth");
-        }
-
-
-        [TestMethod]
-        public void Check_Selected_Topic_Hashes()
-        {
-            CheckTopicHash("client1/building1/level1/sensor1", 0x655D4AF100000000, 0xFFFFFFFFFFFFFFFF);
-            CheckTopicHash("client1/building1/+/sensor1", 0x655D00F100000000, 0xFFFF00FFFFFFFFFF);
-            CheckTopicHash("client1/+/level1/+", 0x65004A0000000000, 0xFF00FF00FFFFFFFF);
-            CheckTopicHash("client1/building1/level1/#", 0x655D4A0000000000, 0xFFFFFF0000000000);
-            CheckTopicHash("client1/+/level1/#", 0x65004A0000000000, 0xFF00FF0000000000);
+            // The mask should have zeroes where the wildcard and ff at the end
+            var hashMaskBytes = GetBytes(topicHashMask);
+            Assert.AreEqual(hashMaskBytes[0], 0xff, "mask 0 mismatch");
+            Assert.AreEqual(hashMaskBytes[1], 0xff, "mask 1 mismatch");
+            Assert.AreEqual(hashMaskBytes[2], 0, "mask 2 mismatch");
+            Assert.AreEqual(hashMaskBytes[3], 0xff, "mask 3 mismatch");
+            Assert.AreEqual(hashMaskBytes[4], 0xff, "mask 4 mismatch");
+            Assert.AreEqual(hashMaskBytes[5], 0xff, "mask 5 mismatch");
+            Assert.AreEqual(hashMaskBytes[6], 0xff, "mask 6 mismatch");
+            Assert.AreEqual(hashMaskBytes[7], 0xff, "mask 7 mismatch");
         }
 
         void CheckTopicHash(string topic, ulong expectedHash, ulong expectedHashMask)
         {
-            ulong topicHash;
-            ulong hashMask;
-            bool hasWildcard;
-
-            MQTTnet.Server.MqttSubscription.CalculateTopicHash(topic, out topicHash, out hashMask, out hasWildcard);
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var hashMask, out var hasWildcard);
 
             Console.WriteLine();
             Console.WriteLine("Topic: " + topic);
-            Console.WriteLine(string.Format("Hash: {0:X8}", topicHash));
-            Console.WriteLine(string.Format("Hash Mask: {0:X8}", hashMask));
+            Console.WriteLine($"Hash: {topicHash:X8}");
+            Console.WriteLine($"Hash Mask: {hashMask:X8}");
 
             Assert.AreEqual(expectedHash, topicHash, "Topic hash not as expected. Has the hash function changed?");
             Assert.AreEqual(expectedHashMask, hashMask, "Topic hash mask not as expected");
         }
 
+        int CheckTopicSubscriptions(MqttSession clientSession, List<string> topicsToFind, string subject)
+        {
+            var matchCount = 0;
+
+            {
+                var resultCount = 0;
+
+                var stopWatch = new Stopwatch();
+                stopWatch.Start();
+                var countUp = 0;
+                var countDown = topicsToFind.Count - 1;
+                for (; countUp < topicsToFind.Count; ++countUp, --countDown)
+                {
+                    var topicToFind = topicsToFind[countDown];
+
+                    MqttSubscription.CalculateTopicHash(topicToFind, out var topicHash, out _, out _);
+
+                    clientSession.TryCheckSubscriptions(topicToFind, topicHash, MqttQualityOfServiceLevel.AtMostOnce, "OtherClient", out var result);
+                    if (result.IsSubscribed)
+                    {
+                        ++matchCount;
+                    }
+
+                    ++resultCount;
+                }
+
+                stopWatch.Stop();
+
+                Console.Write("Match count: " + matchCount + "; ");
+
+                Console.WriteLine(subject + " lookup milliseconds: " + stopWatch.ElapsedMilliseconds);
+            }
+
+            return matchCount;
+        }
+
+        byte[] GetBytes(ulong value)
+        {
+            var bytes = BitConverter.GetBytes(value);
+            // Ensure that highest byte comes first for comparison left to right
+            if (BitConverter.IsLittleEndian)
+            {
+                return bytes.Reverse().ToArray();
+            }
+
+            return bytes;
+        }
+
 
         async Task<List<string>> PrepareTopicHashSubscriptions(TopicHashSelector selector)
         {
-            Dictionary<string, List<string>> topicsByPublisher;
-            Dictionary<string, List<string>> singleWildcardTopicsByPublisher;
-            Dictionary<string, List<string>> multiWildcardTopicsByPublisher;
+            const int numPublishers = 1;
+            const int numTopicsPerPublisher = 10000;
 
-            const int NumPublishers = 1;
-            const int NumTopicsPerPublisher = 10000;
-
-            TopicGenerator.Generate(NumPublishers, NumTopicsPerPublisher, out topicsByPublisher, out singleWildcardTopicsByPublisher, out multiWildcardTopicsByPublisher);
+            TopicGenerator.Generate(
+                numPublishers,
+                numTopicsPerPublisher,
+                out var topicsByPublisher,
+                out var singleWildcardTopicsByPublisher,
+                out var multiWildcardTopicsByPublisher);
 
             var topics = topicsByPublisher.FirstOrDefault().Value;
             var singleWildcardTopics = singleWildcardTopicsByPublisher.FirstOrDefault().Value;
             var multiWildcardTopics = multiWildcardTopicsByPublisher.FirstOrDefault().Value;
 
-            const string ClientId = "Client1";
-            var logger = new Mockups.TestLogger();
-            var serverOptions = new MQTTnet.Server.MqttServerOptions();
-            var eventContainer = new MQTTnet.Server.MqttServerEventContainer();
+            const string clientId = "Client1";
+            var logger = new TestLogger();
+            var serverOptions = new MqttServerOptions();
+            var eventContainer = new MqttServerEventContainer();
             var retainedMessagesManager = new MqttRetainedMessagesManager(eventContainer, logger);
             var sessionManager = new MqttClientSessionsManager(serverOptions, retainedMessagesManager, eventContainer, logger);
-            _clientSession = new MQTTnet.Server.MqttSession(
-                        ClientId,
-                        false,
-                        new Dictionary<object, object>(),
-                        serverOptions,
-                        eventContainer,
-                        retainedMessagesManager,
-                        sessionManager
-                        );
+            _clientSession = new MqttSession(clientId, false, new Dictionary<object, object>(), serverOptions, eventContainer, retainedMessagesManager, sessionManager);
 
             List<string> topicsToSubscribe;
 
@@ -550,63 +573,26 @@ namespace MQTTnet.Tests.Server
                     topicsToSubscribe = topics;
                     break;
             }
+
             foreach (var t in topicsToSubscribe)
             {
-                var subPacket = new Packets.MqttSubscribePacket();
-                var filter = new Packets.MqttTopicFilter();
-                filter.Topic = t;
+                var subPacket = new MqttSubscribePacket();
+                var filter = new MqttTopicFilter
+                {
+                    Topic = t
+                };
                 subPacket.TopicFilters.Add(filter);
-                await _clientSession.SubscriptionsManager.Subscribe(subPacket, default(CancellationToken));
+                await _clientSession.Subscribe(subPacket, default);
             }
 
             return topics;
         }
 
-        int CheckTopicSubscriptions(MQTTnet.Server.MqttSession clientSession, List<string> topicsToFind, string subject)
+        enum TopicHashSelector
         {
-            var matchCount = 0;
-
-            {
-                int resultCount = 0;
-
-                var stopWatch = new System.Diagnostics.Stopwatch();
-                stopWatch.Start();
-                var countUp = 0;
-                var countDown = topicsToFind.Count - 1;
-                for (; countUp < topicsToFind.Count; ++countUp, --countDown)
-                {
-                    var topicToFind = topicsToFind[countDown];
-
-                    UInt64 topicHash;
-                    UInt64 hashMask;
-                    bool hasWildcard;
-                    MQTTnet.Server.MqttSubscription.CalculateTopicHash((string)topicToFind, out topicHash, out hashMask, out hasWildcard);
-
-                    var result = clientSession.SubscriptionsManager.CheckSubscriptions((string)topicToFind, topicHash, MqttQualityOfServiceLevel.AtMostOnce, "OtherClient");
-                    if (result.IsSubscribed)
-                    {
-                        ++matchCount;
-                    }
-                    ++resultCount;
-                }
-
-                stopWatch.Stop();
-
-                Console.Write("Match count: " + matchCount + "; ");
-
-                Console.WriteLine(subject + " lookup milliseconds: " + stopWatch.ElapsedMilliseconds);
-            }
-
-            return matchCount;
-        }
-
-        byte[] GetBytes(UInt64 value)
-        {
-            var bytes = BitConverter.GetBytes(value);
-            // Ensure that highest byte comes first for comparison left to right
-            if (BitConverter.IsLittleEndian)
-                return bytes.Reverse().ToArray();
-            return bytes;
+            NoWildcard,
+            SingleWildcard,
+            MultiWildcard
         }
     }
-}
+}

Source/MQTTnet.Tests/Server/Topic_Alias_Tests.cs

@@ -8,7 +8,7 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Server
 {
@@ -50,7 +50,7 @@ namespace MQTTnet.Tests.Server
                         receivedTopics.Add(e.ApplicationMessage.Topic);
                     }
                     
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync("#");

Source/MQTTnet.Tests/Server/Unsubscribe_Tests.cs

@@ -0,0 +1,32 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Tests.Server
+{
+    [TestClass]
+    public sealed class Unsubscribe_Tests : BaseTestClass
+    {
+        [TestMethod]
+        [ExpectedException(typeof(MqttClientDisconnectedException))]
+        public async Task Disconnect_While_Unsubscribing()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                // The client will be disconnect directly after subscribing!
+                server.ClientUnsubscribedTopicAsync += ev => server.DisconnectClientAsync(ev.ClientId, MqttDisconnectReasonCode.NormalDisconnection);
+
+                var client = await testEnvironment.ConnectClient();
+                await client.SubscribeAsync("#");
+                await client.UnsubscribeAsync("#");
+            }
+        }
+    }
+}

Source/MQTTnet.Tests/Server/User_Properties_Tests.cs

@@ -9,6 +9,7 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
 using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Tests.Mockups;
 
@@ -48,7 +49,7 @@ namespace MQTTnet.Tests.Server
                 receiver.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await sender.PublishAsync(message.Build(), CancellationToken.None);

Source/MQTTnet.Tests/Server/Will_Tests.cs

@@ -2,7 +2,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests.Server
@@ -26,7 +26,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
@@ -53,7 +53,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());

Source/MQTTnet.Tests/TopicGenerator.cs

@@ -1,8 +1,7 @@
 using System;
 using System.Collections.Generic;
-using System.Text;
 
-namespace MQTTnet.Tests.Server
+namespace MQTTnet.Tests
 {
     public class TopicGenerator
     {

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -13,7 +13,6 @@ using MQTTnet.Channel;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 
@@ -23,7 +22,7 @@ namespace MQTTnet.Adapter
     {
         const uint ErrorOperationAborted = 0x800703E3;
         const int ReadBufferSize = 4096;
-        
+
         readonly IMqttChannel _channel;
         readonly byte[] _fixedHeaderBuffer = new byte[2];
         readonly MqttNetSourceLogger _logger;
@@ -73,7 +72,26 @@ namespace MQTTnet.Adapter
 
             try
             {
-                await _channel.ConnectAsync(cancellationToken).ConfigureAwait(false);
+                /*
+                 * We have to implement a small workaround here to support connecting in Xamarin
+                 * with a disabled WiFi network. If the WiFi is disabled the connect method will
+                 * block forever. Even a cancellation token is not supported properly.
+                 */
+
+                var connectTask = _channel.ConnectAsync(cancellationToken);
+
+                var timeout = new TaskCompletionSource<object>();
+                using (cancellationToken.Register(() => timeout.TrySetResult(null)))
+                {
+                    await Task.WhenAny(connectTask, timeout.Task).ConfigureAwait(false);
+                    if (timeout.Task.IsCompleted && !connectTask.IsCompleted)
+                    {
+                        throw new OperationCanceledException("MQTT connect canceled.", cancellationToken);
+                    }
+
+                    // Make sure that the exception from the connect task gets thrown.
+                    await connectTask.ConfigureAwait(false);
+                }
             }
             catch (Exception exception)
             {
@@ -176,7 +194,7 @@ namespace MQTTnet.Adapter
             // This lock makes sure that multiple threads can send packets at the same time.
             // This is required when a disconnect is sent from another thread while the 
             // worker thread is still sending publish packets etc.
-            using (await _syncRoot.WaitAsync(cancellationToken).ConfigureAwait(false))
+            using (await _syncRoot.EnterAsync(cancellationToken).ConfigureAwait(false))
             {
                 // Check for cancellation here again because "WaitAsync" might take some time.
                 cancellationToken.ThrowIfCancellationRequested();
@@ -277,11 +295,29 @@ namespace MQTTnet.Adapter
 
             while (totalBytesRead < buffer.Length)
             {
-                var bytesRead = await _channel.ReadAsync(buffer, totalBytesRead, buffer.Length - totalBytesRead, cancellationToken).ConfigureAwait(false);
-
+                // Check two times for cancellation because the call to _ReadAsync_ might block for some time.
                 if (cancellationToken.IsCancellationRequested)
                 {
-                    return ReadFixedHeaderResult.Cancelled;
+                    return ReadFixedHeaderResult.Canceled;
+                }
+
+                int bytesRead;
+                try
+                {
+                    bytesRead = await _channel.ReadAsync(buffer, totalBytesRead, buffer.Length - totalBytesRead, cancellationToken).ConfigureAwait(false);
+                }
+                catch (OperationCanceledException)
+                {
+                    return ReadFixedHeaderResult.Canceled;
+                }
+                catch (SocketException)
+                {
+                    return ReadFixedHeaderResult.ConnectionClosed;
+                }
+               
+                if (cancellationToken.IsCancellationRequested)
+                {
+                    return ReadFixedHeaderResult.Canceled;
                 }
 
                 if (bytesRead == 0)
@@ -344,7 +380,7 @@ namespace MQTTnet.Adapter
                 var fixedHeader = readFixedHeaderResult.FixedHeader;
                 if (fixedHeader.RemainingLength == 0)
                 {
-                    return new ReceivedMqttPacket(fixedHeader.Flags, PlatformAbstractionLayer.EmptyByteArraySegment, 2);
+                    return new ReceivedMqttPacket(fixedHeader.Flags, EmptyBuffer.ArraySegment, 2);
                 }
 
                 var bodyLength = fixedHeader.RemainingLength;

Source/MQTTnet/Client/Exceptions/MqttClientDisconnectedException.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using MQTTnet.Exceptions;
+
+namespace MQTTnet.Client
+{
+    public sealed class MqttClientDisconnectedException : MqttCommunicationException
+    {
+        public MqttClientDisconnectedException(Exception innerException) : base("The MQTT client is disconnected.", innerException)
+        {
+        }
+    }
+}

Source/MQTTnet/Client/Exceptions/MqttClientUnexpectedDisconnectReceivedException.cs

@@ -3,14 +3,15 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
+using MQTTnet.Exceptions;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
-namespace MQTTnet.Exceptions
+namespace MQTTnet.Client
 {
-    public class MqttUnexpectedDisconnectReceivedException : MqttCommunicationException
+    public sealed class MqttClientUnexpectedDisconnectReceivedException : MqttCommunicationException
     {
-        public MqttUnexpectedDisconnectReceivedException(MqttDisconnectPacket disconnectPacket) 
+        public MqttClientUnexpectedDisconnectReceivedException(MqttDisconnectPacket disconnectPacket) 
             : base($"Unexpected DISCONNECT (Reason code={disconnectPacket.ReasonCode}) received.")
         {
             ReasonCode = disconnectPacket.ReasonCode;

Source/MQTTnet/Client/IMqttClient.cs

@@ -8,6 +8,7 @@ namespace MQTTnet.Client
     public interface IMqttClient : IDisposable
     {
         event Func<MqttApplicationMessageReceivedEventArgs, Task> ApplicationMessageReceivedAsync;
+        
         event Func<MqttClientConnectedEventArgs, Task> ConnectedAsync;
 
         event Func<MqttClientConnectingEventArgs, Task> ConnectingAsync;

Source/MQTTnet/Client/MqttClient.cs

@@ -9,7 +9,6 @@ using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.PacketDispatcher;
 using MQTTnet.Packets;
@@ -33,25 +32,21 @@ namespace MQTTnet.Client
         readonly object _disconnectLock = new object();
         readonly AsyncEvent<InspectMqttPacketEventArgs> _inspectPacketEvent = new AsyncEvent<InspectMqttPacketEventArgs>();
         readonly MqttNetSourceLogger _logger;
-        readonly MqttPacketDispatcher _packetDispatcher = new MqttPacketDispatcher();
         readonly MqttPacketFactories _packetFactories = new MqttPacketFactories();
 
         readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new MqttPacketIdentifierProvider();
         readonly IMqttNetLogger _rootLogger;
 
         IMqttChannelAdapter _adapter;
-
         CancellationTokenSource _backgroundCancellationTokenSource;
         bool _cleanDisconnectInitiated;
         volatile int _connectionStatus;
-
         MqttClientDisconnectReason _disconnectReason;
         string _disconnectReasonString;
         Task _keepAlivePacketsSenderTask;
-
         DateTime _lastPacketSentTimestamp;
+        MqttPacketDispatcher _packetDispatcher;
         Task _packetReceiverTask;
-
         AsyncQueue<MqttPublishPacket> _publishPacketReceiverQueue;
         Task _publishPacketReceiverTask;
 
@@ -129,7 +124,7 @@ namespace MQTTnet.Client
                 }
 
                 _packetIdentifierProvider.Reset();
-                _packetDispatcher.CancelAll();
+                _packetDispatcher = new MqttPacketDispatcher();
 
                 _backgroundCancellationTokenSource = new CancellationTokenSource();
                 var backgroundCancellationToken = _backgroundCancellationTokenSource.Token;
@@ -224,17 +219,17 @@ namespace MQTTnet.Client
 
         public async Task PingAsync(CancellationToken cancellationToken = default)
         {
-            if (!cancellationToken.CanBeCanceled)
+            if (cancellationToken.CanBeCanceled)
+            {
+                await SendAndReceiveAsync<MqttPingRespPacket>(MqttPingReqPacket.Instance, cancellationToken).ConfigureAwait(false);
+            }
+            else
             {
                 using (var timeout = new CancellationTokenSource(Options.Timeout))
                 {
                     await SendAndReceiveAsync<MqttPingRespPacket>(MqttPingReqPacket.Instance, timeout.Token).ConfigureAwait(false);
                 }
             }
-            else
-            {
-                await SendAndReceiveAsync<MqttPingRespPacket>(MqttPingReqPacket.Instance, cancellationToken).ConfigureAwait(false);
-            }
         }
 
         public Task<MqttClientPublishResult> PublishAsync(MqttApplicationMessage applicationMessage, CancellationToken cancellationToken = default)
@@ -369,7 +364,7 @@ namespace MQTTnet.Client
                 throw new MqttProtocolViolationException("Received a not supported QoS level.");
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         async Task<MqttClientConnectResult> AuthenticateAsync(MqttClientOptions options, CancellationToken cancellationToken)
@@ -415,6 +410,10 @@ namespace MQTTnet.Client
                 _publishPacketReceiverQueue = null;
 
                 _adapter?.Dispose();
+                _adapter = null;
+                
+                _packetDispatcher?.Dispose();
+                _packetDispatcher = null;
             }
         }
 
@@ -448,6 +447,8 @@ namespace MQTTnet.Client
 
             try
             {
+                _packetDispatcher.Dispose(new MqttClientDisconnectedException(exception));
+                
                 var receiverTask = WaitForTaskAsync(_packetReceiverTask, sender);
                 var publishPacketReceiverTask = WaitForTaskAsync(_publishPacketReceiverTask, sender);
                 var keepAliveTask = WaitForTaskAsync(_keepAlivePacketsSenderTask, sender);
@@ -490,7 +491,7 @@ namespace MQTTnet.Client
                 return DisconnectCoreAsync(sender, exception, connectResult, clientWasConnected);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         bool DisconnectIsPendingOrFinished()
@@ -550,7 +551,7 @@ namespace MQTTnet.Client
                 return extendedAuthenticationExchangeHandler.HandleRequestAsync(new MqttExtendedAuthenticationExchangeContext(authPacket, this));
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         Task ProcessReceivedDisconnectPacket(MqttDisconnectPacket disconnectPacket)
@@ -559,7 +560,7 @@ namespace MQTTnet.Client
             _disconnectReasonString = disconnectPacket.ReasonString;
 
             // Also dispatch disconnect to waiting threads to generate a proper exception.
-            _packetDispatcher.FailAll(new MqttUnexpectedDisconnectReceivedException(disconnectPacket));
+            _packetDispatcher.Dispose(new MqttClientUnexpectedDisconnectReceivedException(disconnectPacket));
 
             return DisconnectInternalAsync(_packetReceiverTask, null, null);
         }
@@ -604,7 +605,7 @@ namespace MQTTnet.Client
                 return SendAsync(pubRelPacket, cancellationToken);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         Task ProcessReceivedPubRelPacket(MqttPubRelPacket pubRelPacket, CancellationToken cancellationToken)
@@ -777,7 +778,7 @@ namespace MQTTnet.Client
                 }
                 else
                 {
-                    _logger.Error(exception, "Error while receiving packets.");
+                    _logger.Error(exception, $"Error while processing received packet ({packet.GetType().Name}).");
                 }
 
                 _packetDispatcher.FailAll(exception);
@@ -828,6 +829,11 @@ namespace MQTTnet.Client
                     return;
                 }
 
+                if (exception is AggregateException aggregateException)
+                {
+                    exception = aggregateException.GetBaseException();
+                }
+                
                 if (exception is OperationCanceledException)
                 {
                 }

Source/MQTTnet/Client/MqttClientExtensions.cs

@@ -14,26 +14,6 @@ namespace MQTTnet.Client
 {
     public static class MqttClientExtensions
     {
-        public static async Task<bool> TryPingAsync(this IMqttClient client, CancellationToken cancellationToken = default)
-        {
-            if (client == null)
-            {
-                throw new ArgumentNullException(nameof(client));
-            }
-
-            try
-            {
-                await client.PingAsync(cancellationToken).ConfigureAwait(false);
-                return true;
-            }
-            catch
-            {
-                // Ignore errors.
-            }
-
-            return false;
-        }
-        
         public static Task DisconnectAsync(this IMqttClient client, MqttClientDisconnectReason reason = MqttClientDisconnectReason.NormalDisconnection, string reasonString = null)
         {
             if (client == null)
@@ -89,6 +69,17 @@ namespace MQTTnet.Client
             return mqttClient.PublishBinaryAsync(topic, payloadBuffer, qualityOfServiceLevel, retain, cancellationToken);
         }
 
+        public static Task ReconnectAsync(this IMqttClient client, CancellationToken cancellationToken = default)
+        {
+            if (client.Options == null)
+            {
+                throw new InvalidOperationException(
+                    "The MQTT client was not connected before. A reconnect is only permitted when the client was already connected or at least tried to.");
+            }
+
+            return client.ConnectAsync(client.Options, cancellationToken);
+        }
+
         public static Task SendExtendedAuthenticationExchangeDataAsync(this IMqttClient client, MqttExtendedAuthenticationExchangeData data)
         {
             if (client == null)
@@ -137,6 +128,26 @@ namespace MQTTnet.Client
             return mqttClient.SubscribeAsync(subscribeOptions, cancellationToken);
         }
 
+        public static async Task<bool> TryPingAsync(this IMqttClient client, CancellationToken cancellationToken = default)
+        {
+            if (client == null)
+            {
+                throw new ArgumentNullException(nameof(client));
+            }
+
+            try
+            {
+                await client.PingAsync(cancellationToken).ConfigureAwait(false);
+                return true;
+            }
+            catch
+            {
+                // Ignore errors.
+            }
+
+            return false;
+        }
+
         public static Task<MqttClientUnsubscribeResult> UnsubscribeAsync(this IMqttClient mqttClient, string topic, CancellationToken cancellationToken = default)
         {
             if (mqttClient == null)

Source/MQTTnet/Client/Options/MqttClientCertificateValidationEventArgs.cs

@@ -10,12 +10,20 @@ namespace MQTTnet.Client
 {
     public sealed class MqttClientCertificateValidationEventArgs : EventArgs
     {
-        public X509Certificate Certificate { get; set; }
+        public MqttClientCertificateValidationEventArgs(X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors, IMqttClientChannelOptions clientOptions)
+        {
+            Certificate = certificate ?? throw new ArgumentNullException(nameof(certificate));
+            Chain = chain ?? throw new ArgumentNullException(nameof(chain));
+            SslPolicyErrors = sslPolicyErrors;
+            ClientOptions = clientOptions ?? throw new ArgumentNullException(nameof(clientOptions));
+        }
 
-        public X509Chain Chain { get; set; }
+        public X509Certificate Certificate { get; }
 
-        public SslPolicyErrors SslPolicyErrors { get; set; }
+        public X509Chain Chain { get; }
 
-        public IMqttClientChannelOptions ClientOptions { get; set; }
+        public IMqttClientChannelOptions ClientOptions { get; }
+
+        public SslPolicyErrors SslPolicyErrors { get; }
     }
-}
+}

Source/MQTTnet/Client/Options/MqttClientDefaultCertificateValidationHandler.cs

@@ -12,11 +12,18 @@ namespace MQTTnet.Client
     {
         public static bool Handle(MqttClientCertificateValidationEventArgs eventArgs)
         {
-            if (eventArgs.SslPolicyErrors == SslPolicyErrors.None)
+            var sslPolicyErrors = eventArgs.SslPolicyErrors;
+            
+            if (eventArgs.ClientOptions?.TlsOptions?.IgnoreCertificateChainErrors ?? false)
+            {
+                sslPolicyErrors &= ~SslPolicyErrors.RemoteCertificateChainErrors;
+            }
+            
+            if (sslPolicyErrors == SslPolicyErrors.None)
             {
                 return true;
             }
-
+            
             if (eventArgs.Chain.ChainStatus.Any(c =>
                     c.Status == X509ChainStatusFlags.RevocationStatusUnknown || c.Status == X509ChainStatusFlags.Revoked || c.Status == X509ChainStatusFlags.OfflineRevocation))
             {

Source/MQTTnet/Client/Options/MqttClientOptionsBuilderTlsParameters.cs

@@ -13,8 +13,8 @@ namespace MQTTnet.Client
     public sealed class MqttClientOptionsBuilderTlsParameters
     {
         public bool UseTls { get; set; }
-        
-        public Func<MqttClientCertificateValidationEventArgs, bool> CertificateValidationHandler { get; set; }
+
+        public Func<MqttClientCertificateValidationEventArgs, bool> CertificateValidationHandler { get; set; } = MqttClientDefaultCertificateValidationHandler.Handle;
 
 #if NET48 || NETCOREAPP3_1 || NET5 || NET6
         public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12 | SslProtocols.Tls13;

Source/MQTTnet/Client/Options/MqttClientTlsOptions.cs

@@ -11,7 +11,7 @@ namespace MQTTnet.Client
 {
     public sealed class MqttClientTlsOptions
     {
-        public Func<MqttClientCertificateValidationEventArgs, bool> CertificateValidationHandler { get; set; }
+        public Func<MqttClientCertificateValidationEventArgs, bool> CertificateValidationHandler { get; set; } = MqttClientDefaultCertificateValidationHandler.Handle;
 
         public bool UseTls { get; set; }
 

Source/MQTTnet/Client/Publishing/MqttClientPublishResult.cs

@@ -10,29 +10,38 @@ namespace MQTTnet.Client
     public sealed class MqttClientPublishResult
     {
         /// <summary>
-        /// Gets the packet identifier which was used for this publish.
+        ///     Returns if the overall status of the publish is a success. This can be the reason code _Success_ or
+        ///     _NoMatchingSubscribers_. _NoMatchingSubscribers_ usually indicates only that no other client is interested in the topic but overall transmit
+        ///     to the server etc. was a success.
+        /// </summary>
+        public bool IsSuccess => ReasonCode == MqttClientPublishReasonCode.Success || ReasonCode == MqttClientPublishReasonCode.NoMatchingSubscribers;
+
+        /// <summary>
+        ///     Gets the packet identifier which was used for this publish.
         /// </summary>
         public ushort? PacketIdentifier { get; set; }
 
         /// <summary>
-        /// Gets or sets the reason code.
-        /// Hint: MQTT 5 feature only.
+        ///     Gets or sets the reason code.
+        ///     Hint: MQTT 5 feature only.
         /// </summary>
         public MqttClientPublishReasonCode ReasonCode { get; set; } = MqttClientPublishReasonCode.Success;
 
         /// <summary>
-        /// Gets or sets the reason string.
-        /// Hint: MQTT 5 feature only.
+        ///     Gets or sets the reason string.
+        ///     Hint: MQTT 5 feature only.
         /// </summary>
         public string ReasonString { get; set; }
 
         /// <summary>
-        /// Gets or sets the user properties.
-        /// In MQTT 5, user properties are basic UTF-8 string key-value pairs that you can append to almost every type of MQTT packet.
-        /// As long as you don’t exceed the maximum message size, you can use an unlimited number of user properties to add metadata to MQTT messages and pass information between publisher, broker, and subscriber.
-        /// The feature is very similar to the HTTP header concept.
-        /// Hint: MQTT 5 feature only.
+        ///     Gets or sets the user properties.
+        ///     In MQTT 5, user properties are basic UTF-8 string key-value pairs that you can append to almost every type of MQTT
+        ///     packet.
+        ///     As long as you don’t exceed the maximum message size, you can use an unlimited number of user properties to add
+        ///     metadata to MQTT messages and pass information between publisher, broker, and subscriber.
+        ///     The feature is very similar to the HTTP header concept.
+        ///     Hint: MQTT 5 feature only.
         /// </summary>
         public IReadOnlyCollection<MqttUserProperty> UserProperties { get; internal set; }
     }
-}
+}

Source/MQTTnet/Exceptions/MqttCommunicationException.cs

@@ -8,12 +8,8 @@ namespace MQTTnet.Exceptions
 {
     public class MqttCommunicationException : Exception
     {
-        protected MqttCommunicationException()
-        {
-        }
-
         public MqttCommunicationException(Exception innerException)
-            : base(innerException.Message, innerException)
+            : base(innerException?.Message ?? "MQTT communication failed.", innerException)
         {
         }
 

Source/MQTTnet/Formatter/MqttBufferReader.cs

@@ -6,13 +6,13 @@ using System;
 using System.Runtime.CompilerServices;
 using System.Text;
 using MQTTnet.Exceptions;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Formatter
 {
     public sealed class MqttBufferReader
     {
-        byte[] _buffer = PlatformAbstractionLayer.EmptyByteArray;
+        byte[] _buffer = EmptyBuffer.Array;
         int _initialPosition;
         int _length;
         int _position;
@@ -29,7 +29,7 @@ namespace MQTTnet.Formatter
 
             if (length == 0)
             {
-                return PlatformAbstractionLayer.EmptyByteArray;
+                return EmptyBuffer.Array;
             }
 
             ValidateReceiveBuffer(length);
@@ -66,7 +66,7 @@ namespace MQTTnet.Formatter
 
             if (bufferLength == 0)
             {
-                return PlatformAbstractionLayer.EmptyByteArray;
+                return EmptyBuffer.Array;
             }
             
             var buffer = new byte[bufferLength];

Source/MQTTnet/Formatter/MqttPacketBuffer.cs

@@ -5,12 +5,13 @@
 using System;
 using System.Linq;
 using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Formatter
 {
     public readonly struct MqttPacketBuffer
     {
-        static readonly ArraySegment<byte> EmptyPayload = PlatformAbstractionLayer.EmptyByteArraySegment;
+        static readonly ArraySegment<byte> EmptyPayload = EmptyBuffer.ArraySegment;
         
         public MqttPacketBuffer(ArraySegment<byte> packet, ArraySegment<byte> payload)
         {

Source/MQTTnet/Formatter/MqttPacketFactories.cs

@@ -7,6 +7,7 @@ namespace MQTTnet.Formatter
     public sealed class MqttPacketFactories
     {
         public MqttConnAckPacketFactory ConnAck { get; } = new MqttConnAckPacketFactory();
+        
         public MqttConnectPacketFactory Connect { get; } = new MqttConnectPacketFactory();
 
         public MqttDisconnectPacketFactory Disconnect { get; } = new MqttDisconnectPacketFactory();

Source/MQTTnet/Formatter/MqttPubAckPacketFactory.cs

@@ -12,7 +12,10 @@ namespace MQTTnet.Formatter
 {
     public sealed class MqttPubAckPacketFactory
     {
-        public MqttPubAckPacket Create(MqttPublishPacket publishPacket, InterceptingPublishEventArgs interceptingPublishEventArgs)
+        public MqttPubAckPacket Create(
+            MqttPublishPacket publishPacket,
+            InterceptingPublishEventArgs interceptingPublishEventArgs,
+            DispatchApplicationMessageResult dispatchApplicationMessageResult)
         {
             if (publishPacket == null)
             {
@@ -21,10 +24,19 @@ namespace MQTTnet.Formatter
 
             var pubAckPacket = new MqttPubAckPacket
             {
-                PacketIdentifier = publishPacket.PacketIdentifier,
-                ReasonCode = MqttPubAckReasonCode.Success
+                PacketIdentifier = publishPacket.PacketIdentifier
             };
 
+            if (dispatchApplicationMessageResult.MatchingSubscribersCount == 0)
+            {
+                // NoMatchingSubscribers is ONLY sent by the server!
+                pubAckPacket.ReasonCode = MqttPubAckReasonCode.NoMatchingSubscribers;
+            }
+            else
+            {
+                pubAckPacket.ReasonCode = MqttPubAckReasonCode.Success;
+            }
+
             if (interceptingPublishEventArgs != null)
             {
                 pubAckPacket.ReasonCode = (MqttPubAckReasonCode)(int)interceptingPublishEventArgs.Response.ReasonCode;
@@ -42,24 +54,12 @@ namespace MQTTnet.Formatter
                 throw new ArgumentNullException(nameof(applicationMessageReceivedEventArgs));
             }
 
-            var pubAckPacket = Create(applicationMessageReceivedEventArgs.PublishPacket, applicationMessageReceivedEventArgs.ReasonCode);
-            pubAckPacket.UserProperties = applicationMessageReceivedEventArgs.ResponseUserProperties;
-            pubAckPacket.ReasonString = applicationMessageReceivedEventArgs.ResponseReasonString;
-
-            return pubAckPacket;
-        }
-
-        static MqttPubAckPacket Create(MqttPublishPacket publishPacket, MqttApplicationMessageReceivedReasonCode applicationMessageReceivedReasonCode)
-        {
-            if (publishPacket == null)
-            {
-                throw new ArgumentNullException(nameof(publishPacket));
-            }
-
             var pubAckPacket = new MqttPubAckPacket
             {
-                PacketIdentifier = publishPacket.PacketIdentifier,
-                ReasonCode = (MqttPubAckReasonCode)(int)applicationMessageReceivedReasonCode
+                PacketIdentifier = applicationMessageReceivedEventArgs.PublishPacket.PacketIdentifier,
+                ReasonCode = (MqttPubAckReasonCode)(int)applicationMessageReceivedEventArgs.ReasonCode,
+                UserProperties = applicationMessageReceivedEventArgs.ResponseUserProperties,
+                ReasonString = applicationMessageReceivedEventArgs.ResponseReasonString
             };
 
             return pubAckPacket;

Source/MQTTnet/Formatter/MqttPubRecPacketFactory.cs

@@ -25,7 +25,10 @@ namespace MQTTnet.Formatter
             return pubRecPacket;
         }
 
-        public MqttPacket Create(MqttPublishPacket publishPacket, InterceptingPublishEventArgs interceptingPublishEventArgs)
+        public MqttPacket Create(
+            MqttPublishPacket publishPacket,
+            InterceptingPublishEventArgs interceptingPublishEventArgs,
+            DispatchApplicationMessageResult dispatchApplicationMessageResult)
         {
             if (publishPacket == null)
             {
@@ -38,6 +41,11 @@ namespace MQTTnet.Formatter
                 ReasonCode = MqttPubRecReasonCode.Success
             };
 
+            if (dispatchApplicationMessageResult.MatchingSubscribersCount == 0)
+            {
+                pubRecPacket.ReasonCode = MqttPubRecReasonCode.NoMatchingSubscribers;
+            }
+
             if (interceptingPublishEventArgs != null)
             {
                 pubRecPacket.ReasonCode = (MqttPubRecReasonCode)(int)interceptingPublishEventArgs.Response.ReasonCode;

Source/MQTTnet/Formatter/MqttPublishPacketFactory.cs

@@ -5,6 +5,7 @@
 using System;
 using MQTTnet.Exceptions;
 using MQTTnet.Packets;
+using MQTTnet.Server;
 
 namespace MQTTnet.Formatter
 {
@@ -80,5 +81,17 @@ namespace MQTTnet.Formatter
 
             return packet;
         }
+
+        public MqttPublishPacket Create(MqttRetainedMessageMatch retainedMessage)
+        {
+            if (retainedMessage == null)
+            {
+                throw new ArgumentNullException(nameof(retainedMessage));
+            }
+
+            var publishPacket = Create(retainedMessage.ApplicationMessage);
+            publishPacket.QualityOfServiceLevel = retainedMessage.SubscriptionQualityOfServiceLevel;
+            return publishPacket;
+        }
     }
 }

Source/MQTTnet/Formatter/MqttUnsubAckPacketFactory.cs

@@ -10,16 +10,16 @@ namespace MQTTnet.Formatter
 {
     public sealed class MqttUnsubAckPacketFactory
     {
-        public MqttUnsubAckPacket Create(MqttUnsubscribePacket unsubscribePacket, MqttUnsubscribeResult mqttUnsubscribeResult)
+        public MqttUnsubAckPacket Create(MqttUnsubscribePacket unsubscribePacket, UnsubscribeResult unsubscribeResult)
         {
             if (unsubscribePacket == null)
             {
                 throw new ArgumentNullException(nameof(unsubscribePacket));
             }
 
-            if (mqttUnsubscribeResult == null)
+            if (unsubscribeResult == null)
             {
-                throw new ArgumentNullException(nameof(mqttUnsubscribeResult));
+                throw new ArgumentNullException(nameof(unsubscribeResult));
             }
 
             var unsubAckPacket = new MqttUnsubAckPacket
@@ -28,7 +28,7 @@ namespace MQTTnet.Formatter
             };
 
             // MQTTv5.0.0 only.
-            unsubAckPacket.ReasonCodes = mqttUnsubscribeResult.ReasonCodes;
+            unsubAckPacket.ReasonCodes = unsubscribeResult.ReasonCodes;
 
             return unsubAckPacket;
         }

Source/MQTTnet/Formatter/ReadFixedHeaderResult.cs

@@ -6,9 +6,9 @@ namespace MQTTnet.Formatter
 {
     public struct ReadFixedHeaderResult
     {
-        public static ReadFixedHeaderResult Cancelled { get; } = new ReadFixedHeaderResult
+        public static ReadFixedHeaderResult Canceled { get; } = new ReadFixedHeaderResult
         {
-            IsCancelled = true
+            IsCanceled = true
         };
         
         public static ReadFixedHeaderResult ConnectionClosed { get; } = new ReadFixedHeaderResult
@@ -16,7 +16,7 @@ namespace MQTTnet.Formatter
             IsConnectionClosed = true
         };
         
-        public bool IsCancelled { get; set; }
+        public bool IsCanceled { get; set; }
         
         public bool IsConnectionClosed { get; set; }
 

Source/MQTTnet/Implementations/CrossPlatformSocket.cs

@@ -15,14 +15,20 @@ namespace MQTTnet.Implementations
     public sealed class CrossPlatformSocket : IDisposable
     {
         readonly Socket _socket;
+
+#if !NET5_0_OR_GREATER
         readonly Action _socketDisposeAction;
+#endif
 
         NetworkStream _networkStream;
 
         public CrossPlatformSocket(AddressFamily addressFamily)
         {
             _socket = new Socket(addressFamily, SocketType.Stream, ProtocolType.Tcp);
+
+#if !NET5_0_OR_GREATER
             _socketDisposeAction = _socket.Dispose;
+#endif
         }
 
         public CrossPlatformSocket()
@@ -31,17 +37,76 @@ namespace MQTTnet.Implementations
             // will make use of dual mode in the .net framework.
             _socket = new Socket(SocketType.Stream, ProtocolType.Tcp);
 
+#if !NET5_0_OR_GREATER
             _socketDisposeAction = _socket.Dispose;
+#endif
         }
 
-        public CrossPlatformSocket(Socket socket)
+        CrossPlatformSocket(Socket socket)
         {
             _socket = socket ?? throw new ArgumentNullException(nameof(socket));
             _networkStream = new NetworkStream(socket, true);
 
+#if !NET5_0_OR_GREATER
             _socketDisposeAction = _socket.Dispose;
+#endif
         }
 
+        public bool DualMode
+        {
+            get => _socket.DualMode;
+            set => _socket.DualMode = value;
+        }
+
+        public bool IsConnected => _socket.Connected;
+
+        public bool KeepAlive
+        {
+            get => _socket.GetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive) as int? == 1;
+            set => _socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, value ? 1 : 0);
+        }
+
+        public int TcpKeepAliveInterval
+        {
+#if NETCOREAPP3_0_OR_GREATER
+            get => _socket.GetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval) as int? ?? 0;
+            set => _socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval, value);
+#else
+            get { throw new NotSupportedException("TcpKeepAliveInterval requires at least net5.0."); }
+            set { throw new NotSupportedException("TcpKeepAliveInterval requires at least net5.0."); }
+#endif
+        }
+
+        public int TcpKeepAliveRetryCount
+        {
+#if NETCOREAPP3_0_OR_GREATER
+            get => _socket.GetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount) as int? ?? 0;
+            set => _socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount, value);
+#else
+            get { throw new NotSupportedException("TcpKeepAliveRetryCount requires at least net5.0."); }
+            set { throw new NotSupportedException("TcpKeepAliveRetryCount requires at least net5.0."); }
+#endif
+        }
+
+        public int TcpKeepAliveTime
+        {
+#if NETCOREAPP3_0_OR_GREATER
+            get => _socket.GetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime) as int? ?? 0;
+            set => _socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime, value);
+#else
+            get { throw new NotSupportedException("TcpKeepAliveTime requires at least net5.0."); }
+            set { throw new NotSupportedException("TcpKeepAliveTime requires at least net5.0."); }
+#endif
+        }
+
+        public LingerOption LingerState
+        {
+            get => _socket.LingerState;
+            set => _socket.LingerState = value;
+        }
+
+        public EndPoint LocalEndPoint => _socket.LocalEndPoint;
+
         public bool NoDelay
         {
             // We cannot use the _NoDelay_ property from the socket because there is an issue in .NET 4.5.2, 4.6.
@@ -51,24 +116,20 @@ namespace MQTTnet.Implementations
             set => _socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.NoDelay, value ? 1 : 0);
         }
 
-        public LingerOption LingerState
-        {
-            get => _socket.LingerState;
-            set => _socket.LingerState = value;
-        }
-
-        public bool DualMode
-        {
-            get => _socket.DualMode;
-            set => _socket.DualMode = value;
-        }
-
         public int ReceiveBufferSize
         {
             get => _socket.ReceiveBufferSize;
             set => _socket.ReceiveBufferSize = value;
         }
 
+        public EndPoint RemoteEndPoint => _socket.RemoteEndPoint;
+
+        public bool ReuseAddress
+        {
+            get => _socket.GetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress) as int? != 0;
+            set => _socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, value ? 1 : 0);
+        }
+
         public int SendBufferSize
         {
             get => _socket.SendBufferSize;
@@ -81,16 +142,6 @@ namespace MQTTnet.Implementations
             set => _socket.SendTimeout = value;
         }
 
-        public EndPoint RemoteEndPoint => _socket.RemoteEndPoint;
-
-        public bool ReuseAddress
-        {
-            get => (int)_socket.GetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress) != 0;
-            set => _socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, value ? 1 : 0);
-        }
-
-        public bool IsConnected => _socket.Connected;
-
         public async Task<CrossPlatformSocket> AcceptAsync()
         {
             try
@@ -109,8 +160,6 @@ namespace MQTTnet.Implementations
             }
         }
 
-        public EndPoint LocalEndPoint => _socket.LocalEndPoint;
-
         public void Bind(EndPoint localEndPoint)
         {
             if (localEndPoint is null)
@@ -121,11 +170,6 @@ namespace MQTTnet.Implementations
             _socket.Bind(localEndPoint);
         }
 
-        public void Listen(int connectionBacklog)
-        {
-            _socket.Listen(connectionBacklog);
-        }
-
         public async Task ConnectAsync(string host, int port, CancellationToken cancellationToken)
         {
             if (host is null)
@@ -137,11 +181,17 @@ namespace MQTTnet.Implementations
 
             try
             {
-                _networkStream?.Dispose();
-
 #if NET5_0_OR_GREATER
+
+                if (_networkStream != null)
+                {
+                    await _networkStream.DisposeAsync().ConfigureAwait(false);
+                }
+
                 await _socket.ConnectAsync(host, port, cancellationToken).ConfigureAwait(false);
 #else
+                _networkStream?.Dispose();
+
                 // Workaround for: https://github.com/dotnet/corefx/issues/24430
                 using (cancellationToken.Register(_socketDisposeAction))
                 {
@@ -175,20 +225,26 @@ namespace MQTTnet.Implementations
             }
         }
 
-        public async Task SendAsync(ArraySegment<byte> buffer, SocketFlags socketFlags)
+        public void Dispose()
         {
-            try
+            _networkStream?.Dispose();
+            _socket?.Dispose();
+        }
+
+        public NetworkStream GetStream()
+        {
+            var networkStream = _networkStream;
+            if (networkStream == null)
             {
-#if NET452 || NET461
-                await Task.Factory.FromAsync(SocketWrapper.BeginSend, _socket.EndSend, new SocketWrapper(_socket, buffer, socketFlags)).ConfigureAwait(false);
-#else
-                await _socket.SendAsync(buffer, socketFlags).ConfigureAwait(false);
-#endif
-            }
-            catch (ObjectDisposedException)
-            {
-                // This will happen when _socket.EndConnect gets called by Task library but the socket is already disposed.
+                throw new IOException("The socket is not connected.");
             }
+
+            return networkStream;
+        }
+
+        public void Listen(int connectionBacklog)
+        {
+            _socket.Listen(connectionBacklog);
         }
 
         public async Task<int> ReceiveAsync(ArraySegment<byte> buffer, SocketFlags socketFlags)
@@ -208,28 +264,27 @@ namespace MQTTnet.Implementations
             }
         }
 
-        public NetworkStream GetStream()
+        public async Task SendAsync(ArraySegment<byte> buffer, SocketFlags socketFlags)
         {
-            var networkStream = _networkStream;
-            if (networkStream == null)
+            try
             {
-                throw new IOException("The socket is not connected.");
+#if NET452 || NET461
+                await Task.Factory.FromAsync(SocketWrapper.BeginSend, _socket.EndSend, new SocketWrapper(_socket, buffer, socketFlags)).ConfigureAwait(false);
+#else
+                await _socket.SendAsync(buffer, socketFlags).ConfigureAwait(false);
+#endif
+            }
+            catch (ObjectDisposedException)
+            {
+                // This will happen when _socket.EndConnect gets called by Task library but the socket is already disposed.
             }
-
-            return networkStream;
-        }
-
-        public void Dispose()
-        {
-            _networkStream?.Dispose();
-            _socket?.Dispose();
         }
 
 #if NET452 || NET461
         sealed class SocketWrapper
         {
-            readonly Socket _socket;
             readonly ArraySegment<byte> _buffer;
+            readonly Socket _socket;
             readonly SocketFlags _socketFlags;
 
             public SocketWrapper(Socket socket, ArraySegment<byte> buffer, SocketFlags socketFlags)
@@ -239,16 +294,28 @@ namespace MQTTnet.Implementations
                 _socketFlags = socketFlags;
             }
 
-            public static IAsyncResult BeginSend(AsyncCallback callback, object state)
-            {
-                var socketWrapper = (SocketWrapper)state;
-                return socketWrapper._socket.BeginSend(socketWrapper._buffer.Array, socketWrapper._buffer.Offset, socketWrapper._buffer.Count, socketWrapper._socketFlags, callback, state);
-            }
-
             public static IAsyncResult BeginReceive(AsyncCallback callback, object state)
             {
                 var socketWrapper = (SocketWrapper)state;
-                return socketWrapper._socket.BeginReceive(socketWrapper._buffer.Array, socketWrapper._buffer.Offset, socketWrapper._buffer.Count, socketWrapper._socketFlags, callback, state);
+                return socketWrapper._socket.BeginReceive(
+                    socketWrapper._buffer.Array,
+                    socketWrapper._buffer.Offset,
+                    socketWrapper._buffer.Count,
+                    socketWrapper._socketFlags,
+                    callback,
+                    state);
+            }
+
+            public static IAsyncResult BeginSend(AsyncCallback callback, object state)
+            {
+                var socketWrapper = (SocketWrapper)state;
+                return socketWrapper._socket.BeginSend(
+                    socketWrapper._buffer.Array,
+                    socketWrapper._buffer.Offset,
+                    socketWrapper._buffer.Count,
+                    socketWrapper._socketFlags,
+                    callback,
+                    state);
             }
         }
 #endif

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -14,6 +14,7 @@ using System.Runtime.ExceptionServices;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Implementations
 {
@@ -154,7 +155,7 @@ namespace MQTTnet.Implementations
         public Task DisconnectAsync(CancellationToken cancellationToken)
         {
             Dispose();
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public void Dispose()
@@ -263,27 +264,15 @@ namespace MQTTnet.Implementations
             }
         }
 
-        bool InternalUserCertificateValidationCallback(object sender, X509Certificate x509Certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
+        bool InternalUserCertificateValidationCallback(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
         {
             var certificateValidationHandler = _tcpOptions?.TlsOptions?.CertificateValidationHandler;
             if (certificateValidationHandler != null)
             {
-                var eventArgs = new MqttClientCertificateValidationEventArgs
-                {
-                    Certificate = x509Certificate,
-                    Chain = chain,
-                    SslPolicyErrors = sslPolicyErrors,
-                    ClientOptions = _tcpOptions
-                };
-
+                var eventArgs = new MqttClientCertificateValidationEventArgs(certificate, chain, sslPolicyErrors, _tcpOptions);
                 return certificateValidationHandler(eventArgs);
             }
 
-            if (_tcpOptions?.TlsOptions?.IgnoreCertificateChainErrors ?? false)
-            {
-                sslPolicyErrors &= ~SslPolicyErrors.RemoteCertificateChainErrors;
-            }
-
             return sslPolicyErrors == SslPolicyErrors.None;
         }
 

Source/MQTTnet/Implementations/MqttTcpServerAdapter.cs

@@ -13,6 +13,7 @@ using System.Net.Sockets;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Implementations
 {
@@ -56,13 +57,13 @@ namespace MQTTnet.Implementations
                 RegisterListeners(options.TlsEndpointOptions, tlsCertificate, logger, _cancellationTokenSource.Token);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         public Task StopAsync()
         {
             Cleanup();
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         public void Dispose()
@@ -124,7 +125,7 @@ namespace MQTTnet.Implementations
             var clientHandler = ClientHandler;
             if (clientHandler == null)
             {
-                return Task.FromResult(0);
+                return CompletedTask.Instance;
             }
 
             return clientHandler(channelAdapter);

Source/MQTTnet/Implementations/MqttWebSocketChannel.cs

@@ -117,7 +117,7 @@ namespace MQTTnet.Implementations
                 return;
             }
 
-            using (await _sendLock.WaitAsync(cancellationToken).ConfigureAwait(false))
+            using (await _sendLock.EnterAsync(cancellationToken).ConfigureAwait(false))
             {
                 await _webSocket.SendAsync(buffer, WebSocketMessageType.Binary, isEndOfPacket, cancellationToken).ConfigureAwait(false);
             }
@@ -224,14 +224,7 @@ namespace MQTTnet.Implementations
                 clientWebSocket.Options.RemoteCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) =>
                 {
                     // TODO: Find a way to add client options to same callback. Problem is that they have a different type.
-                    var context = new MqttClientCertificateValidationEventArgs
-                    {
-                        Certificate = certificate,
-                        Chain = chain,
-                        SslPolicyErrors = sslPolicyErrors,
-                        ClientOptions = _options
-                    };
-
+                    var context = new MqttClientCertificateValidationEventArgs(certificate, chain, sslPolicyErrors, _options);
                     return certificateValidationHandler(context);
                 };
 #endif

Source/MQTTnet/Implementations/PlatformAbstractionLayer.cs

@@ -1,43 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-// See the LICENSE file in the project root for more information.
-
-using System;
-using System.Threading;
-using System.Threading.Tasks;
-
-namespace MQTTnet.Implementations
-{
-    public static class PlatformAbstractionLayer
-    {
-#if NET452
-        public static Task CompletedTask { get; } = Task.FromResult(0);
-
-        public static byte[] EmptyByteArray { get; } = new byte[0];
-#else
-        public static Task CompletedTask { get; } = Task.CompletedTask;
-
-        public static byte[] EmptyByteArray { get; } = Array.Empty<byte>();
-#endif
-
-        public static ArraySegment<byte> EmptyByteArraySegment { get; } = new ArraySegment<byte>(EmptyByteArray);
-
-        public static void Sleep(TimeSpan timeout)
-        {
-#if !NETSTANDARD1_3 && !WINDOWS_UWP
-            try
-            {
-                System.Threading.Thread.Sleep(timeout);
-            }
-            catch (ThreadAbortException)
-            {
-                // The ThreadAbortException is not actively catched in this project.
-                // So we use a one which is similar and will be catched properly.
-                throw new OperationCanceledException();
-            }
-#else
-            Task.Delay(timeout).Wait();
-#endif
-        }
-    }
-}

Source/MQTTnet/Internal/AsyncEvent.cs

@@ -4,7 +4,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
 
@@ -37,7 +36,7 @@ namespace MQTTnet.Internal
                 _handlers.Add(new AsyncEventInvocator<TEventArgs>(null, handler));
 
                 HasHandlers = true;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers.ToList());
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
             }
         }
 
@@ -53,7 +52,7 @@ namespace MQTTnet.Internal
                 _handlers.Add(new AsyncEventInvocator<TEventArgs>(handler, null));
 
                 HasHandlers = true;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers.ToList());
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
             }
         }
 
@@ -86,7 +85,7 @@ namespace MQTTnet.Internal
                 _handlers.RemoveAll(h => h.WrapsHandler(handler));
 
                 HasHandlers = _handlers.Count > 0;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers.ToList());
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
             }
         }
 
@@ -102,7 +101,7 @@ namespace MQTTnet.Internal
                 _handlers.RemoveAll(h => h.WrapsHandler(handler));
 
                 HasHandlers = _handlers.Count > 0;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers.ToList());
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
             }
         }
 
@@ -124,7 +123,7 @@ namespace MQTTnet.Internal
             }
             catch (Exception exception)
             {
-                logger.Warning(exception, $"Error while invoking event ({typeof(TEventArgs)}).");
+                logger.Warning(exception, $"Error while invoking event with arguments of type {typeof(TEventArgs)}.");
             }
         }
     }

Source/MQTTnet/Internal/AsyncEventInvocator.cs

@@ -36,7 +36,7 @@ namespace MQTTnet.Internal
             if (_handler != null)
             {
                 _handler.Invoke(eventArgs);
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             }
 
             if (_asyncHandler != null)

Source/MQTTnet/Internal/AsyncLock.cs

@@ -4,7 +4,7 @@
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -12,173 +12,151 @@ namespace MQTTnet.Internal
 {
     public sealed class AsyncLock : IDisposable
     {
-        /*
-         * This async supporting lock does not support reentrancy!
-         */
-
-        readonly List<Releaser> _queuedTasks = new List<Releaser>(64);
-        readonly Task<IDisposable> _releaserTaskWithDirectApproval;
-
-        readonly Releaser _releaserWithDirectApproval;
-
+        readonly Task<IDisposable> _completedTask;
+        readonly IDisposable _releaser;
         readonly object _syncRoot = new object();
+        readonly Queue<AsyncLockWaiter> _waiters = new Queue<AsyncLockWaiter>(64);
+
+        volatile bool _isDisposed;
+        bool _isLocked;
 
-        bool _isDisposed;
-        
         public AsyncLock()
         {
-            _releaserWithDirectApproval = new Releaser(this, null, CancellationToken.None);
-            _releaserTaskWithDirectApproval = Task.FromResult((IDisposable)_releaserWithDirectApproval);
+            _releaser = new Releaser(this);
+            _completedTask = Task.FromResult(_releaser);
         }
 
         public void Dispose()
         {
             lock (_syncRoot)
             {
-                foreach (var waitingTask in _queuedTasks)
-                {
-                    waitingTask.Fail(new ObjectDisposedException(nameof(AsyncLock)));
-                }
-
-                _queuedTasks.Clear();
-                
                 _isDisposed = true;
+
+                while (_waiters.Any())
+                {
+                    _waiters.Dequeue().Dispose();
+                }
             }
         }
 
-        public Task<IDisposable> WaitAsync(CancellationToken cancellationToken)
+        public Task<IDisposable> EnterAsync(CancellationToken cancellationToken = default)
         {
-            var hasDirectApproval = false;
-            Releaser releaser;
+            cancellationToken.ThrowIfCancellationRequested();
+            
+            if (_isDisposed)
+            {
+                throw new ObjectDisposedException(nameof(AsyncLock));
+            }
 
             lock (_syncRoot)
             {
-                if (_isDisposed)
+                if (!_isLocked)
                 {
-                    throw new ObjectDisposedException(nameof(AsyncLock));
-                }
-                
-                if (_queuedTasks.Count == 0)
-                {
-                    // There is no other waiting task apart from the current one.
-                    // So we can approve the current task directly.
-                    releaser = _releaserWithDirectApproval;
-                    hasDirectApproval = true;
-                    Debug.WriteLine("AsyncLock: Task -1 directly approved.");
-                }
-                else
-                {
-                    releaser = new Releaser(this, new TaskCompletionSource<IDisposable>(), cancellationToken);
+                    _isLocked = true;
+                    return _completedTask;
                 }
 
-                _queuedTasks.Add(releaser);
+                var waiter = new AsyncLockWaiter(cancellationToken);
+                _waiters.Enqueue(waiter);
+
+                return waiter.Task;
             }
-
-            if (!hasDirectApproval)
-            {
-                return releaser.Task;
-            }
-
-            return _releaserTaskWithDirectApproval;
         }
 
-        void Release(Releaser releaser)
+        void Release()
         {
             lock (_syncRoot)
             {
                 if (_isDisposed)
                 {
-                    // There is no much left to do!
+                    // All waiters have been canceled with a ObjectDisposedException.
+                    // So there is nothing left to do.
                     return;
                 }
 
-                var activeTask = _queuedTasks[0];
-                if (!ReferenceEquals(activeTask, releaser))
-                {
-                    throw new InvalidOperationException("The active task must be the current releaser.");
-                }
+                // Assume that there is no waiter left first.
+                _isLocked = false;
 
-                _queuedTasks.RemoveAt(0);
-                
-                while (_queuedTasks.Count > 0)
+                // Try to find the next waiter which can be approved.
+                // Some of them might be canceled already so it is not
+                // guaranteed that the very next waiter is the correct one.
+                while (_waiters.Any())
                 {
-                    var nextTask = _queuedTasks[0];
-                    if (!nextTask.IsPending)
+                    var waiter = _waiters.Dequeue();
+                    var isApproved = waiter.Approve(_releaser);
+                    waiter.Dispose();
+
+                    if (isApproved)
                     {
-                        // Dequeue all canceled or failed tasks.
-                        _queuedTasks.RemoveAt(0);
-                        continue;
+                        _isLocked = true;
+                        return;
                     }
-
-                    nextTask.Approve();
-                    return;
                 }
-
-                Debug.WriteLine("AsyncLock: No Task pending.");
             }
         }
 
-        sealed class Releaser : IDisposable
+        sealed class AsyncLockWaiter : IDisposable
         {
-            readonly AsyncLock _asyncLock;
-            readonly CancellationToken _cancellationToken;
-            readonly int _id;
-            readonly TaskCompletionSource<IDisposable> _promise;
+            readonly CancellationTokenRegistration _cancellationRegistration;
+            readonly bool _hasCancellationRegistration;
+            readonly AsyncTaskCompletionSource<IDisposable> _promise = new AsyncTaskCompletionSource<IDisposable>();
 
-            // ReSharper disable once FieldCanBeMadeReadOnly.Local
-            CancellationTokenRegistration _cancellationTokenRegistration;
-
-            internal Releaser(AsyncLock asyncLock, TaskCompletionSource<IDisposable> promise, CancellationToken cancellationToken)
+            public AsyncLockWaiter(CancellationToken cancellationToken)
             {
-                _asyncLock = asyncLock ?? throw new ArgumentNullException(nameof(asyncLock));
-                _promise = promise;
-                _cancellationToken = cancellationToken;
-
+                cancellationToken.ThrowIfCancellationRequested();
+                
                 if (cancellationToken.CanBeCanceled)
                 {
-                    _cancellationTokenRegistration = cancellationToken.Register(Cancel);
+                    _cancellationRegistration = cancellationToken.Register(Cancel);
+                    _hasCancellationRegistration = true;
                 }
-
-                _id = promise?.Task.Id ?? -1;
-
-                Debug.WriteLine($"AsyncLock: Task {_id} queued.");
             }
 
-            public bool IsPending => _promise != null && !_promise.Task.IsCanceled && !_promise.Task.IsFaulted && !_promise.Task.IsCompleted;
+            public Task<IDisposable> Task => _promise.Task;
 
-            public Task<IDisposable> Task => _promise?.Task;
-
-            public void Approve()
+            public bool Approve(IDisposable scope)
             {
-                _promise?.TrySetResult(this);
+                if (scope == null)
+                {
+                    throw new ArgumentNullException(nameof(scope));
+                }
 
-                Debug.WriteLine($"AsyncLock: Task {_id} approved.");
+                if (_promise.Task.IsCompleted)
+                {
+                    return false;
+                }
+
+                return _promise.TrySetResult(scope);
             }
 
             public void Dispose()
             {
-                if (_cancellationToken.CanBeCanceled)
+                if (_hasCancellationRegistration)
                 {
-                    _cancellationTokenRegistration.Dispose();
+                    _cancellationRegistration.Dispose();
                 }
 
-                Debug.WriteLine($"AsyncLock: Task {_id} completed.");
-
-                _asyncLock.Release(this);
-            }
-
-            public void Fail(Exception exception)
-            {
-                _promise?.TrySetException(exception);
-
-                Debug.WriteLine($"AsyncLock: Task {_id} failed ({exception.GetType().Name}).");
+                _promise.TrySetException(new ObjectDisposedException(nameof(AsyncLockWaiter)));
             }
 
             void Cancel()
             {
-                _promise?.TrySetCanceled();
+                _promise.TrySetCanceled();
+            }
+        }
 
-                Debug.WriteLine($"AsyncLock: Task {_id} canceled.");
+        readonly struct Releaser : IDisposable
+        {
+            readonly AsyncLock _asyncLock;
+
+            public Releaser(AsyncLock asyncLock)
+            {
+                _asyncLock = asyncLock ?? throw new ArgumentNullException(nameof(asyncLock));
+            }
+
+            public void Dispose()
+            {
+                _asyncLock.Release();
             }
         }
     }

Source/MQTTnet/Internal/AsyncQueue.cs

@@ -11,74 +11,15 @@ namespace MQTTnet.Internal
 {
     public sealed class AsyncQueue<TItem> : IDisposable
     {
+        readonly AsyncSignal _signal = new AsyncSignal();
         readonly object _syncRoot = new object();
-        SemaphoreSlim _semaphore = new SemaphoreSlim(0);
+
         ConcurrentQueue<TItem> _queue = new ConcurrentQueue<TItem>();
 
+        bool _isDisposed;
+        
         public int Count => _queue.Count;
 
-        public void Enqueue(TItem item)
-        {
-            lock (_syncRoot)
-            {
-                _queue.Enqueue(item);
-                _semaphore?.Release();
-            }
-        }
-
-        public async Task<AsyncQueueDequeueResult<TItem>> TryDequeueAsync(CancellationToken cancellationToken)
-        {
-            while (!cancellationToken.IsCancellationRequested)
-            {
-                try
-                {
-                    Task task;
-                    lock (_syncRoot)
-                    {
-                        if (_semaphore == null)
-                        {
-                            return new AsyncQueueDequeueResult<TItem>(false, default);
-                        }
-
-                        task = _semaphore.WaitAsync(cancellationToken);
-                    }
-                    
-                    await task.ConfigureAwait(false);
-
-                    if (cancellationToken.IsCancellationRequested)
-                    {
-                        return new AsyncQueueDequeueResult<TItem>(false, default);
-                    }
-
-                    if (_queue.TryDequeue(out var item))
-                    {
-                        return new AsyncQueueDequeueResult<TItem>(true, item);
-                    }
-                }
-                catch (ArgumentNullException)
-                {
-                    // The semaphore throws this internally sometimes.
-                    return new AsyncQueueDequeueResult<TItem>(false, default);
-                }
-                catch (OperationCanceledException)
-                {
-                    return new AsyncQueueDequeueResult<TItem>(false, default);
-                }
-            }
-
-            return new AsyncQueueDequeueResult<TItem>(false, default);
-        }
-
-        public AsyncQueueDequeueResult<TItem> TryDequeue()
-        {
-            if (_queue.TryDequeue(out var item))
-            {
-                return new AsyncQueueDequeueResult<TItem>(true, item);
-            }
-
-            return new AsyncQueueDequeueResult<TItem>(false, default);
-        }
-
         public void Clear()
         {
             Interlocked.Exchange(ref _queue, new ConcurrentQueue<TItem>());
@@ -88,9 +29,73 @@ namespace MQTTnet.Internal
         {
             lock (_syncRoot)
             {
-                _semaphore?.Dispose();
-                _semaphore = null;
+                _signal.Dispose();
+
+                _isDisposed = true;
             }
         }
+
+        public void Enqueue(TItem item)
+        {
+            lock (_syncRoot)
+            {
+                _queue.Enqueue(item);
+                _signal.Set();
+            }
+        }
+
+        public AsyncQueueDequeueResult<TItem> TryDequeue()
+        {
+            if (_queue.TryDequeue(out var item))
+            {
+                return AsyncQueueDequeueResult<TItem>.Success(item);
+            }
+
+            return AsyncQueueDequeueResult<TItem>.NonSuccess;
+        }
+
+        public async Task<AsyncQueueDequeueResult<TItem>> TryDequeueAsync(CancellationToken cancellationToken)
+        {
+            while (!cancellationToken.IsCancellationRequested)
+            {
+                try
+                {
+                    Task task = null;
+                    lock (_syncRoot)
+                    {
+                        if (_isDisposed)
+                        {
+                            return AsyncQueueDequeueResult<TItem>.NonSuccess;
+                        }
+
+                        if (_queue.IsEmpty)
+                        {
+                            task = _signal.WaitAsync(cancellationToken);
+                        }
+                    }
+
+                    if (task != null)
+                    {
+                        await task.ConfigureAwait(false);    
+                    }
+                    
+                    if (cancellationToken.IsCancellationRequested)
+                    {
+                        return AsyncQueueDequeueResult<TItem>.NonSuccess;
+                    }
+
+                    if (_queue.TryDequeue(out var item))
+                    {
+                        return AsyncQueueDequeueResult<TItem>.Success(item);
+                    }
+                }
+                catch (OperationCanceledException)
+                {
+                    return AsyncQueueDequeueResult<TItem>.NonSuccess;
+                }
+            }
+
+            return AsyncQueueDequeueResult<TItem>.NonSuccess;
+        }
     }
-}
+}

Source/MQTTnet/Internal/AsyncQueueDequeueResult.cs

@@ -4,8 +4,10 @@
 
 namespace MQTTnet.Internal
 {
-    public class AsyncQueueDequeueResult<TItem>
+    public sealed class AsyncQueueDequeueResult<TItem>
     {
+        public static readonly AsyncQueueDequeueResult<TItem> NonSuccess = new AsyncQueueDequeueResult<TItem>(false, default);
+
         public AsyncQueueDequeueResult(bool isSuccess, TItem item)
         {
             IsSuccess = isSuccess;
@@ -15,5 +17,10 @@ namespace MQTTnet.Internal
         public bool IsSuccess { get; }
 
         public TItem Item { get; }
+
+        public static AsyncQueueDequeueResult<TItem> Success(TItem item)
+        {
+            return new AsyncQueueDequeueResult<TItem>(true, item);
+        }
     }
 }

Source/MQTTnet/Internal/AsyncSignal.cs

@@ -0,0 +1,133 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Internal
+{
+    public sealed class AsyncSignal : IDisposable
+    {
+        readonly object _syncRoot = new object();
+
+        bool _isDisposed;
+        bool _isSignaled;
+        AsyncSignalWaiter _waiter;
+
+        public void Dispose()
+        {
+            lock (_syncRoot)
+            {
+                _waiter?.Dispose();
+                _waiter = null;
+
+                _isDisposed = true;
+            }
+        }
+
+        public void Set()
+        {
+            lock (_syncRoot)
+            {
+                _isSignaled = true;
+
+                Cleanup();
+                
+                // If there is already a waiting task let it run.
+                if (_waiter != null)
+                {
+                    _waiter.Approve();
+                    _waiter = null;
+
+                    // Since we already got a waiter the signal must be reset right now!
+                    _isSignaled = false;
+                }
+            }
+        }
+
+        public Task WaitAsync(CancellationToken cancellationToken = default)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            lock (_syncRoot)
+            {
+                ThrowIfDisposed();
+
+                Cleanup();
+
+                if (_isSignaled)
+                {
+                    _isSignaled = false;
+                    return CompletedTask.Instance;
+                }
+
+                if (_waiter != null)
+                {
+                    if (!_waiter.Task.IsCompleted)
+                    {
+                        throw new InvalidOperationException("Only one waiting task is permitted per async signal.");
+                    }
+
+                    _waiter.Dispose();
+                }
+
+                _waiter = new AsyncSignalWaiter(cancellationToken);
+                return _waiter.Task;
+            }
+        }
+
+        void Cleanup()
+        {
+            // Cleanup if the previous waiter was cancelled.
+            if (_waiter != null && _waiter.Task.IsCanceled)
+            {
+                _waiter = null;
+            }
+        }
+
+        void ThrowIfDisposed()
+        {
+            if (_isDisposed)
+            {
+                throw new ObjectDisposedException(nameof(AsyncSignal));
+            }
+        }
+
+        sealed class AsyncSignalWaiter : IDisposable
+        {
+            readonly AsyncTaskCompletionSource<bool> _promise = new AsyncTaskCompletionSource<bool>();
+
+            // ReSharper disable once FieldCanBeMadeReadOnly.Local
+            CancellationTokenRegistration _cancellationTokenRegistration;
+
+            public AsyncSignalWaiter(CancellationToken cancellationToken)
+            {
+                if (cancellationToken.CanBeCanceled)
+                {
+                    _cancellationTokenRegistration = cancellationToken.Register(Cancel);
+                }
+            }
+
+            public Task Task => _promise.Task;
+
+            public void Approve()
+            {
+                _promise.TrySetResult(true);
+            }
+
+            public void Dispose()
+            {
+                _cancellationTokenRegistration.Dispose();
+
+                _promise.TrySetException(new ObjectDisposedException(nameof(AsyncSignalWaiter)));
+            }
+
+            void Cancel()
+            {
+                _promise.TrySetCanceled();
+            }
+        }
+    }
+}

Source/MQTTnet/Internal/AsyncTaskCompletionSource.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace MQTTnet.Internal
@@ -22,7 +23,7 @@ namespace MQTTnet.Internal
 
         public Task<TResult> Task => _taskCompletionSource.Task;
 
-        public bool TrySetCanceled()
+        public void TrySetCanceled()
         {
 #if NET452
             // To prevent deadlocks it is required to call the _TrySetCanceled_ method
@@ -31,19 +32,14 @@ namespace MQTTnet.Internal
             // do it. But _this_ thread is also reading incoming packets -> deadlock.
             // NET452 does not support RunContinuationsAsynchronously
             System.Threading.Tasks.Task.Run(() => _taskCompletionSource.TrySetCanceled());
-            return true;
+            SpinWait.SpinUntil(() => _taskCompletionSource.Task.IsCompleted);
 #else
-            return _taskCompletionSource.TrySetCanceled();
+            _taskCompletionSource.TrySetCanceled();
 #endif
         }
 
-        public bool TrySetException(Exception exception)
+        public void TrySetException(Exception exception)
         {
-            if (exception == null)
-            {
-                throw new ArgumentNullException(nameof(exception));
-            }
-
 #if NET452
             // To prevent deadlocks it is required to call the _TrySetException_ method
             // from a new thread because the awaiting code will not(!) be executed in
@@ -51,9 +47,9 @@ namespace MQTTnet.Internal
             // do it. But _this_ thread is also reading incoming packets -> deadlock.
             // NET452 does not support RunContinuationsAsynchronously
             System.Threading.Tasks.Task.Run(() => _taskCompletionSource.TrySetException(exception));
-            return true;
+            SpinWait.SpinUntil(() => _taskCompletionSource.Task.IsCompleted);
 #else
-            return _taskCompletionSource.TrySetException(exception);
+            _taskCompletionSource.TrySetException(exception);
 #endif
         }
 
@@ -65,7 +61,14 @@ namespace MQTTnet.Internal
             // a new thread automatically (due to await). Furthermore _this_ thread will
             // do it. But _this_ thread is also reading incoming packets -> deadlock.
             // NET452 does not support RunContinuationsAsynchronously
+            if (_taskCompletionSource.Task.IsCompleted)
+            {
+                return false;
+            }
+            
             System.Threading.Tasks.Task.Run(() => _taskCompletionSource.TrySetResult(result));
+            SpinWait.SpinUntil(() => _taskCompletionSource.Task.IsCompleted);
+
             return true;
 #else
             return _taskCompletionSource.TrySetResult(result);

Source/MQTTnet/Internal/CompletedTask.cs

@@ -0,0 +1,17 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Threading.Tasks;
+
+namespace MQTTnet.Internal
+{
+    public static class CompletedTask
+    {
+#if NET452
+        public static readonly Task Instance = Task.FromResult(true);
+#else
+        public static readonly Task Instance = Task.CompletedTask;
+#endif
+    }
+}

Source/MQTTnet/Internal/EmptyBuffer.cs

@@ -0,0 +1,19 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace MQTTnet.Internal
+{
+    public static class EmptyBuffer
+    {
+#if NET452
+        public static readonly byte[] Array = new byte[0];
+#else
+        public static readonly byte[] Array = System.Array.Empty<byte>();
+#endif
+
+        public static readonly ArraySegment<byte> ArraySegment = new ArraySegment<byte>(Array, 0, 0);
+    }
+}