Add samples.

.github/workflows/ReleaseNotes.md

@@ -1,2 +0,0 @@
-* [Core] Fixed dead lock and race conditions in new _AsyncLock_ implementation (#1542).
-* [Client] Fixed connection freeze when using Xamarin etc.

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ name: CI
 on: [push, pull_request]
 
 env:
-  VERSION: "4.1.2.${{github.run_number}}"
+  VERSION: "4.1.0.${{github.run_number}}"
 
 jobs:
   build:

MQTTnet.sln.DotSettings

@@ -210,7 +210,6 @@
         </Or>
       </Entry.Match>
         <Entry.SortBy>
-            <Access Order="Public Internal Protected Private" />
             <Name />
         </Entry.SortBy>
     </Entry>
@@ -241,6 +240,5 @@ 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_002EMigrateBlankLinesAroundFieldToBlankLinesAroundProperty/@EntryIndexedValue">True</s:Boolean>
 	<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/=Tnet/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=unsub/@EntryIndexedValue">True</s:Boolean></wpf:ResourceDictionary>

README.md

@@ -12,7 +12,7 @@
 
 # MQTTnet
 
-MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker) and supports the MQTT protocol up to version 5. It is compatible with mostly any supported .NET Framework version and CPU architecture.
+MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker) and supports the MQTT protocol up to version 5.
 
 ## Features
 
@@ -50,6 +50,39 @@ MQTTnet is a high performance .NET library for MQTT based communication. It prov
 * A custom message interceptor can be added which allows transforming or extending every received application message
 * Validate subscriptions and deny subscribing of certain topics depending on requesting clients
 
+## Supported frameworks
+
+| Framwork           | Version     |
+| ------------------ | ----------- |
+|.NET                | 5.0+        |
+|.NET Framework      | 4.5.2+      |
+|.NET Standard       | 1.3+        |
+|.NET Core           | 1.1+        |
+|.NET Core App       | 1.1+        |
+| Mono               | 5.2+        |
+| UWP                | 10.0.10240+ |
+| Xamarin.Android    | 7.5+        |
+| Xamarin.iOS        | 10.14+      |
+| Blazor WebAssembly | 3.2.0+      |
+
+## References
+
+This library is used in the following projects:
+
+* Azure Functions MQTT Bindings, <https://github.com/keesschollaart81/CaseOnline.Azure.WebJobs.Extensions.Mqtt>)
+* HA4IoT (Open Source Home Automation system for .NET, <https://github.com/chkr1011/HA4IoT>)
+* MQTT Client Rx (Wrapper for Reactive Extensions, <https://github.com/1iveowl/MQTTClient.rx>)
+* MQTT Client Rx (Managed Client Wrapper for Reactive Extensions, <https://github.com/mmuecke/RxMQTTnet>)
+* MQTT Tester (MQTT client test app for [Android](https://play.google.com/store/apps/details?id=com.liveowl.mqtttester) and [iOS](https://itunes.apple.com/us/app/mqtt-tester/id1278621826?mt=8))
+* MQTTnet App (Cross platform client application for MQTT debugging, inspection etc., <https://github.com/chkr1011/MQTTnetApp>)
+* Wirehome.Core (Open Source Home Automation system for .NET Core, <https://github.com/chkr1011/Wirehome.Core>)
+* SparkplugNet (Sparkplug library for .Net, <https://github.com/SeppPenner/SparkplugNet>)
+* Silverback (Framework to build event-driven applications - support for MQTT, Kafka & RabbitMQ) <https://github.com/BEagle1984/silverback>
+
+Further projects using this project can be found under https://github.com/dotnet/MQTTnet/network/dependents.
+
+If you use this library and want to see your project here please create a pull request.
+
 ## Code of Conduct
 
 This project has adopted the code of conduct defined by the Contributor Covenant to clarify expected behavior in our community.

Samples/Client/Client_Connection_Samples.cs

@@ -10,7 +10,7 @@ using System.Security.Authentication;
 using MQTTnet.Client;
 using MQTTnet.Extensions.WebSocket4Net;
 using MQTTnet.Formatter;
-using MQTTnet.Samples.Helpers;
+using MQTTnet.Samples.Shared;
 
 namespace MQTTnet.Samples.Client;
 
@@ -134,17 +134,15 @@ public static class Client_Connection_Samples
         }
     }
 
-    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())
         {
@@ -158,15 +156,17 @@ 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())
         {
@@ -214,49 +214,6 @@ 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()
     {
         /*
@@ -364,12 +321,14 @@ public static class Client_Connection_Samples
                     {
                         try
                         {
-                            // This code will also do the very first connect! So no call to _ConnectAsync_ is required in the first place.
+                            // This code will also do the very first connect! So no call to _ConnectAsync_ is required
-                            if (!await mqttClient.TryPingAsync())
+                            // in the first place.
+                            if (!mqttClient.IsConnected)
                             {
                                 await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
 
                                 // Subscribe to topics when session is clean etc.
+
                                 Console.WriteLine("The MQTT client is connected.");
                             }
                         }

Samples/Client/Client_Publish_Samples.cs

@@ -40,53 +40,6 @@ public static class Client_Publish_Samples
 
             await mqttClient.PublishAsync(applicationMessage, CancellationToken.None);
 
-            await mqttClient.DisconnectAsync();
-            
-            Console.WriteLine("MQTT application message is published.");
-        }
-    }
-    
-    public static async Task Publish_Multiple_Application_Messages()
-    {
-        /*
-         * This sample pushes multiple simple application message including a topic and a payload.
-         *
-         * See sample _Publish_Application_Message_ for more details.
-         */
-
-        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 applicationMessage = new MqttApplicationMessageBuilder()
-                .WithTopic("samples/temperature/living_room")
-                .WithPayload("19.5")
-                .Build();
-
-            await mqttClient.PublishAsync(applicationMessage, CancellationToken.None);
-            
-            applicationMessage = new MqttApplicationMessageBuilder()
-                .WithTopic("samples/temperature/living_room")
-                .WithPayload("20.0")
-                .Build();
-            
-            await mqttClient.PublishAsync(applicationMessage, CancellationToken.None);
-
-            applicationMessage = new MqttApplicationMessageBuilder()
-                .WithTopic("samples/temperature/living_room")
-                .WithPayload("21.0")
-                .Build();
-            
-            await mqttClient.PublishAsync(applicationMessage, CancellationToken.None);
-
-            await mqttClient.DisconnectAsync();
-            
             Console.WriteLine("MQTT application message is published.");
         }
     }

Samples/Client/Client_Subscribe_Samples.cs

@@ -7,9 +7,7 @@
 // ReSharper disable InconsistentNaming
 
 using MQTTnet.Client;
-using MQTTnet.Packets;
+using MQTTnet.Samples.Shared;
-using MQTTnet.Protocol;
-using MQTTnet.Samples.Helpers;
 
 namespace MQTTnet.Samples.Client;
 
@@ -25,7 +23,9 @@ 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
@@ -41,11 +41,7 @@ public static class Client_Subscribe_Samples
             await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
 
             var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
-                .WithTopicFilter(
+                .WithTopicFilter(f => { f.WithTopic("mqttnet/samples/topic/2"); })
-                    f =>
-                    {
-                        f.WithTopic("mqttnet/samples/topic/2");
-                    })
                 .Build();
 
             await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);
@@ -57,95 +53,6 @@ public static class Client_Subscribe_Samples
         }
     }
 
-    public static async Task Send_Responses()
-    {
-        /*
-         * This sample subscribes to a topic and sends a response to the broker. This requires at least QoS level 1 to work!
-         */
-
-        var mqttFactory = new MqttFactory();
-
-        using (var mqttClient = mqttFactory.CreateMqttClient())
-        {
-            mqttClient.ApplicationMessageReceivedAsync += delegate(MqttApplicationMessageReceivedEventArgs args)
-            {
-                // Do some work with the message...
-
-                // 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();
-
-            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 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()
     {
         /*
@@ -156,16 +63,14 @@ 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();
 
             await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
 
             var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
-                .WithTopicFilter(
+                .WithTopicFilter(f => { f.WithTopic("mqttnet/samples/topic/1"); })
-                    f =>
-                    {
-                        f.WithTopic("mqttnet/samples/topic/1");
-                    })
                 .Build();
 
             var response = await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);

Samples/Diagnostics/Logger_Samples.cs

@@ -8,6 +8,7 @@
 
 using System.Text;
 using MQTTnet.Diagnostics;
+using MQTTnet.Samples.Shared;
 
 namespace MQTTnet.Samples.Diagnostics;
 
@@ -22,9 +23,7 @@ public static class Logger_Samples
 
         var mqttFactory = new MqttFactory(new MyLogger());
 
-        var mqttClientOptions = mqttFactory.CreateClientOptionsBuilder()
+        var mqttClientOptions = mqttFactory.CreateClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
-            .WithTcpServer("broker.hivemq.com")
-            .Build();
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
@@ -32,13 +31,31 @@ public static class Logger_Samples
 
             Console.WriteLine("MQTT client is connected.");
 
-            var mqttClientDisconnectOptions = mqttFactory.CreateClientDisconnectOptionsBuilder()
+            var mqttClientDisconnectOptions = mqttFactory.CreateClientDisconnectOptionsBuilder().Build();
-                .Build();
 
             await mqttClient.DisconnectAsync(mqttClientDisconnectOptions, CancellationToken.None);
         }
     }
 
+    public static async Task Log_To_Console()
+    {
+        /*
+       * This sample covers the creation of a custom logger which is used to log to the console.
+       */
+
+        var mqttFactory = new MqttFactory(new ConsoleLogger());
+
+        var mqttClientOptions = mqttFactory.CreateClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
+
+        using (var mqttClient = mqttFactory.CreateMqttClient())
+        {
+            await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
+
+            Console.WriteLine("MQTT client is connected. Press any key to exit.");
+            Console.ReadKey(true);
+        }
+    }
+
     public static async Task Use_Event_Logger()
     {
         /*
@@ -64,9 +81,7 @@ public static class Logger_Samples
 
         var mqttFactory = new MqttFactory(mqttEventLogger);
 
-        var mqttClientOptions = mqttFactory.CreateClientOptionsBuilder()
+        var mqttClientOptions = mqttFactory.CreateClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
-            .WithTcpServer("broker.hivemq.com")
-            .Build();
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
@@ -74,8 +89,7 @@ public static class Logger_Samples
 
             Console.WriteLine("MQTT client is connected.");
 
-            var mqttClientDisconnectOptions = mqttFactory.CreateClientDisconnectOptionsBuilder()
+            var mqttClientDisconnectOptions = mqttFactory.CreateClientDisconnectOptionsBuilder().Build();
-                .Build();
 
             await mqttClient.DisconnectAsync(mqttClientDisconnectOptions, CancellationToken.None);
         }
@@ -83,7 +97,7 @@ public static class Logger_Samples
 
     sealed class MyLogger : IMqttNetLogger
     {
-        public bool IsEnabled { get; set; } = true;
+        public bool IsEnabled { get; } = true;
 
         public void Publish(MqttNetLogLevel logLevel, string source, string message, object[] parameters, Exception exception)
         {

Samples/Program.cs

@@ -8,6 +8,7 @@ Console.WriteLine("Welcome to MQTTnet samples!");
 Console.WriteLine();
 
 var sampleClasses = Assembly.GetExecutingAssembly().GetExportedTypes().OrderBy(c => c.Name).ToList();
+sampleClasses.RemoveAll(c => c.Namespace.Contains(".Shared"));
 
 var index = 0;
 foreach (var sampleClass in sampleClasses)

Samples/Server/Server_Simple_Samples.cs

@@ -6,8 +6,8 @@
 // ReSharper disable UnusedMember.Global
 // ReSharper disable InconsistentNaming
 
-using MQTTnet.Diagnostics;
 using MQTTnet.Protocol;
+using MQTTnet.Samples.Shared;
 using MQTTnet.Server;
 
 namespace MQTTnet.Samples.Server;
@@ -170,50 +170,4 @@ public static class Server_Simple_Samples
         await server.StartAsync();
         return server;
     }
-
-    class ConsoleLogger : IMqttNetLogger
-    {
-        readonly object _consoleSyncRoot = new();
-
-        public bool IsEnabled => true;
-
-        public void Publish(MqttNetLogLevel logLevel, string source, string message, object[]? parameters, Exception? exception)
-        {
-            var foregroundColor = ConsoleColor.White;
-            switch (logLevel)
-            {
-                case MqttNetLogLevel.Verbose:
-                    foregroundColor = ConsoleColor.White;
-                    break;
-
-                case MqttNetLogLevel.Info:
-                    foregroundColor = ConsoleColor.Green;
-                    break;
-
-                case MqttNetLogLevel.Warning:
-                    foregroundColor = ConsoleColor.DarkYellow;
-                    break;
-
-                case MqttNetLogLevel.Error:
-                    foregroundColor = ConsoleColor.Red;
-                    break;
-            }
-
-            if (parameters?.Length > 0)
-            {
-                message = string.Format(message, parameters);
-            }
-
-            lock (_consoleSyncRoot)
-            {
-                Console.ForegroundColor = foregroundColor;
-                Console.WriteLine(message);
-
-                if (exception != null)
-                {
-                    Console.WriteLine(exception);
-                }
-            }
-        }
-    }
 }

Samples/Shared/ConsoleLogger.cs

@@ -0,0 +1,53 @@
+// 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 MQTTnet.Diagnostics;
+
+namespace MQTTnet.Samples.Shared;
+
+public sealed class ConsoleLogger : IMqttNetLogger
+{
+    readonly object _consoleSyncRoot = new();
+
+    public bool IsEnabled => true;
+
+    public void Publish(MqttNetLogLevel logLevel, string source, string message, object[]? parameters, Exception? exception)
+    {
+        var foregroundColor = ConsoleColor.White;
+        switch (logLevel)
+        {
+            case MqttNetLogLevel.Verbose:
+                foregroundColor = ConsoleColor.White;
+                break;
+
+            case MqttNetLogLevel.Info:
+                foregroundColor = ConsoleColor.Green;
+                break;
+
+            case MqttNetLogLevel.Warning:
+                foregroundColor = ConsoleColor.DarkYellow;
+                break;
+
+            case MqttNetLogLevel.Error:
+                foregroundColor = ConsoleColor.Red;
+                break;
+        }
+
+        if (parameters?.Length > 0)
+        {
+            message = string.Format(message, parameters);
+        }
+
+        lock (_consoleSyncRoot)
+        {
+            Console.ForegroundColor = foregroundColor;
+            Console.WriteLine(message);
+
+            if (exception != null)
+            {
+                Console.WriteLine(exception);
+            }
+        }
+    }
+}

Samples/Shared/ObjectExtensions.cs

@@ -4,7 +4,7 @@
 
 using System.Text.Json;
 
-namespace MQTTnet.Samples.Helpers;
+namespace MQTTnet.Samples.Shared;
 
 internal static class ObjectExtensions
 {

Source/MQTTnet.AspnetCore/MqttConnectionContext.cs

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

Source/MQTTnet.Benchmarks/AsyncLockBenchmark.cs

@@ -1,66 +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;
-using BenchmarkDotNet.Attributes;
-using BenchmarkDotNet.Jobs;
-using MQTTnet.Internal;
-
-namespace MQTTnet.Benchmarks
-{
-    [SimpleJob(RuntimeMoniker.Net60)]
-    [MemoryDiagnoser]
-    public class AsyncLockBenchmark : BaseBenchmark
-    {
-        [Benchmark]
-        public async Task Synchronize_100_Tasks()
-        {
-            const int tasksCount = 100;
-
-            var tasks = new Task[tasksCount];
-            var asyncLock = new AsyncLock();
-            var globalI = 0;
-
-            for (var i = 0; i < tasksCount; i++)
-            {
-                tasks[i] = Task.Run(
-                    async () =>
-                    {
-                        using (await asyncLock.EnterAsync().ConfigureAwait(false))
-                        {
-                            var localI = globalI;
-                            await Task.Delay(5).ConfigureAwait(false); // Increase the chance for wrong data.
-                            localI++;
-                            globalI = localI;
-                        }
-                    });
-            }
-
-            await Task.WhenAll(tasks).ConfigureAwait(false);
-
-            if (globalI != tasksCount)
-            {
-                throw new Exception($"Code is broken ({globalI})!");
-            }
-        }
-        
-        [Benchmark]
-        public async Task Wait_100_000_Times()
-        {
-            var asyncLock = new AsyncLock();
-
-            using (var cancellationToken = new CancellationTokenSource())
-            {
-                for (var i = 0; i < 100000; i++)
-                {
-                    using (await asyncLock.EnterAsync(cancellationToken.Token).ConfigureAwait(false))
-                    {
-                    }
-                }
-            }
-        }
-    }
-}

Source/MQTTnet.Benchmarks/MQTTnet.Benchmarks.csproj

@@ -12,10 +12,14 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="BenchmarkDotNet" Version="0.13.2" />
+        <PackageReference Include="BenchmarkDotNet" Version="0.13.1" />
         <PackageReference Include="Microsoft.AspNetCore" Version="2.2.0" />
     </ItemGroup>
 
+    <ItemGroup Condition="'$(TargetFramework)' != 'net5.0'">
+        <PackageReference Include="System.IO.Pipelines" Version="4.7.2" />
+    </ItemGroup>
+
     <ItemGroup Condition="'$(TargetFramework)' == 'net5.0'">
         <PackageReference Include="System.IO.Pipelines" Version="5.0.0" />
     </ItemGroup>

Source/MQTTnet.Benchmarks/MqttTcpChannelBenchmark.cs

@@ -2,7 +2,6 @@
 // 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 BenchmarkDotNet.Attributes;
 using MQTTnet.Channel;
 using MQTTnet.Implementations;
@@ -71,8 +70,8 @@ namespace MQTTnet.Benchmarks
 
             while (read < expected)
             {
-                var readResult = await _clientChannel.ReadAsync(new byte[size], 0, size, CancellationToken.None).ConfigureAwait(false);
+                var readresult = await _clientChannel.ReadAsync(new byte[size], 0, size, CancellationToken.None).ConfigureAwait(false);
-                read += readResult;
+                read += readresult;
             }
         }
 
@@ -80,11 +79,9 @@ namespace MQTTnet.Benchmarks
         {
             await Task.Yield();
 
-            var buffer = new ArraySegment<byte>(new byte[size]);
-            
             for (var i = 0; i < iterations; i++)
             {
-                await _serverChannel.WriteAsync(buffer, true, CancellationToken.None).ConfigureAwait(false);
+                await _serverChannel.WriteAsync(new byte[size], 0, size, CancellationToken.None).ConfigureAwait(false);
             }
         }
     }

Source/MQTTnet.Benchmarks/Program.cs

@@ -29,7 +29,6 @@ namespace MQTTnet.Benchmarks
             Console.WriteLine("c = SubscribeBenchmark");
             Console.WriteLine("d = UnsubscribeBenchmark");
             Console.WriteLine("e = MessageDeliveryBenchmark");
-            Console.WriteLine("f = AsyncLockBenchmark");
 
             var pressedKey = Console.ReadKey(true);
             switch (pressedKey.KeyChar)
@@ -76,9 +75,6 @@ namespace MQTTnet.Benchmarks
                 case 'e':
                     BenchmarkRunner.Run<MessageDeliveryBenchmark>();
                     break;
-                case 'f':
-                    BenchmarkRunner.Run<AsyncLockBenchmark>();
-                    break;
             }
 
             Console.ReadLine();

Source/MQTTnet.Benchmarks/SerializerBenchmark.cs

@@ -104,7 +104,7 @@ namespace MQTTnet.Benchmarks
                 return Task.FromResult(count);
             }
 
-            public Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken)
+            public Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
             {
                 throw new NotSupportedException();
             }

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs

@@ -167,7 +167,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
             try
             {
-                using (await _messageQueueLock.EnterAsync().ConfigureAwait(false))
+                using (await _messageQueueLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
                 {
                     if (_messageQueue.Count >= Options.MaxPendingMessages)
                     {
@@ -296,7 +296,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
             _subscriptionsQueuedSignal.Release();
 
-            return CompletedTask.Instance;
+            return Task.FromResult(0);
         }
 
         public Task UnsubscribeAsync(ICollection<string> topics)
@@ -319,7 +319,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
             _subscriptionsQueuedSignal.Release();
 
-            return CompletedTask.Instance;
+            return Task.FromResult(0);
         }
 
         protected override void Dispose(bool disposing)
@@ -693,7 +693,7 @@ namespace MQTTnet.Extensions.ManagedClient
             {
                 await InternalClient.PublishAsync(message.ApplicationMessage).ConfigureAwait(false);
 
-                using (await _messageQueueLock.EnterAsync().ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
+                using (await _messageQueueLock.WaitAsync(CancellationToken.None).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 +724,7 @@ 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.EnterAsync().ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
+                    using (await _messageQueueLock.WaitAsync(CancellationToken.None).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.EnterAsync().ConfigureAwait(false))
+            using (await _messagesLock.WaitAsync(CancellationToken.None).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.EnterAsync().ConfigureAwait(false))
+            using (await _messagesLock.WaitAsync(CancellationToken.None).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.Internal;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Extensions.Rpc
 {
@@ -116,7 +116,7 @@ namespace MQTTnet.Extensions.Rpc
         {
             if (!_waitingCalls.TryRemove(eventArgs.ApplicationMessage.Topic, out var awaitable))
             {
-                return CompletedTask.Instance;
+                return PlatformAbstractionLayer.CompletedTask;
             }
 
 #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 CompletedTask.Instance;
+            return PlatformAbstractionLayer.CompletedTask;
         }
 
         public void Dispose()

Source/MQTTnet.Extensions.WebSocket4Net/WebSocket4NetMqttChannel.cs

@@ -14,7 +14,6 @@ using System.Threading.Tasks;
 using MQTTnet.Channel;
 using MQTTnet.Client;
 using MQTTnet.Exceptions;
-using MQTTnet.Internal;
 using SuperSocket.ClientEngine;
 using WebSocket4Net;
 
@@ -125,7 +124,7 @@ namespace MQTTnet.Extensions.WebSocket4Net
                 _webSocket.Close();
             }
             
-            return CompletedTask.Instance;
+            return Task.FromResult(0);
         }
 
         public Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
@@ -155,10 +154,10 @@ namespace MQTTnet.Extensions.WebSocket4Net
             return Task.FromResult(readBytes);
         }
 
-        public Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken)
+        public Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
         {
-            _webSocket.Send(buffer.Array, buffer.Offset, buffer.Count);
+            _webSocket.Send(buffer, offset, count);
-            return CompletedTask.Instance;
+            return Task.FromResult(0);
         }
 
         public void Dispose()

Source/MQTTnet.TestApp/AsyncLockTest.cs

@@ -1,28 +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.Threading;
-using System.Threading.Tasks;
-using MQTTnet.Internal;
-
-namespace MQTTnet.TestApp
-{
-    public sealed class AsyncLockTest
-    {
-        public async Task Run()
-        {
-            var asyncLock = new AsyncLock();
-
-            using (var cancellationToken = new CancellationTokenSource())
-            {
-                for (var i = 0; i < 100000; i++)
-                {
-                    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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.TestApp
@@ -40,7 +40,7 @@ namespace MQTTnet.TestApp
                     Console.WriteLine($"+ Retain = {e.ApplicationMessage.Retain}");
                     Console.WriteLine();
                     
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.TestApp
@@ -42,7 +42,7 @@ namespace MQTTnet.TestApp
                 managedClient.ApplicationMessageReceivedAsync += e =>
                 {
                     Console.WriteLine(">> RECEIVED: " + e.ApplicationMessage.Topic);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await managedClient.StartAsync(options);
@@ -85,7 +85,7 @@ namespace MQTTnet.TestApp
             public Task SaveQueuedMessagesAsync(IList<ManagedMqttApplicationMessage> messages)
             {
                 File.WriteAllText(Filename, JsonConvert.SerializeObject(messages));
-                return CompletedTask.Instance;
+                return Task.FromResult(0);
             }
 
             public Task<IList<ManagedMqttApplicationMessage>> LoadQueuedMessagesAsync()

Source/MQTTnet.TestApp/MessageThroughputTest.cs

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

Source/MQTTnet.TestApp/Program.cs

@@ -28,7 +28,6 @@ namespace MQTTnet.TestApp
             Console.WriteLine("c = Start QoS 0 benchmark");
             Console.WriteLine("d = Start server with logging");
             Console.WriteLine("e = Start Message Throughput Test");
-            Console.WriteLine("f = Start AsyncLock Test");
 
             var pressedKey = Console.ReadKey(true);
             if (pressedKey.KeyChar == '1')
@@ -89,10 +88,6 @@ namespace MQTTnet.TestApp
             {
                 Task.Run(new MessageThroughputTest().Run);
             }
-            else if (pressedKey.KeyChar == 'f')
-            {
-                Task.Run(new AsyncLockTest().Run);
-            }
 
             Thread.Sleep(Timeout.Infinite);
         }

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

Source/MQTTnet.TestApp/ServerTest.cs

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

Source/MQTTnet.Tests/Client/ManagedMqttClient_Tests.cs

@@ -11,10 +11,12 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Diagnostics;
 using MQTTnet.Extensions.ManagedClient;
+using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 using MQTTnet.Tests.Mockups;
+using MqttClient = MQTTnet.Client.MqttClient;
 
 namespace MQTTnet.Tests.Client
 {
@@ -37,7 +39,7 @@ namespace MQTTnet.Tests.Client
                 managedClient.ConnectingFailedAsync += args =>
                 {
                     connectingFailedEventArgs = args;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await managedClient.StartAsync(
@@ -115,7 +117,7 @@ namespace MQTTnet.Tests.Client
                 receivingClient.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await receivingClient.SubscribeAsync("My/last/will");
@@ -279,7 +281,7 @@ namespace MQTTnet.Tests.Client
                     });
 
                 // Wait a bit for the retained message to be available
-                await LongTestDelay();
+                await Task.Delay(1000);
 
                 await sendingClient.DisconnectAsync();
 
@@ -288,29 +290,28 @@ 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 CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await managedClient.SubscribeAsync("topic");
 
                 await managedClient.StartAsync(new ManagedMqttClientOptionsBuilder().WithClientOptions(clientOptions).WithAutoReconnectDelay(TimeSpan.FromSeconds(1)).Build());
 
-                await LongTestDelay();
+                await Task.Delay(1000);
 
                 Assert.AreEqual(1, receivedManagedMessages.Count);
 
                 await managedClient.StopAsync();
 
-                await LongTestDelay();
+                await Task.Delay(500);
 
                 await managedClient.StartAsync(new ManagedMqttClientOptionsBuilder().WithClientOptions(clientOptions).WithAutoReconnectDelay(TimeSpan.FromSeconds(1)).Build());
 
-                await LongTestDelay();
+                await Task.Delay(1000);
 
                 // After reconnect and then some delay, the retained message must not be received,
                 // showing that the subscriptions were cleared
@@ -318,9 +319,7 @@ 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);
             }
         }
@@ -429,7 +428,7 @@ namespace MQTTnet.Tests.Client
             managedClient.ConnectedAsync += e =>
             {
                 connected.TrySetResult(true);
-                return CompletedTask.Instance;
+                return PlatformAbstractionLayer.CompletedTask;
             };
 
             return connected.Task;
@@ -458,7 +457,7 @@ namespace MQTTnet.Tests.Client
                     result.TrySetResult(receivedMessages);
                 }
 
-                return CompletedTask.Instance;
+                return PlatformAbstractionLayer.CompletedTask;
             };
 
             return result.Task;
@@ -487,7 +486,7 @@ namespace MQTTnet.Tests.Client
         public Task SaveQueuedMessagesAsync(IList<ManagedMqttApplicationMessage> messages)
         {
             _messages = messages;
-            return CompletedTask.Instance;
+            return Task.FromResult(0);
         }
     }
 }

Source/MQTTnet.Tests/Client/MqttClient_Tests.cs

@@ -14,17 +14,15 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
-using MQTTnet.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Tests.Mockups;
 
-// ReSharper disable InconsistentNaming
-
 namespace MQTTnet.Tests.Client
 {
     [TestClass]
-    public sealed class MqttClient_Tests : BaseTestClass
+    public sealed class Client_Tests : BaseTestClass
     {
         [DataTestMethod]
         [DataRow(MqttQualityOfServiceLevel.ExactlyOnce)]
@@ -58,7 +56,7 @@ namespace MQTTnet.Tests.Client
                     }
 
                     _ = InvokeInternal();
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var publishes = Task.WhenAll(publisher.PublishStringAsync("a", null, qos), publisher.PublishStringAsync("b", null, qos));
@@ -109,7 +107,7 @@ namespace MQTTnet.Tests.Client
                     client.DisconnectedAsync += args =>
                     {
                         disconnectHandlerCalled = true;
-                        return CompletedTask.Instance;
+                        return PlatformAbstractionLayer.CompletedTask;
                     };
 
                     await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("1.2.3.4").Build());
@@ -123,7 +121,7 @@ namespace MQTTnet.Tests.Client
                     //Assert.IsInstanceOfType(exception.InnerException, typeof(SocketException));
                 }
 
-                await LongTestDelay(); // disconnected handler is called async
+                await Task.Delay(100); // disconnected handler is called async
                 Assert.IsTrue(disconnectHandlerCalled);
             }
         }
@@ -138,7 +136,7 @@ namespace MQTTnet.Tests.Client
                 client.DisconnectedAsync += e =>
                 {
                     ex = e.Exception;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 try
@@ -147,7 +145,6 @@ namespace MQTTnet.Tests.Client
                 }
                 catch
                 {
-                    // Ignore errors.
                 }
 
                 await Task.Delay(500);
@@ -170,7 +167,7 @@ namespace MQTTnet.Tests.Client
                 server.InterceptingPublishAsync += c =>
                 {
                     i++;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client.SendAsync(
@@ -211,7 +208,7 @@ namespace MQTTnet.Tests.Client
                 client.DisconnectedAsync += e =>
                 {
                     handlerFired = true;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await server.StopAsync();
@@ -242,10 +239,10 @@ namespace MQTTnet.Tests.Client
 
                 for (var i = 0; i < 98; i++)
                 {
-                    Assert.IsFalse(clients[i].TryPingAsync().GetAwaiter().GetResult(), $"clients[{i}] is not connected");
+                    Assert.IsFalse(clients[i].IsConnected, $"clients[{i}] is not connected");
                 }
 
-                Assert.IsTrue(clients[99].TryPingAsync().GetAwaiter().GetResult());
+                Assert.IsTrue(clients[99].IsConnected);
 
                 Assert.AreEqual(1, clientStatus.Count);
                 Assert.AreEqual(1, sessionStatus.Count);
@@ -255,7 +252,7 @@ namespace MQTTnet.Tests.Client
                 receiveClient.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedPayload = e.ApplicationMessage.ConvertPayloadToString();
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await receiveClient.SubscribeAsync("x");
@@ -313,7 +310,7 @@ namespace MQTTnet.Tests.Client
                 receiver.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await sender.PublishAsync(message.Build(), CancellationToken.None);
@@ -335,7 +332,7 @@ namespace MQTTnet.Tests.Client
 
                 var client = await testEnvironment.ConnectClient();
 
-                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
+                Assert.IsTrue(client.IsConnected);
             }
         }
 
@@ -346,11 +343,11 @@ namespace MQTTnet.Tests.Client
             {
                 await testEnvironment.StartServer();
                 var client = await testEnvironment.ConnectClient();
-                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
+                Assert.IsTrue(client.IsConnected);
 
                 await client.DisconnectAsync();
 
-                Assert.IsFalse(client.TryPingAsync().GetAwaiter().GetResult());
+                Assert.IsFalse(client.IsConnected);
             }
         }
 
@@ -456,7 +453,7 @@ namespace MQTTnet.Tests.Client
                         receivedValues.Add(value);
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 }
 
                 client1.ApplicationMessageReceivedAsync += Handler1;
@@ -534,7 +531,7 @@ namespace MQTTnet.Tests.Client
                 client2.ApplicationMessageReceivedAsync += e =>
                 {
                     client2TopicResults.Add(Encoding.UTF8.GetString(e.ApplicationMessage.Payload));
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client2.SubscribeAsync(client2Topic);
@@ -569,7 +566,7 @@ namespace MQTTnet.Tests.Client
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client1.SubscribeAsync("a");
@@ -594,17 +591,17 @@ namespace MQTTnet.Tests.Client
                 var client = await testEnvironment.ConnectClient();
 
                 await Task.Delay(500);
-                Assert.IsTrue(await client.TryPingAsync());
+                Assert.IsTrue(client.IsConnected);
 
                 await server.StopAsync();
                 await Task.Delay(500);
-                Assert.IsFalse(await client.TryPingAsync());
+                Assert.IsFalse(client.IsConnected);
 
                 await server.StartAsync();
                 await Task.Delay(500);
 
                 await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort).Build());
-                Assert.IsTrue(await client.TryPingAsync());
+                Assert.IsTrue(client.IsConnected);
             }
         }
 
@@ -640,7 +637,6 @@ namespace MQTTnet.Tests.Client
                 }
                 catch
                 {
-                    // Ignore errors.
                 }
 
                 SpinWait.SpinUntil(() => tries >= maxTries, 10000);
@@ -660,11 +656,11 @@ namespace MQTTnet.Tests.Client
                 var client = await testEnvironment.ConnectClient();
 
                 await Task.Delay(500);
-                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
+                Assert.IsTrue(client.IsConnected);
 
                 await server.StopAsync();
                 await Task.Delay(500);
-                Assert.IsFalse(client.TryPingAsync().GetAwaiter().GetResult());
+                Assert.IsFalse(client.IsConnected);
 
                 for (var i = 0; i < 5; i++)
                 {
@@ -675,7 +671,6 @@ namespace MQTTnet.Tests.Client
                     }
                     catch
                     {
-                        // Ignore errors.
                     }
                 }
 
@@ -683,7 +678,7 @@ namespace MQTTnet.Tests.Client
                 await Task.Delay(500);
 
                 await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort).Build());
-                await client.PingAsync();
+                Assert.IsTrue(client.IsConnected);
             }
         }
 
@@ -728,7 +723,7 @@ namespace MQTTnet.Tests.Client
                         replies.Add(eventArgs.ApplicationMessage.Topic);
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 }
 
                 client2.ApplicationMessageReceivedAsync += Handler2;
@@ -766,7 +761,7 @@ namespace MQTTnet.Tests.Client
                         replyReceived = true;
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 client2.ApplicationMessageReceivedAsync += async e =>
@@ -817,7 +812,7 @@ namespace MQTTnet.Tests.Client
                         replyReceived = true;
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client.PublishStringAsync("request", null, MqttQualityOfServiceLevel.AtLeastOnce);
@@ -833,14 +828,14 @@ namespace MQTTnet.Tests.Client
         {
             using (var testEnvironment = new TestEnvironment(TestContext))
             {
-                await testEnvironment.StartServer();
+                var server = await testEnvironment.StartServer();
                 var client = await testEnvironment.ConnectClient();
 
-                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
+                Assert.IsTrue(client.IsConnected);
                 client.DisconnectedAsync += e =>
                 {
                     Assert.IsTrue(e.ClientWasConnected);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client.DisconnectAsync();
@@ -856,11 +851,11 @@ namespace MQTTnet.Tests.Client
                 var server = await testEnvironment.StartServer();
                 var client = await testEnvironment.ConnectClient();
 
-                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
+                Assert.IsTrue(client.IsConnected);
                 client.DisconnectedAsync += e =>
                 {
                     Assert.IsTrue(e.ClientWasConnected);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await server.StopAsync();
@@ -897,7 +892,7 @@ namespace MQTTnet.Tests.Client
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("localhost", testEnvironment.ServerPort).Build());
@@ -922,14 +917,14 @@ namespace MQTTnet.Tests.Client
                 client1.DisconnectedAsync += c =>
                 {
                     disconnectedFired = true;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var messageReceived = false;
                 client1.ApplicationMessageReceivedAsync += e =>
                 {
                     messageReceived = true;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("topic1").WithExactlyOnceQoS().Build());
@@ -946,7 +941,7 @@ namespace MQTTnet.Tests.Client
                 await Task.Delay(500);
 
                 Assert.IsTrue(messageReceived);
-                Assert.IsTrue(client1.TryPingAsync().GetAwaiter().GetResult());
+                Assert.IsTrue(client1.IsConnected);
                 Assert.IsFalse(disconnectedFired);
             }
         }

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.Internal;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Tests.Diagnostics
 {
@@ -33,7 +33,7 @@ namespace MQTTnet.Tests.Diagnostics
                     mqttClient.InspectPackage += eventArgs =>
                     {
                         packets.Add(eventArgs.Direction + ":" + Convert.ToBase64String(eventArgs.Buffer));
-                        return CompletedTask.Instance;
+                        return PlatformAbstractionLayer.CompletedTask;
                     };
                 
                     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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Server;
 
 namespace MQTTnet.Tests.Factory
@@ -56,7 +56,7 @@ namespace MQTTnet.Tests.Factory
                 managedClient.ConnectingFailedAsync += e =>
                 {
                     tcs.TrySetResult(null);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
                 
                 await Task.WhenAny(Task.Delay(managedClient.Options.ClientOptions.Timeout), tcs.Task);

Source/MQTTnet.Tests/Internal/AsyncEvent_Tests.cs

@@ -6,6 +6,7 @@ using System;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Implementations;
 using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Internal
@@ -15,28 +16,6 @@ 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()
         {
@@ -49,19 +28,6 @@ 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()
         {
@@ -75,62 +41,22 @@ namespace MQTTnet.Tests.Internal
             Assert.AreEqual(0, _testEventAsyncCount);
         }
 
-        [TestMethod]
-        public void Has_Handlers()
-        {
-            var testClass = new TestClass();
-            testClass.TestEventAsync += OnTestEventAsync;
-
-            Assert.AreEqual(true, testClass.HasTestHandlers);
-        }
-
-        [TestMethod]
-        public void No_Handlers()
-        {
-            var testClass = new TestClass();
-
-            Assert.AreEqual(false, testClass.HasTestHandlers);
-        }
-
-        [TestMethod]
-        public void Remove_Handlers()
-        {
-            var testClass = new TestClass();
-            testClass.TestEventAsync += OnTestEventAsync;
-            testClass.TestEventAsync -= OnTestEventAsync;
-
-            Assert.AreEqual(false, testClass.HasTestHandlers);
-        }
-
-        void OnTestEvent(EventArgs arg)
-        {
-            Interlocked.Increment(ref _testEventAsyncCount);
-        }
-
         Task OnTestEventAsync(EventArgs arg)
         {
             Interlocked.Increment(ref _testEventAsyncCount);
-            return CompletedTask.Instance;
+            return PlatformAbstractionLayer.CompletedTask;
         }
 
         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);
                 remove => _testEvent.RemoveHandler(value);
             }
 
-            public bool HasTestHandlers => _testEvent.HasHandlers;
-
             public Task FireEventAsync(EventArgs eventArgs)
             {
                 return _testEvent.InvokeAsync(eventArgs);

Source/MQTTnet.Tests/Internal/AsyncLock_Tests.cs

@@ -3,7 +3,6 @@
 // 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;
@@ -12,186 +11,8 @@ using MQTTnet.Internal;
 namespace MQTTnet.Tests.Internal
 {
     [TestClass]
-    public sealed class AsyncLock_Tests
+    public class AsyncLock_Tests
     {
-        [TestMethod]
-        public async Task Cancellation_Of_Awaiter()
-        {
-            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 < taskCount; i++)
-            {
-#pragma warning disable CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
-                tasks[i] = Task.Run(
-                    async () =>
-                    {
-                        using (await @lock.EnterAsync())
-                        {
-                            var localI = globalI;
-                            await Task.Delay(5); // Increase the chance for wrong data.
-                            localI++;
-                            globalI = localI;
-                        }
-                    });
-#pragma warning restore CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
-            }
-
-            Task.WaitAll(tasks);
-            Assert.AreEqual(taskCount, globalI);
-        }
-
-        [TestMethod]
-        public void Lock_10_Parallel_Tasks_With_Dispose_Doesnt_Lockup()
-        {
-            const int ThreadsCount = 10;
-
-            var threads = new Task[ThreadsCount];
-            var @lock = new AsyncLock();
-            var globalI = 0;
-            for (var i = 0; i < ThreadsCount; 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(
-                        async () =>
-                        {
-                            using (await @lock.EnterAsync())
-                            {
-                                var localI = globalI;
-                                await Task.Delay(10); // Increase the chance for wrong data.
-                                localI++;
-                                globalI = localI;
-                            }
-                        })
-                    .ContinueWith(
-                        x =>
-                        {
-                            if (globalI == 5)
-                            {
-                                @lock.Dispose();
-                                @lock = new AsyncLock();
-                            }
-
-                            if (x.Exception != null)
-                            {
-                                Debug.WriteLine(x.Exception.GetBaseException().GetType().Name);
-                            }
-                        });
-#pragma warning restore CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
-            }
-
-            Task.WaitAll(threads);
-
-            // Expect only 6 because the others are failing due to disposal (if (globalI == 5)).
-            Assert.AreEqual(6, globalI);
-        }
-
         [TestMethod]
         public async Task Lock_Serial_Calls()
         {
@@ -200,7 +21,7 @@ namespace MQTTnet.Tests.Internal
             var @lock = new AsyncLock();
             for (var i = 0; i < 100; i++)
             {
-                using (await @lock.EnterAsync().ConfigureAwait(false))
+                using (await @lock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
                 {
                     sum++;
                 }
@@ -216,76 +37,66 @@ namespace MQTTnet.Tests.Internal
             var @lock = new AsyncLock();
 
             // This call will never "release" the lock due to missing _using_.
-            await @lock.EnterAsync().ConfigureAwait(false);
+            await @lock.WaitAsync(CancellationToken.None).ConfigureAwait(false);
 
             using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
             {
-                await @lock.EnterAsync(cts.Token).ConfigureAwait(false);
+                await @lock.WaitAsync(cts.Token).ConfigureAwait(false);
             }
         }
 
-        [TestMethod]
+        //[TestMethod]
-        public async Task Test_Cancellation_With_Later_Access()
+        //public async Task Test_Cancellation_With_Later_Access()
-        {
+        //{
-            var asyncLock = new AsyncLock();
+        //    var @lock = new AsyncLock();
 
-            var releaser = await asyncLock.EnterAsync().ConfigureAwait(false);
+        //    var releaser = await @lock.WaitAsync().ConfigureAwait(false);
 
-            try
+        //    try
-            {
+        //    {
-                using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
+        //        using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
-                {
+        //        {
-                    await asyncLock.EnterAsync(timeout.Token).ConfigureAwait(false);
+        //            await @lock.WaitAsync(cts.Token).ConfigureAwait(false);
-                }
+        //        }
+        //    }
+        //    catch (OperationCanceledException)
+        //    {
+        //    }
             
-                Assert.Fail("Exception should be thrown!");
+        //    releaser.Dispose();
-            }
-            catch (OperationCanceledException)
-            {
-            }
 
-            releaser.Dispose();
+        //    using (await @lock.WaitAsync().ConfigureAwait(false))
-
+        //    {
-            using (await asyncLock.EnterAsync(CancellationToken.None).ConfigureAwait(false))
+        //        // When the method finished, the thread got access.
-            {
+        //    }
-                // When the method finished, the thread got access.
+        //}
-            }
-        }
 
         [TestMethod]
-        public async Task Use_After_Cancellation()
+        public void Lock_10_Parallel_Tasks()
         {
+            const int ThreadsCount = 10;
+
+            var threads = new Task[ThreadsCount];
             var @lock = new AsyncLock();
-
+            var globalI = 0;
-            // This call will not yet "release" the lock due to missing _using_.
+            for (var i = 0; i < ThreadsCount; i++)
-            var releaser = await @lock.EnterAsync().ConfigureAwait(false);
-
-            try
             {
-                using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
+#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(async () =>
                 {
-                    await @lock.EnterAsync(cts.Token).ConfigureAwait(false);
+                    using (var releaser = await @lock.WaitAsync(CancellationToken.None))
-                }
+                    {
-            }
+                        var localI = globalI;
-            catch (OperationCanceledException)
+                        await Task.Delay(10); // Increase the chance for wrong data.
-            {
+                        localI++;
-                // Expected exception!
+                        globalI = localI;
+                    }
+                });
+#pragma warning restore CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
             }
 
-            releaser.Dispose();
+            Task.WaitAll(threads);
-
+            Assert.AreEqual(ThreadsCount, globalI);
-            // 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/MQTTnet.Tests.csproj

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

Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs

@@ -8,9 +8,10 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using MQTTnet.Tests.Mockups;
 
 namespace MQTTnet.Tests.MQTTv5
 {
@@ -42,7 +43,7 @@ namespace MQTTnet.Tests.MQTTv5
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client.PublishAsync(new MqttApplicationMessageBuilder()
@@ -244,7 +245,7 @@ namespace MQTTnet.Tests.MQTTv5
                 receiver.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests.MQTTv5
@@ -31,7 +31,7 @@ namespace MQTTnet.Tests.MQTTv5
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
                 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
@@ -171,53 +171,11 @@ namespace MQTTnet.Tests.MQTTv5
                 // Session should be present
 
                 Assert.IsTrue(!result3.IsSessionPresent, "Session is present when it should not");
+
             }
         }
 
-        [TestMethod]
+        MqttClientOptions CreateClientOptions(TestEnvironment testEnvironment, string clientId, bool cleanSession, uint sessionExpiryInterval)
-        public async Task Disconnect_with_Reason()
-        {
-            using (var testEnvironment = CreateTestEnvironment())
-            {
-                var disconnectReason = MqttClientDisconnectReason.UnspecifiedError;
-
-                string testClientId = null;
-
-                await testEnvironment.StartServer();
-
-                testEnvironment.Server.ClientConnectedAsync += e =>
-                {
-                    testClientId = e.ClientId;
-                    return CompletedTask.Instance;
-                };
-
-                var client = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500));
-
-                client.DisconnectedAsync += e =>
-                {
-                    disconnectReason = e.Reason;
-                    return CompletedTask.Instance;
-                };
-
-                await LongTestDelay();
-
-                // Test client should be connected now
-
-                Assert.IsTrue(testClientId != null);
-
-                // Have the server disconnect the client with AdministrativeAction reason
-
-                await testEnvironment.Server.DisconnectClientAsync(testClientId, MqttDisconnectReasonCode.AdministrativeAction);
-
-                await LongTestDelay();
-
-                // The reason should be returned to the client in the DISCONNECT packet
-
-                Assert.AreEqual(MqttClientDisconnectReason.AdministrativeAction, disconnectReason);
-            }
-        }
-
-        static MqttClientOptions CreateClientOptions(TestEnvironment testEnvironment, string clientId, bool cleanSession, uint sessionExpiryInterval)
         {
             return testEnvironment.Factory.CreateClientOptionsBuilder()
                 .WithProtocolVersion(MqttProtocolVersion.V500)

Source/MQTTnet.Tests/Mockups/MemoryMqttChannel.cs

@@ -2,13 +2,11 @@
 // 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.IO;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Channel;
-using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Mockups
 {
@@ -34,12 +32,12 @@ namespace MQTTnet.Tests.Mockups
 
         public Task ConnectAsync(CancellationToken cancellationToken)
         {
-            return CompletedTask.Instance;
+            return Task.FromResult(0);
         }
 
         public Task DisconnectAsync(CancellationToken cancellationToken)
         {
-            return CompletedTask.Instance;
+            return Task.FromResult(0);
         }
 
         public Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
@@ -47,9 +45,9 @@ namespace MQTTnet.Tests.Mockups
             return _stream.ReadAsync(buffer, offset, count, cancellationToken);
         }
 
-        public Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken)
+        public Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
         {
-            return _stream.WriteAsync(buffer.Array, buffer.Offset, buffer.Count, cancellationToken);
+            return _stream.WriteAsync(buffer, offset, count, cancellationToken);
         }
 
         public void Dispose()

Source/MQTTnet.Tests/Mockups/TestApplicationMessageReceivedHandler.cs

@@ -7,7 +7,7 @@ using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
-using MQTTnet.Internal;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Tests.Mockups
 {
@@ -46,7 +46,7 @@ namespace MQTTnet.Tests.Mockups
                 _receivedEventArgs.Add(eventArgs);
             }
 
-            return CompletedTask.Instance;
+            return PlatformAbstractionLayer.CompletedTask;
         }
     }
 }

Source/MQTTnet.Tests/Mockups/TestEnvironment.cs

@@ -13,7 +13,7 @@ using MQTTnet.Client;
 using MQTTnet.Diagnostics;
 using MQTTnet.Extensions.Rpc;
 using MQTTnet.Formatter;
-using MQTTnet.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.LowLevelClient;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
@@ -220,7 +220,7 @@ namespace MQTTnet.Tests.Mockups
                         }
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 return client;
@@ -261,7 +261,7 @@ namespace MQTTnet.Tests.Mockups
                     }
                 }
 
-                return CompletedTask.Instance;
+                return PlatformAbstractionLayer.CompletedTask;
             };
 
             return Server;

Source/MQTTnet.Tests/RPC_Tests.cs

@@ -13,7 +13,6 @@ using MQTTnet.Extensions.Rpc;
 using MQTTnet.Protocol;
 using MQTTnet.Formatter;
 using MQTTnet.Implementations;
-using MQTTnet.Internal;
 
 namespace MQTTnet.Tests
 {
@@ -190,7 +189,7 @@ namespace MQTTnet.Tests
                 responseSender.ApplicationMessageReceivedAsync += async e =>
                 {
                     Assert.IsNull(e.ApplicationMessage.ResponseTopic);
-                    await CompletedTask.Instance;
+                    await PlatformAbstractionLayer.CompletedTask;
                 };
 
                 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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Tests.Mockups;
 
 namespace MQTTnet.Tests
@@ -33,7 +33,7 @@ namespace MQTTnet.Tests
                 receiverClient.ApplicationMessageReceivedAsync += e => 
                 {
                     response?.TrySetResult(e.ApplicationMessage.ConvertPayloadToString());
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests.Server
@@ -47,20 +47,20 @@ namespace MQTTnet.Tests.Server
                         e.ReasonCode = MqttConnectReasonCode.Success;
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
                 
                 testEnvironment.Server.ClientConnectedAsync += args =>
                 {
                     serverConnectedClientId = args.ClientId;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 testEnvironment.Server.ClientDisconnectedAsync += args =>
                 {
                     serverDisconnectedClientId = args.ClientId;
                     disconnectedMre.Set();
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 // Arrange client
@@ -68,7 +68,7 @@ namespace MQTTnet.Tests.Server
                 client.ConnectedAsync += args =>
                 {
                     clientAssignedClientId = args.ConnectResult.AssignedClientIdentifier;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 // 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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 
@@ -27,7 +27,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientConnectedAsync += e =>
                 {
                     eventArgs = e;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await testEnvironment.ConnectClient(o => o.WithCredentials("TheUser"));
@@ -54,7 +54,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientDisconnectedAsync += e =>
                 {
                     eventArgs = e;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client = await testEnvironment.ConnectClient(o => o.WithCredentials("TheUser"));
@@ -81,7 +81,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientSubscribedTopicAsync += e =>
                 {
                     eventArgs = e;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -108,7 +108,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientUnsubscribedTopicAsync += e =>
                 {
                     eventArgs = e;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
         
                 var client = await testEnvironment.ConnectClient();
@@ -134,7 +134,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     eventArgs = e;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
         
                 var client = await testEnvironment.ConnectClient();
@@ -161,7 +161,7 @@ namespace MQTTnet.Tests.Server
                 server.StartedAsync += e =>
                 {
                     eventArgs = e;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
                 
                 await server.StartAsync();
@@ -183,7 +183,7 @@ namespace MQTTnet.Tests.Server
                 server.StoppedAsync += e =>
                 {
                     eventArgs = e;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await server.StopAsync();

Source/MQTTnet.Tests/Server/General.cs

@@ -11,7 +11,8 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Adapter;
 using MQTTnet.Client;
-using MQTTnet.Internal;
+using MQTTnet.Formatter;
+using MQTTnet.Implementations;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
@@ -98,7 +99,7 @@ namespace MQTTnet.Tests.Server
                 server.ValidatingConnectionAsync += e =>
                 {
                     e.ReasonCode = MqttConnectReasonCode.NotAuthorized;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var connectingFailedException = await Assert.ThrowsExceptionAsync<MqttConnectingFailedException>(() => testEnvironment.ConnectClient());
@@ -121,7 +122,7 @@ namespace MQTTnet.Tests.Server
                         e.Response.ReasonCode = MqttSubscribeReasonCode.UnspecifiedError;
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 // Prepare some retained messages.
@@ -145,7 +146,7 @@ namespace MQTTnet.Tests.Server
                         buffer.Append(e.ApplicationMessage.Topic);
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client2.SubscribeAsync("y");
@@ -171,13 +172,13 @@ namespace MQTTnet.Tests.Server
                 server.ClientConnectedAsync += e =>
                 {
                     Interlocked.Increment(ref clientConnectedCalled);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 server.ClientDisconnectedAsync += e =>
                 {
                     Interlocked.Increment(ref clientDisconnectedCalled);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var c1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder());
@@ -224,7 +225,7 @@ namespace MQTTnet.Tests.Server
                                         // Clear retained message.
                                         await client.PublishAsync(
                                             new MqttApplicationMessageBuilder().WithTopic("r" + i2)
-                                                .WithPayload(EmptyBuffer.Array)
+                                                .WithPayload(PlatformAbstractionLayer.EmptyByteArray)
                                                 .WithRetainFlag()
                                                 .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce)
                                                 .Build());
@@ -275,7 +276,7 @@ namespace MQTTnet.Tests.Server
                 {
                     e.ApplicationMessage = new MqttApplicationMessage { Topic = "new_topic" };
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 string receivedTopic = null;
@@ -285,7 +286,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += a =>
                 {
                     receivedTopic = a.ApplicationMessage.Topic;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("original_topic").Build());
@@ -304,7 +305,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     e.ApplicationMessage.Payload = Encoding.ASCII.GetBytes("extended");
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var c1 = await testEnvironment.ConnectClient();
@@ -315,7 +316,7 @@ namespace MQTTnet.Tests.Server
                 c2.ApplicationMessageReceivedAsync += e =>
                 {
                     isIntercepted = string.Compare("extended", Encoding.UTF8.GetString(e.ApplicationMessage.Payload), StringComparison.Ordinal) == 0;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("test").Build());
@@ -338,7 +339,7 @@ namespace MQTTnet.Tests.Server
                 server.ApplicationMessageNotConsumedAsync += e =>
                 {
                     undeliverd = e.ApplicationMessage.Topic;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -375,7 +376,7 @@ namespace MQTTnet.Tests.Server
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await Task.Delay(500);
@@ -399,7 +400,7 @@ namespace MQTTnet.Tests.Server
                 s.RetainedMessageChangedAsync += e =>
                 {
                     savedRetainedMessages = e.StoredRetainedMessages;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var c1 = await testEnvironment.ConnectClient();
@@ -439,7 +440,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedTopic = e.ApplicationMessage.Topic;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.SubscribeAsync("#");
@@ -486,7 +487,7 @@ namespace MQTTnet.Tests.Server
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var message = new MqttApplicationMessageBuilder().WithTopic("a").WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce).Build();
@@ -520,13 +521,13 @@ namespace MQTTnet.Tests.Server
                 c2.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 c3.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c2.SubscribeAsync(new MqttTopicFilter { Topic = "a", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce }).ConfigureAwait(false);
@@ -580,7 +581,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("c");
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 server.ClientDisconnectedAsync += e =>
@@ -590,7 +591,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("d");
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var clientOptionsBuilder = new MqttClientOptionsBuilder().WithClientId(Guid.NewGuid().ToString());
@@ -620,7 +621,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("r");
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c2.SubscribeAsync("topic");
@@ -668,7 +669,7 @@ namespace MQTTnet.Tests.Server
                 server.ValidatingConnectionAsync += e =>
                 {
                     ConnectionValidationHandler(e);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var events = new List<string>();
@@ -680,7 +681,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("c");
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 server.ClientDisconnectedAsync += e =>
@@ -690,7 +691,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("d");
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var clientOptions = new MqttClientOptionsBuilder().WithClientId("same_id");
@@ -705,7 +706,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("x");
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 c1.ApplicationMessageReceivedAsync += e =>
@@ -715,7 +716,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("r");
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 c1.SubscribeAsync("topic").Wait();
@@ -784,7 +785,7 @@ namespace MQTTnet.Tests.Server
                 client1.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedBody = e.ApplicationMessage.Payload;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client1.SubscribeAsync("string");
@@ -821,7 +822,7 @@ namespace MQTTnet.Tests.Server
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await Task.Delay(500);
@@ -849,7 +850,7 @@ namespace MQTTnet.Tests.Server
                 c1.DisconnectedAsync += e =>
                 {
                     disconnectCalled++;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await Task.Delay(100);
@@ -935,61 +936,6 @@ namespace MQTTnet.Tests.Server
             }
         }
         
-        [TestMethod]
-        public async Task Disconnect_Client_with_Reason()
-        {
-            using (var testEnvironment = CreateTestEnvironment())
-            {
-                var disconnectPacketReceived = false;
-
-                string testClientId = null;
-
-                await testEnvironment.StartServer();
-
-                testEnvironment.Server.ClientConnectedAsync += e =>
-                {
-                    testClientId = e.ClientId;
-                    return CompletedTask.Instance;
-                };
-
-                var client = testEnvironment.CreateClient();
-
-                client.InspectPackage += e =>
-                {
-                    if (e.Buffer.Length > 0)
-                    {
-                        if (e.Buffer[0] == (byte)MqttControlPacketType.Disconnect << 4)
-                        {
-                            disconnectPacketReceived = true;
-                        }
-                    }
-                    return CompletedTask.Instance;
-                };
-
-                await client.ConnectAsync(
-                    new MqttClientOptionsBuilder().WithTcpServer("localhost", testEnvironment.ServerPort)
-                    .WithProtocolVersion(MQTTnet.Formatter.MqttProtocolVersion.V311)
-                    .Build()
-                    );
-
-                await LongTestDelay();
-
-                // Test client should be connected now
-
-                Assert.IsTrue(testClientId != null);
-
-                // Have the server disconnect the client with SessionTakenOver reason; no DISCONNECT should be sent to MQTT 3.1.1 clients
-
-                await testEnvironment.Server.DisconnectClientAsync(testClientId, Protocol.MqttDisconnectReasonCode.SessionTakenOver);
-
-                await LongTestDelay();
-
-                // MQTT 3.1.1 client should not receive a disconnect packet
-
-                Assert.IsFalse(disconnectPacketReceived, "Disconnect packet received when none is expected");
-            }
-        }
-
         void ConnectionValidationHandler(ValidatingConnectionEventArgs eventArgs)
         {
             if (_connected.ContainsKey(eventArgs.ClientId))

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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
@@ -26,7 +26,7 @@ namespace MQTTnet.Tests.Server
                     receiverClient.ApplicationMessageReceivedAsync += e =>
                     {
                         Interlocked.Increment(ref receivedMessages);
-                        return CompletedTask.Instance;
+                        return PlatformAbstractionLayer.CompletedTask;
                     };
 
                     await receiverClient.SubscribeAsync("t/+");
@@ -74,7 +74,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessages);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
                 
                 for (var i = 0; i < 100; i++)
@@ -137,7 +137,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessages);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 for (var i = 0; i < 100; i++)

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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 
@@ -26,7 +26,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientAcknowledgedPublishPacketAsync += args =>
                 {
                     eventArgs = args;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client1 = await testEnvironment.ConnectClient();
@@ -53,7 +53,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientAcknowledgedPublishPacketAsync += args =>
                 {
                     eventArgs = args;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client1 = await testEnvironment.ConnectClient();
@@ -88,7 +88,7 @@ namespace MQTTnet.Tests.Server
                         eventArgs.Add(args);
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client1 = await testEnvironment.ConnectClient();

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

Source/MQTTnet.Tests/Server/Server_Reference_Tests.cs

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

Source/MQTTnet.Tests/Server/Session_Tests.cs

@@ -10,8 +10,9 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Protocol;
+using MQTTnet.Server;
 using MQTTnet.Tests.Mockups;
 
 namespace MQTTnet.Tests.Server
@@ -28,7 +29,7 @@ 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
 
@@ -37,7 +38,7 @@ 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
@@ -56,7 +57,7 @@ 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();
 
 
@@ -82,7 +83,7 @@ namespace MQTTnet.Tests.Server
                 server.SessionDeletedAsync += e =>
                 {
                     deletedEventFired = true;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -110,7 +111,7 @@ namespace MQTTnet.Tests.Server
                     e.SessionItems["can_subscribe_x"] = true;
                     e.SessionItems["default_payload"] = "Hello World";
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await testEnvironment.ConnectClient();
@@ -122,12 +123,8 @@ namespace MQTTnet.Tests.Server
             }
         }
 
-        #if !NET452
-        
         [TestMethod]
-        [DataRow(MqttProtocolVersion.V311)]
+        public async Task Handle_Parallel_Connection_Attempts()
-        [DataRow(MqttProtocolVersion.V500)]
-        public async Task Handle_Parallel_Connection_Attempts(MqttProtocolVersion protocolVersion)
         {
             using (var testEnvironment = CreateTestEnvironment())
             {
@@ -135,7 +132,7 @@ namespace MQTTnet.Tests.Server
 
                 await testEnvironment.StartServer();
 
-                var options = new MqttClientOptionsBuilder().WithClientId("1").WithTimeout(TimeSpan.FromSeconds(1)).WithProtocolVersion(protocolVersion);
+                var options = new MqttClientOptionsBuilder().WithClientId("1").WithKeepAlivePeriod(TimeSpan.FromSeconds(5));
 
                 var hasReceive = false;
 
@@ -149,15 +146,12 @@ 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)));
-
+                var connectedClients = clients.Where(c => c?.IsConnected ?? false).ToList();
-                await Task.Delay(5000);
-                
-                var connectedClients = clients.Where(c => c?.TryPingAsync().GetAwaiter().GetResult() == true).ToList();
-
-                await LongTestDelay();
 
                 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");
@@ -167,7 +161,25 @@ namespace MQTTnet.Tests.Server
                 Assert.AreEqual(true, hasReceive);
             }
         }
-#endif
+
+        [TestMethod]
+        public async Task Manage_Session_MaxParallel()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                testEnvironment.IgnoreClientLogErrors = true;
+                var serverOptions = new MqttServerOptionsBuilder();
+                await testEnvironment.StartServer(serverOptions);
+
+                var options = new MqttClientOptionsBuilder().WithClientId("1");
+
+                var clients = await Task.WhenAll(Enumerable.Range(0, 10).Select(i => TryConnect(testEnvironment, options)));
+
+                var connectedClients = clients.Where(c => c?.IsConnected ?? false).ToList();
+
+                Assert.AreEqual(1, connectedClients.Count);
+            }
+        }
 
         [DataTestMethod]
         [DataRow(MqttQualityOfServiceLevel.ExactlyOnce)]
@@ -188,7 +200,7 @@ namespace MQTTnet.Tests.Server
                     c.AutoAcknowledge = false;
                     ++count;
                     Console.WriteLine("process");
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await subscriber.SubscribeAsync("#", qos);
@@ -225,7 +237,7 @@ namespace MQTTnet.Tests.Server
                 client1.DisconnectedAsync += c =>
                 {
                     disconnectReason = c.Reason;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client2 = await testEnvironment.ConnectClient(options);
@@ -251,7 +263,7 @@ namespace MQTTnet.Tests.Server
                     e.SessionItems["can_subscribe_x"] = true;
                     e.SessionItems["default_payload"] = "Hello World";
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 server.InterceptingSubscriptionAsync += e =>
@@ -264,13 +276,13 @@ namespace MQTTnet.Tests.Server
                         }
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 server.InterceptingPublishAsync += e =>
                 {
                     e.ApplicationMessage.Payload = Encoding.UTF8.GetBytes(e.SessionItems["default_payload"] as string ?? string.Empty);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 string receivedPayload = null;
@@ -279,7 +291,7 @@ namespace MQTTnet.Tests.Server
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedPayload = e.ApplicationMessage.ConvertPayloadToString();
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var subscribeResult = await client.SubscribeAsync("x");
@@ -329,7 +341,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
@@ -337,10 +349,10 @@ namespace MQTTnet.Tests.Server
                 // C2 has the last will defined.
                 var clientOptions = new MqttClientOptionsBuilder().WithWillTopic("My/last/will").WithClientId("WillOwner");
 
-                await testEnvironment.ConnectClient(clientOptions);
+                var c2 = await testEnvironment.ConnectClient(clientOptions);
 
                 // C3 will do the connection takeover.
-                await testEnvironment.ConnectClient(clientOptions);
+                var c3 = await testEnvironment.ConnectClient(clientOptions);
 
                 await Task.Delay(1000);
 
@@ -348,29 +360,41 @@ namespace MQTTnet.Tests.Server
             }
         }
 
-        static async Task<IMqttClient> ConnectAndSubscribe(TestEnvironment testEnvironment, MqttClientOptionsBuilder options, Action onReceive)
+        async Task<IMqttClient> ConnectAndSubscribe(TestEnvironment testEnvironment, MqttClientOptionsBuilder options, Action onReceive)
         {
             try
             {
-                var client = await testEnvironment.ConnectClient(options).ConfigureAwait(false);
+                var sendClient = await testEnvironment.ConnectClient(options);
 
-                client.ApplicationMessageReceivedAsync += e =>
+                sendClient.ApplicationMessageReceivedAsync += e =>
                 {
                     onReceive();
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
-                using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5)))
+                using (var subscribeTimeout = new CancellationTokenSource(TimeSpan.FromSeconds(5)))
                 {
-                    await client.SubscribeAsync("aaa", MqttQualityOfServiceLevel.AtMostOnce, timeout.Token).ConfigureAwait(false);
+                    await sendClient.SubscribeAsync("aaa", MqttQualityOfServiceLevel.AtMostOnce, subscribeTimeout.Token).ConfigureAwait(false);
                 }
 
-                return client;
+                return sendClient;
             }
             catch (Exception)
             {
                 return null;
             }
         }
+
+        async Task<IMqttClient> TryConnect(TestEnvironment testEnvironment, MqttClientOptionsBuilder options)
+        {
+            try
+            {
+                return await testEnvironment.ConnectClient(options);
+            }
+            catch
+            {
+                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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
@@ -33,7 +33,7 @@ namespace MQTTnet.Tests.Server
                         e.Response.ReasonCode = MqttSubscribeReasonCode.TopicFilterInvalid;
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -56,7 +56,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientSubscribedTopicAsync += e =>
                 {
                     server.InjectApplicationMessage(new InjectedMqttApplicationMessage(new MqttApplicationMessageBuilder().WithTopic("test_topic").Build()));
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -81,7 +81,7 @@ namespace MQTTnet.Tests.Server
                 {
                     // Set the topic to "a" regards what the client wants to subscribe.
                     e.TopicFilter.Topic = "a";
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var topicAReceived = false;
@@ -99,7 +99,7 @@ namespace MQTTnet.Tests.Server
                         topicBReceived = true;
                     }
 
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await client.SubscribeAsync("b");
@@ -146,7 +146,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 for (var i = 0; i < 500; i++)
@@ -189,7 +189,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var optionsBuilder = new MqttClientSubscribeOptionsBuilder();
@@ -229,7 +229,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.SubscribeAsync(new MqttClientSubscribeOptionsBuilder().WithTopicFilter("a").Build());
@@ -267,7 +267,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.SubscribeAsync(new MqttClientSubscribeOptionsBuilder().WithTopicFilter("a").WithTopicFilter("b").WithTopicFilter("c").Build());
@@ -301,7 +301,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 var c2 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("c2"));
@@ -316,7 +316,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientSubscribedTopicAsync += e =>
                 {
                     subscribeEventCalled = e.TopicFilter.Topic == "a" && e.ClientId == c1.Options.ClientId;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilter { Topic = "a", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce });
@@ -331,7 +331,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientUnsubscribedTopicAsync += e =>
                 {
                     unsubscribeEventCalled = e.TopicFilter == "a" && e.ClientId == c1.Options.ClientId;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.UnsubscribeAsync("a");

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.Internal;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Tests.Server
 {
@@ -50,7 +50,7 @@ namespace MQTTnet.Tests.Server
                         receivedTopics.Add(e.ApplicationMessage.Topic);
                     }
                     
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.SubscribeAsync("#");

Source/MQTTnet.Tests/Server/User_Properties_Tests.cs

@@ -9,7 +9,6 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
 using MQTTnet.Implementations;
-using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Tests.Mockups;
 
@@ -49,7 +48,7 @@ namespace MQTTnet.Tests.Server
                 receiver.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 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.Internal;
+using MQTTnet.Implementations;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests.Server
@@ -26,7 +26,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
@@ -53,7 +53,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return CompletedTask.Instance;
+                    return PlatformAbstractionLayer.CompletedTask;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -73,26 +73,7 @@ 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 cancelled.", cancellationToken);
-                    }
-
-                    // Make sure that the exception from the connect task gets thrown.
-                    await connectTask.ConfigureAwait(false);
-                }
             }
             catch (Exception exception)
             {
@@ -195,7 +176,7 @@ 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.EnterAsync(cancellationToken).ConfigureAwait(false))
+            using (await _syncRoot.WaitAsync(cancellationToken).ConfigureAwait(false))
             {
                 // Check for cancellation here again because "WaitAsync" might take some time.
                 cancellationToken.ThrowIfCancellationRequested();
@@ -207,14 +188,11 @@ namespace MQTTnet.Adapter
 
                     _logger.Verbose("TX ({0} bytes) >>> {1}", packetBuffer.Length, packet);
 
+                    await _channel.WriteAsync(packetBuffer.Packet.Array, packetBuffer.Packet.Offset, packetBuffer.Packet.Count, cancellationToken).ConfigureAwait(false);
+
                     if (packetBuffer.Payload.Count > 0)
                     {
-                        await _channel.WriteAsync(packetBuffer.Packet, false, cancellationToken).ConfigureAwait(false);
+                        await _channel.WriteAsync(packetBuffer.Payload.Array, packetBuffer.Payload.Offset, packetBuffer.Payload.Count, cancellationToken).ConfigureAwait(false);
-                        await _channel.WriteAsync(packetBuffer.Payload, true, cancellationToken).ConfigureAwait(false);
-                    }
-                    else
-                    {
-                        await _channel.WriteAsync(packetBuffer.Packet, true, cancellationToken).ConfigureAwait(false);
                     }
 
                     Interlocked.Add(ref _bytesReceived, packetBuffer.Length);
@@ -363,7 +341,7 @@ namespace MQTTnet.Adapter
                 var fixedHeader = readFixedHeaderResult.FixedHeader;
                 if (fixedHeader.RemainingLength == 0)
                 {
-                    return new ReceivedMqttPacket(fixedHeader.Flags, EmptyBuffer.ArraySegment, 2);
+                    return new ReceivedMqttPacket(fixedHeader.Flags, PlatformAbstractionLayer.EmptyByteArraySegment, 2);
                 }
 
                 var bodyLength = fixedHeader.RemainingLength;

Source/MQTTnet/Channel/IMqttChannel.cs

@@ -12,17 +12,13 @@ namespace MQTTnet.Channel
     public interface IMqttChannel : IDisposable
     {
         string Endpoint { get; }
-        
         bool IsSecureConnection { get; }
-        
         X509Certificate2 ClientCertificate { get; }
 
         Task ConnectAsync(CancellationToken cancellationToken);
-        
         Task DisconnectAsync(CancellationToken cancellationToken);
 
         Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken);
-        
+        Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken);
-        Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken);
     }
 }

Source/MQTTnet/Client/IMqttClient.cs

@@ -7,14 +7,14 @@ namespace MQTTnet.Client
 {
     public interface IMqttClient : IDisposable
     {
-        event Func<MqttApplicationMessageReceivedEventArgs, Task> ApplicationMessageReceivedAsync;
-        
         event Func<MqttClientConnectedEventArgs, Task> ConnectedAsync;
         
         event Func<MqttClientConnectingEventArgs, Task> ConnectingAsync;
         
         event Func<MqttClientDisconnectedEventArgs, Task> DisconnectedAsync;
         
+        event Func<MqttApplicationMessageReceivedEventArgs, Task> ApplicationMessageReceivedAsync;
+        
         event Func<InspectMqttPacketEventArgs, Task> InspectPackage;
         
         bool IsConnected { get; }
@@ -27,12 +27,12 @@ namespace MQTTnet.Client
         
         Task PingAsync(CancellationToken cancellationToken = default);
         
-        Task<MqttClientPublishResult> PublishAsync(MqttApplicationMessage applicationMessage, CancellationToken cancellationToken = default);
-
         Task SendExtendedAuthenticationExchangeDataAsync(MqttExtendedAuthenticationExchangeData data, CancellationToken cancellationToken = default);
         
         Task<MqttClientSubscribeResult> SubscribeAsync(MqttClientSubscribeOptions options, CancellationToken cancellationToken = default);
         
         Task<MqttClientUnsubscribeResult> UnsubscribeAsync(MqttClientUnsubscribeOptions options, CancellationToken cancellationToken = default);
+        
+        Task<MqttClientPublishResult> PublishAsync(MqttApplicationMessage applicationMessage, CancellationToken cancellationToken = default);
     }
 }

Source/MQTTnet/Client/MqttClient.cs

@@ -2,57 +2,59 @@
 // 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;
 using MQTTnet.Adapter;
-using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
-using MQTTnet.Formatter;
 using MQTTnet.Internal;
 using MQTTnet.PacketDispatcher;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using MQTTnet.Diagnostics;
+using MQTTnet.Formatter;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Client
 {
     public sealed class MqttClient : Disposable, IMqttClient
     {
-        readonly IMqttClientAdapterFactory _adapterFactory;
+        readonly MqttPacketFactories _packetFactories = new MqttPacketFactories();
-        readonly MqttApplicationMessageFactory _applicationMessageFactory = new MqttApplicationMessageFactory();
-        readonly AsyncEvent<MqttApplicationMessageReceivedEventArgs> _applicationMessageReceivedEvent = new AsyncEvent<MqttApplicationMessageReceivedEventArgs>();
         
         readonly MqttClientPublishResultFactory _clientPublishResultFactory = new MqttClientPublishResultFactory();
         readonly MqttClientSubscribeResultFactory _clientSubscribeResultFactory = new MqttClientSubscribeResultFactory();
         readonly MqttClientUnsubscribeResultFactory _clientUnsubscribeResultFactory = new MqttClientUnsubscribeResultFactory();
+        readonly MqttApplicationMessageFactory _applicationMessageFactory = new MqttApplicationMessageFactory();
+        
+        readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new MqttPacketIdentifierProvider();
+        readonly MqttPacketDispatcher _packetDispatcher = new MqttPacketDispatcher();
+        readonly object _disconnectLock = new object();
 
         readonly AsyncEvent<MqttClientConnectedEventArgs> _connectedEvent = new AsyncEvent<MqttClientConnectedEventArgs>();
         readonly AsyncEvent<MqttClientConnectingEventArgs> _connectingEvent = new AsyncEvent<MqttClientConnectingEventArgs>();
         readonly AsyncEvent<MqttClientDisconnectedEventArgs> _disconnectedEvent = new AsyncEvent<MqttClientDisconnectedEventArgs>();
-        readonly object _disconnectLock = new object();
+        readonly AsyncEvent<MqttApplicationMessageReceivedEventArgs> _applicationMessageReceivedEvent = new AsyncEvent<MqttApplicationMessageReceivedEventArgs>();
         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 IMqttClientAdapterFactory _adapterFactory;
         readonly IMqttNetLogger _rootLogger;
-
+        readonly MqttNetSourceLogger _logger;
-        IMqttChannelAdapter _adapter;
 
         CancellationTokenSource _backgroundCancellationTokenSource;
+        Task _packetReceiverTask;
+        Task _keepAlivePacketsSenderTask;
+        Task _publishPacketReceiverTask;
+
+        AsyncQueue<MqttPublishPacket> _publishPacketReceiverQueue;
+
+        IMqttChannelAdapter _adapter;
         bool _cleanDisconnectInitiated;
         volatile int _connectionStatus;
 
         MqttClientDisconnectReason _disconnectReason;
-        string _disconnectReasonString;
-        Task _keepAlivePacketsSenderTask;
 
         DateTime _lastPacketSentTimestamp;
-        Task _packetReceiverTask;
+        string _disconnectReasonString;
-
-        AsyncQueue<MqttPublishPacket> _publishPacketReceiverQueue;
-        Task _publishPacketReceiverTask;
         
         public MqttClient(IMqttClientAdapterFactory channelFactory, IMqttNetLogger logger)
         {
@@ -61,12 +63,6 @@ namespace MQTTnet.Client
             _logger = logger.WithSource(nameof(MqttClient));
         }
         
-        public event Func<MqttApplicationMessageReceivedEventArgs, Task> ApplicationMessageReceivedAsync
-        {
-            add => _applicationMessageReceivedEvent.AddHandler(value);
-            remove => _applicationMessageReceivedEvent.RemoveHandler(value);
-        }
-
         public event Func<MqttClientConnectedEventArgs, Task> ConnectedAsync
         {
             add => _connectedEvent.AddHandler(value);
@@ -85,6 +81,12 @@ namespace MQTTnet.Client
             remove => _disconnectedEvent.RemoveHandler(value);
         }
         
+        public event Func<MqttApplicationMessageReceivedEventArgs, Task> ApplicationMessageReceivedAsync
+        {
+            add => _applicationMessageReceivedEvent.AddHandler(value);
+            remove => _applicationMessageReceivedEvent.RemoveHandler(value);
+        }
+        
         public event Func<InspectMqttPacketEventArgs, Task> InspectPackage
         {
             add => _inspectPacketEvent.AddHandler(value);
@@ -97,15 +99,8 @@ namespace MQTTnet.Client
 
         public async Task<MqttClientConnectResult> ConnectAsync(MqttClientOptions options, CancellationToken cancellationToken = default)
         {
-            if (options == null)
+            if (options == null) throw new ArgumentNullException(nameof(options));
-            {
+            if (options.ChannelOptions == null) throw new ArgumentException("ChannelOptions are not set.");
-                throw new ArgumentNullException(nameof(options));
-            }
-
-            if (options.ChannelOptions == null)
-            {
-                throw new ArgumentException("ChannelOptions are not set.");
-            }
 
             ThrowIfConnected("It is not allowed to connect with a server after the connection is established.");
 
@@ -191,10 +186,7 @@ namespace MQTTnet.Client
 
         public async Task DisconnectAsync(MqttClientDisconnectOptions options, CancellationToken cancellationToken = default)
         {
-            if (options is null)
+            if (options is null) throw new ArgumentNullException(nameof(options));
-            {
-                throw new ArgumentNullException(nameof(options));
-            }
 
             ThrowIfDisposed();
 
@@ -221,19 +213,63 @@ namespace MQTTnet.Client
             }
         }
 
-        public async Task PingAsync(CancellationToken cancellationToken = default)
+        public Task PingAsync(CancellationToken cancellationToken = default)
         {
-            if (cancellationToken.CanBeCanceled)
+            return SendAndReceiveAsync<MqttPingRespPacket>(MqttPingReqPacket.Instance, cancellationToken);
+        }
+
+        public Task SendExtendedAuthenticationExchangeDataAsync(MqttExtendedAuthenticationExchangeData data, CancellationToken cancellationToken = default)
+        {
+            if (data == null) throw new ArgumentNullException(nameof(data));
+
+            ThrowIfDisposed();
+            ThrowIfNotConnected();
+
+            var authPacket = new MqttAuthPacket
             {
-                await SendAndReceiveAsync<MqttPingRespPacket>(MqttPingReqPacket.Instance, cancellationToken).ConfigureAwait(false);
+                // This must always be equal to the value from the CONNECT packet. So we use it here to ensure that.
-            }
+                AuthenticationMethod = Options.AuthenticationMethod,
-            else
+                AuthenticationData = data.AuthenticationData,
+                ReasonString = data.ReasonString,
+                UserProperties = data.UserProperties
+            };
+
+            return SendAsync(authPacket, cancellationToken);
+        }
+
+        public async Task<MqttClientSubscribeResult> SubscribeAsync(MqttClientSubscribeOptions options, CancellationToken cancellationToken = default)
+        {
+            if (options == null) throw new ArgumentNullException(nameof(options));
+
+            foreach (var topicFilter in options.TopicFilters)
             {
-                using (var timeout = new CancellationTokenSource(Options.Timeout))
+                MqttTopicValidator.ThrowIfInvalidSubscribe(topicFilter.Topic);
-                {
-                    await SendAndReceiveAsync<MqttPingRespPacket>(MqttPingReqPacket.Instance, timeout.Token).ConfigureAwait(false);
-                }
             }
+
+            ThrowIfDisposed();
+            ThrowIfNotConnected();
+
+            var subscribePacket = _packetFactories.Subscribe.Create(options);
+            subscribePacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
+
+            var subAckPacket = await SendAndReceiveAsync<MqttSubAckPacket>(subscribePacket, cancellationToken).ConfigureAwait(false);
+
+            return _clientSubscribeResultFactory.Create(subscribePacket, subAckPacket);
+        }
+
+        public async Task<MqttClientUnsubscribeResult> UnsubscribeAsync(MqttClientUnsubscribeOptions options, CancellationToken cancellationToken = default)
+        {
+            if (options == null) throw new ArgumentNullException(nameof(options));
+
+            ThrowIfDisposed();
+            ThrowIfNotConnected();
+            
+            var unsubscribePacket = _packetFactories.Unsubscribe.Create(options);
+            unsubscribePacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
+
+            var unsubAckPacket = await SendAndReceiveAsync<MqttUnsubAckPacket>(unsubscribePacket, cancellationToken).ConfigureAwait(false);
+
+            return _clientUnsubscribeResultFactory.Create(unsubscribePacket, unsubAckPacket);
         }
 
         public Task<MqttClientPublishResult> PublishAsync(MqttApplicationMessage applicationMessage, CancellationToken cancellationToken = default)
@@ -268,67 +304,22 @@ namespace MQTTnet.Client
             }
         }
 
-        public Task SendExtendedAuthenticationExchangeDataAsync(MqttExtendedAuthenticationExchangeData data, CancellationToken cancellationToken = default)
+        void Cleanup()
         {
-            if (data == null)
+            try
             {
-                throw new ArgumentNullException(nameof(data));
+                _backgroundCancellationTokenSource?.Cancel(false);
             }
-
+            finally
-            ThrowIfDisposed();
-            ThrowIfNotConnected();
-
-            var authPacket = new MqttAuthPacket
             {
-                // This must always be equal to the value from the CONNECT packet. So we use it here to ensure that.
+                _backgroundCancellationTokenSource?.Dispose();
-                AuthenticationMethod = Options.AuthenticationMethod,
+                _backgroundCancellationTokenSource = null;
-                AuthenticationData = data.AuthenticationData,
-                ReasonString = data.ReasonString,
-                UserProperties = data.UserProperties
-            };
 
-            return SendAsync(authPacket, cancellationToken);
+                _publishPacketReceiverQueue?.Dispose();
-        }
+                _publishPacketReceiverQueue = null;
 
-        public async Task<MqttClientSubscribeResult> SubscribeAsync(MqttClientSubscribeOptions options, CancellationToken cancellationToken = default)
+                _adapter?.Dispose();
-        {
-            if (options == null)
-            {
-                throw new ArgumentNullException(nameof(options));
             }
-
-            foreach (var topicFilter in options.TopicFilters)
-            {
-                MqttTopicValidator.ThrowIfInvalidSubscribe(topicFilter.Topic);
-            }
-
-            ThrowIfDisposed();
-            ThrowIfNotConnected();
-
-            var subscribePacket = _packetFactories.Subscribe.Create(options);
-            subscribePacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
-
-            var subAckPacket = await SendAndReceiveAsync<MqttSubAckPacket>(subscribePacket, cancellationToken).ConfigureAwait(false);
-
-            return _clientSubscribeResultFactory.Create(subscribePacket, subAckPacket);
-        }
-
-        public async Task<MqttClientUnsubscribeResult> UnsubscribeAsync(MqttClientUnsubscribeOptions options, CancellationToken cancellationToken = default)
-        {
-            if (options == null)
-            {
-                throw new ArgumentNullException(nameof(options));
-            }
-
-            ThrowIfDisposed();
-            ThrowIfNotConnected();
-
-            var unsubscribePacket = _packetFactories.Unsubscribe.Create(options);
-            unsubscribePacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
-
-            var unsubAckPacket = await SendAndReceiveAsync<MqttUnsubAckPacket>(unsubscribePacket, cancellationToken).ConfigureAwait(false);
-
-            return _clientUnsubscribeResultFactory.Create(unsubscribePacket, unsubAckPacket);
         }
 
         protected override void Dispose(bool disposing)
@@ -341,36 +332,6 @@ namespace MQTTnet.Client
             base.Dispose(disposing);
         }
 
-        Task AcknowledgeReceivedPublishPacket(MqttApplicationMessageReceivedEventArgs eventArgs, CancellationToken cancellationToken)
-        {
-            if (eventArgs.PublishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtMostOnce)
-            {
-                // no response required
-            }
-            else if (eventArgs.PublishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtLeastOnce)
-            {
-                if (!eventArgs.ProcessingFailed)
-                {
-                    var pubAckPacket = _packetFactories.PubAck.Create(eventArgs);
-                    return SendAsync(pubAckPacket, cancellationToken);
-                }
-            }
-            else if (eventArgs.PublishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.ExactlyOnce)
-            {
-                if (!eventArgs.ProcessingFailed)
-                {
-                    var pubRecPacket = _packetFactories.PubRec.Create(eventArgs);
-                    return SendAsync(pubRecPacket, cancellationToken);
-                }
-            }
-            else
-            {
-                throw new MqttProtocolViolationException("Received a not supported QoS level.");
-            }
-
-            return CompletedTask.Instance;
-        }
-
         async Task<MqttClientConnectResult> AuthenticateAsync(MqttClientOptions options, CancellationToken cancellationToken)
         {
             MqttClientConnectResult result;
@@ -399,27 +360,32 @@ namespace MQTTnet.Client
             return result;
         }
 
-        void Cleanup()
+        void ThrowIfNotConnected()
         {
-            try
+            if (!IsConnected)
             {
-                _backgroundCancellationTokenSource?.Cancel(false);
+                throw new MqttCommunicationException("The client is not connected.");
-            }
-            finally
-            {
-                _backgroundCancellationTokenSource?.Dispose();
-                _backgroundCancellationTokenSource = null;
-
-                _publishPacketReceiverQueue?.Dispose();
-                _publishPacketReceiverQueue = null;
-
-                _adapter?.Dispose();
             }
         }
 
-        MqttClientConnectionStatus CompareExchangeConnectionStatus(MqttClientConnectionStatus value, MqttClientConnectionStatus comparand)
+        void ThrowIfConnected(string message)
         {
-            return (MqttClientConnectionStatus)Interlocked.CompareExchange(ref _connectionStatus, (int)value, (int)comparand);
+            if (IsConnected)
+            {
+                throw new MqttProtocolViolationException(message);
+            }
+        }
+
+        Task DisconnectInternalAsync(Task sender, Exception exception, MqttClientConnectResult connectResult)
+        {
+            var clientWasConnected = IsConnected;
+            
+            if (!DisconnectIsPendingOrFinished())
+            {
+                return DisconnectCoreAsync(sender, exception, connectResult, clientWasConnected);
+            }
+            
+            return PlatformAbstractionLayer.CompletedTask;
         }
 
         async Task DisconnectCoreAsync(Task sender, Exception exception, MqttClientConnectResult connectResult, bool clientWasConnected)
@@ -480,166 +446,28 @@ namespace MQTTnet.Client
             }
         }
 
-        Task DisconnectInternalAsync(Task sender, Exception exception, MqttClientConnectResult connectResult)
+        void TryInitiateDisconnect()
         {
-            var clientWasConnected = IsConnected;
+            lock (_disconnectLock)
-
-            if (!DisconnectIsPendingOrFinished())
-            {
-                return DisconnectCoreAsync(sender, exception, connectResult, clientWasConnected);
-            }
-
-            return CompletedTask.Instance;
-        }
-
-        bool DisconnectIsPendingOrFinished()
-        {
-            var connectionStatus = (MqttClientConnectionStatus)_connectionStatus;
-
-            do
-            {
-                switch (connectionStatus)
-                {
-                    case MqttClientConnectionStatus.Disconnected:
-                    case MqttClientConnectionStatus.Disconnecting:
-                        return true;
-                    case MqttClientConnectionStatus.Connected:
-                    case MqttClientConnectionStatus.Connecting:
-                        // This will compare the _connectionStatus to old value and set it to "MqttClientConnectionStatus.Disconnecting" afterwards.
-                        // So the first caller will get a "false" and all subsequent ones will get "true".
-                        var curStatus = CompareExchangeConnectionStatus(MqttClientConnectionStatus.Disconnecting, connectionStatus);
-                        if (curStatus == connectionStatus)
-                        {
-                            return false;
-                        }
-
-                        connectionStatus = curStatus;
-                        break;
-                }
-            } while (true);
-        }
-
-        void EnqueueReceivedPublishPacket(MqttPublishPacket publishPacket)
-        {
-            try
-            {
-                _publishPacketReceiverQueue.Enqueue(publishPacket);
-            }
-            catch (Exception exception)
-            {
-                _logger.Error(exception, "Error while queueing application message.");
-            }
-        }
-
-        async Task<MqttApplicationMessageReceivedEventArgs> HandleReceivedApplicationMessageAsync(MqttPublishPacket publishPacket)
-        {
-            var applicationMessage = _applicationMessageFactory.Create(publishPacket);
-            var eventArgs = new MqttApplicationMessageReceivedEventArgs(Options.ClientId, applicationMessage, publishPacket, AcknowledgeReceivedPublishPacket);
-
-            await _applicationMessageReceivedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
-
-            return eventArgs;
-        }
-
-        Task ProcessReceivedAuthPacket(MqttAuthPacket authPacket)
-        {
-            var extendedAuthenticationExchangeHandler = Options.ExtendedAuthenticationExchangeHandler;
-            if (extendedAuthenticationExchangeHandler != null)
-            {
-                return extendedAuthenticationExchangeHandler.HandleRequestAsync(new MqttExtendedAuthenticationExchangeContext(authPacket, this));
-            }
-
-            return CompletedTask.Instance;
-        }
-
-        Task ProcessReceivedDisconnectPacket(MqttDisconnectPacket disconnectPacket)
-        {
-            _disconnectReason = (MqttClientDisconnectReason)disconnectPacket.ReasonCode;
-            _disconnectReasonString = disconnectPacket.ReasonString;
-
-            // Also dispatch disconnect to waiting threads to generate a proper exception.
-            _packetDispatcher.FailAll(new MqttUnexpectedDisconnectReceivedException(disconnectPacket));
-
-            return DisconnectInternalAsync(_packetReceiverTask, null, null);
-        }
-
-        async Task ProcessReceivedPublishPackets(CancellationToken cancellationToken)
-        {
-            while (!cancellationToken.IsCancellationRequested)
             {
                 try
                 {
-                    var publishPacketDequeueResult = await _publishPacketReceiverQueue.TryDequeueAsync(cancellationToken).ConfigureAwait(false);
+                    _backgroundCancellationTokenSource?.Cancel(false);
-                    if (!publishPacketDequeueResult.IsSuccess)
-                    {
-                        return;
-                    }
-
-                    var publishPacket = publishPacketDequeueResult.Item;
-                    var eventArgs = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
-
-                    if (eventArgs.AutoAcknowledge)
-                    {
-                        await eventArgs.AcknowledgeAsync(cancellationToken).ConfigureAwait(false);
-                    }
-                }
-                catch (OperationCanceledException)
-                {
                 }
                 catch (Exception exception)
                 {
-                    _logger.Error(exception, "Error while handling application message.");
+                    _logger.Warning(exception, "Error while initiating disconnect.");
                 }
             }
         }
 
-        Task ProcessReceivedPubRecPacket(MqttPubRecPacket pubRecPacket, CancellationToken cancellationToken)
+        Task SendAsync(MqttPacket packet, CancellationToken cancellationToken)
         {
-            if (!_packetDispatcher.TryDispatch(pubRecPacket))
+            cancellationToken.ThrowIfCancellationRequested();
-            {
-                // The packet is unknown. Probably due to a restart of the client.
-                // So wen send this to the server to trigger a full resend of the message.
-                var pubRelPacket = _packetFactories.PubRel.Create(pubRecPacket, MqttApplicationMessageReceivedReasonCode.PacketIdentifierNotFound);
-                return SendAsync(pubRelPacket, cancellationToken);
-            }
 
-            return CompletedTask.Instance;
+            _lastPacketSentTimestamp = DateTime.UtcNow;
-        }
 
-        Task ProcessReceivedPubRelPacket(MqttPubRelPacket pubRelPacket, CancellationToken cancellationToken)
+            return _adapter.SendPacketAsync(packet, cancellationToken);
-        {
-            var pubCompPacket = _packetFactories.PubComp.Create(pubRelPacket, MqttApplicationMessageReceivedReasonCode.Success);
-            return SendAsync(pubCompPacket, cancellationToken);
-        }
-
-        async Task<MqttClientPublishResult> PublishAtLeastOnceAsync(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
-        {
-            publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
-
-            var pubAckPacket = await SendAndReceiveAsync<MqttPubAckPacket>(publishPacket, cancellationToken).ConfigureAwait(false);
-
-            return _clientPublishResultFactory.Create(pubAckPacket);
-        }
-
-        async Task<MqttClientPublishResult> PublishAtMostOnce(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
-        {
-            // No packet identifier is used for QoS 0 [3.3.2.2 Packet Identifier]
-            await SendAsync(publishPacket, cancellationToken).ConfigureAwait(false);
-
-            return _clientPublishResultFactory.Create(null);
-        }
-
-        async Task<MqttClientPublishResult> PublishExactlyOnceAsync(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
-        {
-            publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
-
-            var pubRecPacket = await SendAndReceiveAsync<MqttPubRecPacket>(publishPacket, cancellationToken).ConfigureAwait(false);
-
-            var pubRelPacket = _packetFactories.PubRel.Create(pubRecPacket, MqttApplicationMessageReceivedReasonCode.Success);
-
-            var pubCompPacket = await SendAndReceiveAsync<MqttPubCompPacket>(pubRelPacket, cancellationToken).ConfigureAwait(false);
-
-            return _clientPublishResultFactory.Create(pubRecPacket, pubCompPacket);
         }
 
         async Task<TResponsePacket> SendAndReceiveAsync<TResponsePacket>(MqttPacket requestPacket, CancellationToken cancellationToken) where TResponsePacket : MqttPacket
@@ -680,43 +508,120 @@ namespace MQTTnet.Client
             }
         }
 
-        Task SendAsync(MqttPacket packet, CancellationToken cancellationToken)
+        async Task TrySendKeepAliveMessagesAsync(CancellationToken cancellationToken)
         {
-            cancellationToken.ThrowIfCancellationRequested();
+            try
-
-            _lastPacketSentTimestamp = DateTime.UtcNow;
-
-            return _adapter.SendPacketAsync(packet, cancellationToken);
-        }
-
-        void ThrowIfConnected(string message)
-        {
-            if (IsConnected)
             {
-                throw new MqttProtocolViolationException(message);
+                _logger.Verbose("Start sending keep alive packets.");
+
+                var keepAlivePeriod = Options.KeepAlivePeriod;
+     
+                while (!cancellationToken.IsCancellationRequested)
+                {
+                    // Values described here: [MQTT-3.1.2-24].
+                    var timeWithoutPacketSent = DateTime.UtcNow - _lastPacketSentTimestamp;
+
+                    if (timeWithoutPacketSent > keepAlivePeriod)
+                    {
+                        await PingAsync(cancellationToken).ConfigureAwait(false);
+                    }
+
+                    // Wait a fixed time in all cases. Calculation of the remaining time is complicated
+                    // due to some edge cases and was buggy in the past. Now we wait several ms because the
+                    // min keep alive value is one second so that the server will wait 1.5 seconds for a PING
+                    // packet.
+                    await Task.Delay(250, cancellationToken).ConfigureAwait(false);
+                }
+            }
+            catch (Exception exception)
+            {
+                if (_cleanDisconnectInitiated)
+                {
+                    return;
+                }
+
+                if (exception is OperationCanceledException)
+                {
+                    return;
+                }
+                else if (exception is MqttCommunicationException)
+                {
+                    _logger.Warning(exception, "Communication error while sending/receiving keep alive packets.");
+                }
+                else
+                {
+                    _logger.Error(exception, "Error exception while sending/receiving keep alive packets.");
+                }
+
+                await DisconnectInternalAsync(_keepAlivePacketsSenderTask, exception, null).ConfigureAwait(false);
+            }
+            finally
+            {
+                _logger.Verbose("Stopped sending keep alive packets.");
             }
         }
 
-        void ThrowIfNotConnected()
+        async Task TryReceivePacketsAsync(CancellationToken cancellationToken)
         {
-            if (!IsConnected)
+            try
             {
-                throw new MqttCommunicationException("The client is not connected.");
+                _logger.Verbose("Start receiving packets.");
-            }
-        }
 
-        void TryInitiateDisconnect()
+                while (!cancellationToken.IsCancellationRequested)
-        {
+                {
-            lock (_disconnectLock)
+                    MqttPacket packet;
+                    var packetTask = _adapter.ReceivePacketAsync(cancellationToken);
+
+                    if (packetTask.IsCompleted)
+                    {
+                        packet = packetTask.Result;
+                    }
+                    else
+                    {
+                        packet = await packetTask.ConfigureAwait(false);
+                    }
+
+                    if (cancellationToken.IsCancellationRequested)
+                    {
+                        return;
+                    }
+
+                    if (packet == null)
+                    {
+                        await DisconnectInternalAsync(_packetReceiverTask, null, null).ConfigureAwait(false);
+
+                        return;
+                    }
+
+                    await TryProcessReceivedPacketAsync(packet, cancellationToken).ConfigureAwait(false);
+                }
+            }
+            catch (Exception exception)
             {
-                try
+                if (_cleanDisconnectInitiated)
                 {
-                    _backgroundCancellationTokenSource?.Cancel(false);
+                    return;
                 }
-                catch (Exception exception)
+
+                if (exception is OperationCanceledException)
                 {
-                    _logger.Warning(exception, "Error while initiating disconnect.");
                 }
+                else if (exception is MqttCommunicationException)
+                {
+                    _logger.Warning(exception, "Communication error while receiving packets.");
+                }
+                else
+                {
+                    _logger.Error(exception, "Error while receiving packets.");
+                }
+
+                _packetDispatcher.FailAll(exception);
+
+                await DisconnectInternalAsync(_packetReceiverTask, exception, null).ConfigureAwait(false);
+            }
+            finally
+            {
+                _logger.Verbose("Stopped receiving packets.");
             }
         }
 
@@ -785,125 +690,158 @@ namespace MQTTnet.Client
             }
         }
 
-        async Task TryReceivePacketsAsync(CancellationToken cancellationToken)
+        void EnqueueReceivedPublishPacket(MqttPublishPacket publishPacket)
         {
             try
             {
-                _logger.Verbose("Start receiving packets.");
+                _publishPacketReceiverQueue.Enqueue(publishPacket);
-
-                while (!cancellationToken.IsCancellationRequested)
-                {
-                    MqttPacket packet;
-                    var packetTask = _adapter.ReceivePacketAsync(cancellationToken);
-
-                    if (packetTask.IsCompleted)
-                    {
-                        packet = packetTask.Result;
-                    }
-                    else
-                    {
-                        packet = await packetTask.ConfigureAwait(false);
-                    }
-
-                    if (cancellationToken.IsCancellationRequested)
-                    {
-                        return;
-                    }
-
-                    if (packet == null)
-                    {
-                        await DisconnectInternalAsync(_packetReceiverTask, null, null).ConfigureAwait(false);
-
-                        return;
-                    }
-
-                    await TryProcessReceivedPacketAsync(packet, cancellationToken).ConfigureAwait(false);
-                }
             }
             catch (Exception exception)
             {
-                if (_cleanDisconnectInitiated)
+                _logger.Error(exception, "Error while queueing application message.");
-                {
-                    return;
-                }
-
-                if (exception is OperationCanceledException)
-                {
-                }
-                else if (exception is MqttCommunicationException)
-                {
-                    _logger.Warning(exception, "Communication error while receiving packets.");
-                }
-                else
-                {
-                    _logger.Error(exception, "Error while receiving packets.");
-                }
-
-                _packetDispatcher.FailAll(exception);
-
-                await DisconnectInternalAsync(_packetReceiverTask, exception, null).ConfigureAwait(false);
-            }
-            finally
-            {
-                _logger.Verbose("Stopped receiving packets.");
             }
         }
 
-        async Task TrySendKeepAliveMessagesAsync(CancellationToken cancellationToken)
+        async Task ProcessReceivedPublishPackets(CancellationToken cancellationToken)
         {
-            try
+            while (!cancellationToken.IsCancellationRequested)
             {
-                _logger.Verbose("Start sending keep alive packets.");
+                try
-
-                var keepAlivePeriod = Options.KeepAlivePeriod;
-
-                while (!cancellationToken.IsCancellationRequested)
                 {
-                    // Values described here: [MQTT-3.1.2-24].
+                    var publishPacketDequeueResult = await _publishPacketReceiverQueue.TryDequeueAsync(cancellationToken).ConfigureAwait(false);
-                    var timeWithoutPacketSent = DateTime.UtcNow - _lastPacketSentTimestamp;
+                    if (!publishPacketDequeueResult.IsSuccess)
-
-                    if (timeWithoutPacketSent > keepAlivePeriod)
                     {
-                        using (var timeoutCancellationTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken))
+                        return;
-                        {
-                            timeoutCancellationTokenSource.CancelAfter(Options.Timeout);
-                            await PingAsync(timeoutCancellationTokenSource.Token).ConfigureAwait(false);
-                        }
                     }
 
-                    // Wait a fixed time in all cases. Calculation of the remaining time is complicated
+                    var publishPacket = publishPacketDequeueResult.Item;
-                    // due to some edge cases and was buggy in the past. Now we wait several ms because the
+                    var eventArgs = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
-                    // min keep alive value is one second so that the server will wait 1.5 seconds for a PING
-                    // packet.
-                    await Task.Delay(250, cancellationToken).ConfigureAwait(false);
-                }
-            }
-            catch (Exception exception)
-            {
-                if (_cleanDisconnectInitiated)
-                {
-                    return;
-                }
 
-                if (exception is OperationCanceledException)
+                    if (eventArgs.AutoAcknowledge)
-                {
+                    {
-                    return;
+                        await eventArgs.AcknowledgeAsync(cancellationToken).ConfigureAwait(false);
+                    }
                 }
-                else if (exception is MqttCommunicationException)
+                catch (OperationCanceledException)
                 {
-                    _logger.Warning(exception, "Communication error while sending/receiving keep alive packets.");
                 }
-                else
+                catch (Exception exception)
                 {
-                    _logger.Error(exception, "Error exception while sending/receiving keep alive packets.");
+                    _logger.Error(exception, "Error while handling application message.");
                 }
+            }
+        }
 
-                await DisconnectInternalAsync(_keepAlivePacketsSenderTask, exception, null).ConfigureAwait(false);
+        Task AcknowledgeReceivedPublishPacket(MqttApplicationMessageReceivedEventArgs eventArgs, CancellationToken cancellationToken)
-            }
+        {
-            finally
+            if (eventArgs.PublishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtMostOnce)
             {
-                _logger.Verbose("Stopped sending keep alive packets.");
+                // no response required
             }
+            else if (eventArgs.PublishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtLeastOnce)
+            {
+                if (!eventArgs.ProcessingFailed)
+                {
+                    var pubAckPacket = _packetFactories.PubAck.Create(eventArgs);
+                    return SendAsync(pubAckPacket, cancellationToken);
+                }
+            }
+            else if (eventArgs.PublishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.ExactlyOnce)
+            {
+                if (!eventArgs.ProcessingFailed)
+                {
+                    var pubRecPacket = _packetFactories.PubRec.Create(eventArgs);
+                    return SendAsync(pubRecPacket, cancellationToken);
+                }
+            }
+            else
+            {
+                throw new MqttProtocolViolationException("Received a not supported QoS level.");
+            }
+
+            return PlatformAbstractionLayer.CompletedTask;
+        }
+
+        Task ProcessReceivedPubRecPacket(MqttPubRecPacket pubRecPacket, CancellationToken cancellationToken)
+        {
+            if (!_packetDispatcher.TryDispatch(pubRecPacket))
+            {
+                // The packet is unknown. Probably due to a restart of the client.
+                // So wen send this to the server to trigger a full resend of the message.
+                var pubRelPacket = _packetFactories.PubRel.Create(pubRecPacket, MqttApplicationMessageReceivedReasonCode.PacketIdentifierNotFound);
+                return SendAsync(pubRelPacket, cancellationToken);
+            }
+
+            return PlatformAbstractionLayer.CompletedTask;
+        }
+
+        Task ProcessReceivedPubRelPacket(MqttPubRelPacket pubRelPacket, CancellationToken cancellationToken)
+        {
+            var pubCompPacket = _packetFactories.PubComp.Create(pubRelPacket, MqttApplicationMessageReceivedReasonCode.Success);
+            return SendAsync(pubCompPacket, cancellationToken);
+        }
+
+        Task ProcessReceivedDisconnectPacket(MqttDisconnectPacket disconnectPacket)
+        {
+            _disconnectReason = (MqttClientDisconnectReason) disconnectPacket.ReasonCode;
+            _disconnectReasonString = disconnectPacket.ReasonString;
+            
+            // Also dispatch disconnect to waiting threads to generate a proper exception.
+            _packetDispatcher.FailAll(new MqttUnexpectedDisconnectReceivedException(disconnectPacket));
+
+            return DisconnectInternalAsync(_packetReceiverTask, null, null);
+        }
+
+        Task ProcessReceivedAuthPacket(MqttAuthPacket authPacket)
+        {
+            var extendedAuthenticationExchangeHandler = Options.ExtendedAuthenticationExchangeHandler;
+            if (extendedAuthenticationExchangeHandler != null)
+            {
+                return extendedAuthenticationExchangeHandler.HandleRequestAsync(new MqttExtendedAuthenticationExchangeContext(authPacket, this));
+            }
+
+            return PlatformAbstractionLayer.CompletedTask;
+        }
+
+        async Task<MqttClientPublishResult> PublishAtMostOnce(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
+        {
+            // No packet identifier is used for QoS 0 [3.3.2.2 Packet Identifier]
+            await SendAsync(publishPacket, cancellationToken)
+                .ConfigureAwait(false);
+
+            return _clientPublishResultFactory.Create(null);
+        }
+
+        async Task<MqttClientPublishResult> PublishAtLeastOnceAsync(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
+        {
+            publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
+            
+            var pubAckPacket = await SendAndReceiveAsync<MqttPubAckPacket>(publishPacket, cancellationToken).ConfigureAwait(false);
+
+            return _clientPublishResultFactory.Create(pubAckPacket);
+        }
+
+        async Task<MqttClientPublishResult> PublishExactlyOnceAsync(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
+        {
+            publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
+
+            var pubRecPacket = await SendAndReceiveAsync<MqttPubRecPacket>(publishPacket, cancellationToken).ConfigureAwait(false);
+
+            var pubRelPacket = _packetFactories.PubRel.Create(pubRecPacket, MqttApplicationMessageReceivedReasonCode.Success);
+
+            var pubCompPacket = await SendAndReceiveAsync<MqttPubCompPacket>(pubRelPacket, cancellationToken).ConfigureAwait(false);
+
+            return _clientPublishResultFactory.Create(pubRecPacket, pubCompPacket);
+        }
+
+        async Task<MqttApplicationMessageReceivedEventArgs> HandleReceivedApplicationMessageAsync(MqttPublishPacket publishPacket)
+        {
+            var applicationMessage = _applicationMessageFactory.Create(publishPacket);
+            var eventArgs = new MqttApplicationMessageReceivedEventArgs(Options.ClientId, applicationMessage, publishPacket, AcknowledgeReceivedPublishPacket);
+            
+            await _applicationMessageReceivedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
+
+            return eventArgs;
         }
 
         async Task WaitForTaskAsync(Task task, Task sender)
@@ -936,5 +874,37 @@ namespace MQTTnet.Client
             {
             }
         }
+
+        bool DisconnectIsPendingOrFinished()
+        {
+            var connectionStatus = (MqttClientConnectionStatus)_connectionStatus;
+            
+            do
+            {
+                switch (connectionStatus)
+                {
+                    case MqttClientConnectionStatus.Disconnected:
+                    case MqttClientConnectionStatus.Disconnecting:
+                        return true;
+                    case MqttClientConnectionStatus.Connected:
+                    case MqttClientConnectionStatus.Connecting:
+                        // This will compare the _connectionStatus to old value and set it to "MqttClientConnectionStatus.Disconnecting" afterwards.
+                        // So the first caller will get a "false" and all subsequent ones will get "true".
+                        var curStatus = CompareExchangeConnectionStatus(MqttClientConnectionStatus.Disconnecting, connectionStatus);
+                        if (curStatus == connectionStatus)
+                        {
+                            return false;
+                        }
+                        
+                        connectionStatus = curStatus;
+                        break;
+                }
+            } while (true);
+        }
+
+        MqttClientConnectionStatus CompareExchangeConnectionStatus(MqttClientConnectionStatus value, MqttClientConnectionStatus comparand)
+        {
+            return (MqttClientConnectionStatus)Interlocked.CompareExchange(ref _connectionStatus, (int)value, (int)comparand);
+        }
     }
 }

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)

Source/MQTTnet/Client/Receiving/MqttApplicationMessageReceivedEventArgs.cs

@@ -24,58 +24,55 @@ namespace MQTTnet.Client
         {
             ClientId = clientId;
             ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
-            PublishPacket = publishPacket ?? throw new ArgumentNullException(nameof(publishPacket));
+            PublishPacket = publishPacket;
+            PacketIdentifier = publishPacket.PacketIdentifier;
             _acknowledgeHandler = acknowledgeHandler;
         }
 
-        public MqttApplicationMessage ApplicationMessage { get; }
+        internal MqttPublishPacket PublishPacket { get; set; }
         
         /// <summary>
-        ///     Gets or sets whether the library should send MQTT ACK packets automatically if required.
+        /// Gets the client identifier.
-        /// </summary>
+        /// Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
-        public bool AutoAcknowledge { get; set; } = true;
-
-        /// <summary>
-        ///     Gets the client identifier.
-        ///     Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
         /// </summary>
         public string ClientId { get; }
 
-        /// <summary>
+        public MqttApplicationMessage ApplicationMessage { get; }
-        ///     Gets or sets whether this message was handled.
-        ///     This value can be used in user code for custom control flow.
-        /// </summary>
-        public bool IsHandled { get; set; }
 
-        /// <summary>
-        ///     Gets the identifier of the MQTT packet
-        /// </summary>
-        public ushort PacketIdentifier => PublishPacket.PacketIdentifier;
-
-        /// <summary>
-        ///     Indicates if the processing of this PUBLISH packet has failed.
-        ///     If the processing has failed the client will not send an ACK packet etc.
-        /// </summary>
         public bool ProcessingFailed { get; set; }
 
         /// <summary>
-        ///     Gets or sets the reason code which will be sent to the server.
+        /// Gets or sets the reason code which will be sent to the server.
         /// </summary>
         public MqttApplicationMessageReceivedReasonCode ReasonCode { get; set; } = MqttApplicationMessageReceivedReasonCode.Success;
 
         /// <summary>
-        ///     Gets or sets the reason string which will be sent to the server in the ACK packet.
+        /// Gets or sets the user properties which will be sent to the server in the ACK packet etc.
+        /// </summary>
+        public List<MqttUserProperty> ResponseUserProperties { get; } = new List<MqttUserProperty>();
+
+        /// <summary>
+        /// Gets or sets the reason string which will be sent to the server in the ACK packet.
         /// </summary>
         public string ResponseReasonString { get; set; }
         
         /// <summary>
-        ///     Gets or sets the user properties which will be sent to the server in the ACK packet etc.
+        /// Gets or sets whether this message was handled.
+        /// This value can be used in user code for custom control flow.
         /// </summary>
-        public List<MqttUserProperty> ResponseUserProperties { get; } = new List<MqttUserProperty>();
+        public bool IsHandled { get; set; }
+
+        /// <summary>
+        /// Gets or sets whether the library should send MQTT ACK packets automatically if required.
+        /// </summary>
+        public bool AutoAcknowledge { get; set; } = true;
         
         public object Tag { get; set; }
 
-        internal MqttPublishPacket PublishPacket { get; set; }
+        /// <summary>
+        /// Gets the indentifier of MQTT packet
+        /// </summary>
+        public ushort PacketIdentifier { get; }
 
         public Task AcknowledgeAsync(CancellationToken cancellationToken) 
         {

Source/MQTTnet/Formatter/MqttBufferReader.cs

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

Source/MQTTnet/Formatter/MqttPacketBuffer.cs

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

Source/MQTTnet/Implementations/CrossPlatformSocket.cs

@@ -15,20 +15,14 @@ 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()
@@ -37,76 +31,17 @@ 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
         }
 
-        CrossPlatformSocket(Socket socket)
+        public 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.
@@ -116,20 +51,24 @@ 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;
@@ -142,6 +81,16 @@ 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
@@ -160,6 +109,8 @@ namespace MQTTnet.Implementations
             }
         }
 
+        public EndPoint LocalEndPoint => _socket.LocalEndPoint;
+
         public void Bind(EndPoint localEndPoint)
         {
             if (localEndPoint is null)
@@ -170,6 +121,11 @@ 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)
@@ -181,17 +137,11 @@ namespace MQTTnet.Implementations
 
             try
             {
-#if NET5_0_OR_GREATER
-
-                if (_networkStream != null)
-                {
-                    await _networkStream.DisposeAsync().ConfigureAwait(false);
-                }
-
-                await _socket.ConnectAsync(host, port, cancellationToken).ConfigureAwait(false);
-#else
                 _networkStream?.Dispose();
 
+#if NET5_0_OR_GREATER
+                await _socket.ConnectAsync(host, port, cancellationToken).ConfigureAwait(false);
+#else
                 // Workaround for: https://github.com/dotnet/corefx/issues/24430
                 using (cancellationToken.Register(_socketDisposeAction))
                 {
@@ -225,26 +175,20 @@ namespace MQTTnet.Implementations
             }
         }
 
-        public void Dispose()
+        public async Task SendAsync(ArraySegment<byte> buffer, SocketFlags socketFlags)
         {
-            _networkStream?.Dispose();
+            try
-            _socket?.Dispose();
-        }
-
-        public NetworkStream GetStream()
-        {
-            var networkStream = _networkStream;
-            if (networkStream == null)
             {
-                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 Listen(int connectionBacklog)
-        {
-            _socket.Listen(connectionBacklog);
         }
 
         public async Task<int> ReceiveAsync(ArraySegment<byte> buffer, SocketFlags socketFlags)
@@ -264,27 +208,28 @@ namespace MQTTnet.Implementations
             }
         }
 
-        public async Task SendAsync(ArraySegment<byte> buffer, SocketFlags socketFlags)
+        public NetworkStream GetStream()
         {
-            try
+            var networkStream = _networkStream;
+            if (networkStream == null)
             {
-#if NET452 || NET461
+                throw new IOException("The socket is not connected.");
-                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 ArraySegment<byte> _buffer;
             readonly Socket _socket;
+            readonly ArraySegment<byte> _buffer;
             readonly SocketFlags _socketFlags;
 
             public SocketWrapper(Socket socket, ArraySegment<byte> buffer, SocketFlags socketFlags)
@@ -294,28 +239,16 @@ namespace MQTTnet.Implementations
                 _socketFlags = socketFlags;
             }
 
-            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);
-            }
-
             public static IAsyncResult BeginSend(AsyncCallback callback, object state)
             {
                 var socketWrapper = (SocketWrapper)state;
-                return socketWrapper._socket.BeginSend(
+                return socketWrapper._socket.BeginSend(socketWrapper._buffer.Array, socketWrapper._buffer.Offset, socketWrapper._buffer.Count, socketWrapper._socketFlags, callback, state);
-                    socketWrapper._buffer.Array,
+            }
-                    socketWrapper._buffer.Offset,
+
-                    socketWrapper._buffer.Count,
+            public static IAsyncResult BeginReceive(AsyncCallback callback, object state)
-                    socketWrapper._socketFlags,
+            {
-                    callback,
+                var socketWrapper = (SocketWrapper)state;
-                    state);
+                return socketWrapper._socket.BeginReceive(socketWrapper._buffer.Array, socketWrapper._buffer.Offset, socketWrapper._buffer.Count, socketWrapper._socketFlags, callback, state);
             }
         }
 #endif

Source/MQTTnet/Implementations/MqttTcpChannel.Uwp.cs

@@ -108,12 +108,12 @@ namespace MQTTnet.Implementations
             return _readStream.ReadAsync(buffer, offset, count, cancellationToken);
         }
 
-        public Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken)
+        public Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
         {
             // In the write method only the internal buffer will be filled. So here is no
             // async/await required. The real network transmit is done when calling the
             // Flush method.
-            _writeStream.Write(buffer.Array, buffer.Offset, buffer.Count);
+            _writeStream.Write(buffer, offset, count);
             return _writeStream.FlushAsync(cancellationToken);
         }
 

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -14,7 +14,6 @@ using System.Runtime.ExceptionServices;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
-using MQTTnet.Internal;
 
 namespace MQTTnet.Implementations
 {
@@ -155,7 +154,7 @@ namespace MQTTnet.Implementations
         public Task DisconnectAsync(CancellationToken cancellationToken)
         {
             Dispose();
-            return CompletedTask.Instance;
+            return Task.FromResult(0);
         }
 
         public void Dispose()
@@ -226,7 +225,7 @@ namespace MQTTnet.Implementations
             }
         }
 
-        public async Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken)
+        public async Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
         {
             cancellationToken.ThrowIfCancellationRequested();
 
@@ -240,12 +239,12 @@ namespace MQTTnet.Implementations
                 }
 
 #if NETCOREAPP3_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
-                await stream.WriteAsync(buffer.AsMemory(), cancellationToken).ConfigureAwait(false);
+                await stream.WriteAsync(buffer.AsMemory(offset, count), cancellationToken).ConfigureAwait(false);
 #else
                 // Workaround for: https://github.com/dotnet/corefx/issues/24430
                 using (cancellationToken.Register(_disposeAction))
                 {
-                    await stream.WriteAsync(buffer.Array, buffer.Offset, buffer.Count, cancellationToken).ConfigureAwait(false);
+                    await stream.WriteAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
                 }
 #endif
             }

Source/MQTTnet/Implementations/MqttTcpServerAdapter.cs

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

Source/MQTTnet/Implementations/MqttTcpServerListener.cs

@@ -134,7 +134,7 @@ namespace MQTTnet.Implementations
                         continue;
                     }
 
-                    _ = Task.Factory.StartNew(() => TryHandleClientConnectionAsync(clientSocket), cancellationToken, TaskCreationOptions.PreferFairness, TaskScheduler.Default).ConfigureAwait(false);
+                    Task.Run(() => TryHandleClientConnectionAsync(clientSocket), cancellationToken).RunInBackground(_logger);
                 }
                 catch (OperationCanceledException)
                 {

Source/MQTTnet/Implementations/MqttWebSocketChannel.cs

@@ -2,15 +2,15 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using MQTTnet.Channel;
+using MQTTnet.Internal;
 using System;
 using System.Net;
 using System.Net.WebSockets;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
-using MQTTnet.Channel;
 using MQTTnet.Client;
-using MQTTnet.Internal;
 
 namespace MQTTnet.Implementations
 {
@@ -35,12 +35,12 @@ namespace MQTTnet.Implementations
             ClientCertificate = clientCertificate;
         }
 
-        public X509Certificate2 ClientCertificate { get; }
-
         public string Endpoint { get; }
 
         public bool IsSecureConnection { get; private set; }
 
+        public X509Certificate2 ClientCertificate { get; }
+
         public async Task ConnectAsync(CancellationToken cancellationToken)
         {
             var uri = _options.Uri;
@@ -89,26 +89,14 @@ namespace MQTTnet.Implementations
             Cleanup();
         }
 
-        public void Dispose()
-        {
-            Cleanup();
-        }
-
         public async Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
         {
             var response = await _webSocket.ReceiveAsync(new ArraySegment<byte>(buffer, offset, count), cancellationToken).ConfigureAwait(false);
             return response.Count;
         }
 
-        public async Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken)
+        public async Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
         {
-            cancellationToken.ThrowIfCancellationRequested();
-            
-#if NET5_0_OR_GREATER
-            // MQTT Control Packets MUST be sent in WebSocket binary data frames. If any other type of data frame is received the recipient MUST close the Network Connection [MQTT-6.0.0-1].
-            // A single WebSocket data frame can contain multiple or partial MQTT Control Packets. The receiver MUST NOT assume that MQTT Control Packets are aligned on WebSocket frame boundaries [MQTT-6.0.0-2].
-            await _webSocket.SendAsync(buffer, WebSocketMessageType.Binary, isEndOfPacket, cancellationToken).ConfigureAwait(false);
-#else
             // The lock is required because the client will throw an exception if _SendAsync_ is 
             // called from multiple threads at the same time. But this issue only happens with several
             // framework versions.
@@ -117,11 +105,88 @@ namespace MQTTnet.Implementations
                 return;
             }
 
-            using (await _sendLock.EnterAsync(cancellationToken).ConfigureAwait(false))
+            using (await _sendLock.WaitAsync(cancellationToken).ConfigureAwait(false))
             {
-                await _webSocket.SendAsync(buffer, WebSocketMessageType.Binary, isEndOfPacket, cancellationToken).ConfigureAwait(false);
+                await _webSocket.SendAsync(new ArraySegment<byte>(buffer, offset, count), WebSocketMessageType.Binary, true, cancellationToken).ConfigureAwait(false);
             }
+        }
+
+        public void Dispose()
+        {
+            Cleanup();
+        }
+
+        void SetupClientWebSocket(ClientWebSocket clientWebSocket)
+        {
+            if (_options.ProxyOptions != null)
+            {
+                clientWebSocket.Options.Proxy = CreateProxy();
+            }
+
+            if (_options.RequestHeaders != null)
+            {
+                foreach (var requestHeader in _options.RequestHeaders)
+                {
+                    clientWebSocket.Options.SetRequestHeader(requestHeader.Key, requestHeader.Value);
+                }
+            }
+
+            if (_options.SubProtocols != null)
+            {
+                foreach (var subProtocol in _options.SubProtocols)
+                {
+                    clientWebSocket.Options.AddSubProtocol(subProtocol);
+                }
+            }
+
+            if (_options.CookieContainer != null)
+            {
+                clientWebSocket.Options.Cookies = _options.CookieContainer;
+            }
+
+            if (_options.TlsOptions?.UseTls == true && _options.TlsOptions?.Certificates != null)
+            {
+                clientWebSocket.Options.ClientCertificates = new X509CertificateCollection();
+                foreach (var certificate in _options.TlsOptions.Certificates)
+                {
+#if WINDOWS_UWP
+                    clientWebSocket.Options.ClientCertificates.Add(new X509Certificate(certificate));
+#else
+                    clientWebSocket.Options.ClientCertificates.Add(certificate);
 #endif
+
+                }
+            }
+
+            var certificateValidationHandler = _options.TlsOptions?.CertificateValidationHandler;
+            if (certificateValidationHandler != null)
+            {
+#if NETSTANDARD1_3
+                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'netstandard1.3'.");
+#elif NETSTANDARD2_0
+                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'netstandard2.0'.");
+#elif WINDOWS_UWP
+                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'uap10.0'.");
+#elif NET452
+                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'net452'.");
+#elif NET461
+                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'net461'.");
+#else
+                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
+                    };
+
+                    return certificateValidationHandler(context);
+                };
+#endif
+            }
         }
 
         void Cleanup()
@@ -165,77 +230,5 @@ namespace MQTTnet.Implementations
             return new WebProxy(proxyUri, _options.ProxyOptions.BypassOnLocal, _options.ProxyOptions.BypassList);
 #endif
         }
-
-        void SetupClientWebSocket(ClientWebSocket clientWebSocket)
-        {
-            if (_options.ProxyOptions != null)
-            {
-                clientWebSocket.Options.Proxy = CreateProxy();
-            }
-
-            if (_options.RequestHeaders != null)
-            {
-                foreach (var requestHeader in _options.RequestHeaders)
-                {
-                    clientWebSocket.Options.SetRequestHeader(requestHeader.Key, requestHeader.Value);
-                }
-            }
-
-            if (_options.SubProtocols != null)
-            {
-                foreach (var subProtocol in _options.SubProtocols)
-                {
-                    clientWebSocket.Options.AddSubProtocol(subProtocol);
-                }
-            }
-
-            if (_options.CookieContainer != null)
-            {
-                clientWebSocket.Options.Cookies = _options.CookieContainer;
-            }
-
-            if (_options.TlsOptions?.UseTls == true && _options.TlsOptions?.Certificates != null)
-            {
-                clientWebSocket.Options.ClientCertificates = new X509CertificateCollection();
-                foreach (var certificate in _options.TlsOptions.Certificates)
-                {
-#if WINDOWS_UWP
-                    clientWebSocket.Options.ClientCertificates.Add(new X509Certificate(certificate));
-#else
-                    clientWebSocket.Options.ClientCertificates.Add(certificate);
-#endif
-                }
-            }
-
-            var certificateValidationHandler = _options.TlsOptions?.CertificateValidationHandler;
-            if (certificateValidationHandler != null)
-            {
-#if NETSTANDARD1_3
-                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'netstandard1.3'.");
-#elif NETSTANDARD2_0
-                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'netstandard2.0'.");
-#elif WINDOWS_UWP
-                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'uap10.0'.");
-#elif NET452
-                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'net452'.");
-#elif NET461
-                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'net461'.");
-#else
-                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
-                    };
-
-                    return certificateValidationHandler(context);
-                };
-#endif
-            }
-        }
     }
 }

Source/MQTTnet/Implementations/PlatformAbstractionLayer.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;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Implementations
+{
+    public static class PlatformAbstractionLayer
+    {
+#if NET452
+        public static Task CompletedTask => Task.FromResult(0);
+
+        public static byte[] EmptyByteArray { get; } = new byte[0];
+#else
+        public static Task CompletedTask => 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

@@ -13,16 +13,7 @@ namespace MQTTnet.Internal
     {
         readonly List<AsyncEventInvocator<TEventArgs>> _handlers = new List<AsyncEventInvocator<TEventArgs>>();
 
-        ICollection<AsyncEventInvocator<TEventArgs>> _handlersForInvoke;
+        public bool HasHandlers => _handlers.Count > 0;
-
-        public AsyncEvent()
-        {
-            _handlersForInvoke = _handlers;
-        }
-
-        // Track the existence of handlers in a separate field so that checking it all the time will not
-        // require locking the actual list (_handlers).
-        public bool HasHandlers { get; private set; }
 
         public void AddHandler(Func<TEventArgs, Task> handler)
         {
@@ -31,13 +22,7 @@ namespace MQTTnet.Internal
                 throw new ArgumentNullException(nameof(handler));
             }
 
-            lock (_handlers)
+            _handlers.Add(new AsyncEventInvocator<TEventArgs>(null, handler));
-            {
-                _handlers.Add(new AsyncEventInvocator<TEventArgs>(null, handler));
-
-                HasHandlers = true;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
-            }
         }
 
         public void AddHandler(Action<TEventArgs> handler)
@@ -47,27 +32,12 @@ namespace MQTTnet.Internal
                 throw new ArgumentNullException(nameof(handler));
             }
 
-            lock (_handlers)
+            _handlers.Add(new AsyncEventInvocator<TEventArgs>(handler, null));
-            {
-                _handlers.Add(new AsyncEventInvocator<TEventArgs>(handler, null));
-
-                HasHandlers = true;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
-            }
         }
 
         public async Task InvokeAsync(TEventArgs eventArgs)
         {
-            if (!HasHandlers)
+            foreach (var handler in _handlers)
-            {
-                return;
-            }
-
-            // Adding or removing handlers will produce a new list instance all the time.
-            // So locking here is not required since only the reference to an immutable list
-            // of handlers is used.
-            var handlers = _handlersForInvoke;
-            foreach (var handler in handlers)
             {
                 await handler.InvokeAsync(eventArgs).ConfigureAwait(false);
             }
@@ -80,13 +50,7 @@ namespace MQTTnet.Internal
                 throw new ArgumentNullException(nameof(handler));
             }
 
-            lock (_handlers)
+            _handlers.RemoveAll(h => h.WrapsHandler(handler));
-            {
-                _handlers.RemoveAll(h => h.WrapsHandler(handler));
-
-                HasHandlers = _handlers.Count > 0;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
-            }
         }
 
         public void RemoveHandler(Action<TEventArgs> handler)
@@ -96,13 +60,7 @@ namespace MQTTnet.Internal
                 throw new ArgumentNullException(nameof(handler));
             }
 
-            lock (_handlers)
+            _handlers.RemoveAll(h => h.WrapsHandler(handler));
-            {
-                _handlers.RemoveAll(h => h.WrapsHandler(handler));
-
-                HasHandlers = _handlers.Count > 0;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
-            }
         }
 
         public async Task TryInvokeAsync(TEventArgs eventArgs, MqttNetSourceLogger logger)

Source/MQTTnet/Internal/AsyncEventInvocator.cs

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

Source/MQTTnet/Internal/AsyncLock.cs

@@ -3,8 +3,6 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Collections.Generic;
-using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -12,151 +10,76 @@ namespace MQTTnet.Internal
 {
     public sealed class AsyncLock : IDisposable
     {
-        readonly Task<IDisposable> _completedTask;
-        readonly IDisposable _releaser;
         readonly object _syncRoot = new object();
-        readonly Queue<AsyncLockWaiter> _waiters = new Queue<AsyncLockWaiter>(64);
+        readonly Task<IDisposable> _releaser;
 
-        volatile bool _isDisposed;
+        SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
-        bool _isLocked;
 
         public AsyncLock()
         {
-            _releaser = new Releaser(this);
+            _releaser = Task.FromResult((IDisposable)new Releaser(this));
-            _completedTask = Task.FromResult(_releaser);
+        }
+
+        public Task<IDisposable> WaitAsync(CancellationToken cancellationToken)
+        {
+            Task task;
+
+            // This lock is required to avoid ObjectDisposedExceptions.
+            // These are fired when this lock gets disposed (and thus the semaphore)
+            // and a worker thread tries to call this method at the same time.
+            // Another way would be catching all ObjectDisposedExceptions but this situation happens
+            // quite often when clients are disconnecting.
+            lock (_syncRoot)
+            {
+                task = _semaphore?.WaitAsync(cancellationToken);
+            }
+
+            if (task == null)
+            {
+                throw new ObjectDisposedException("The AsyncLock is disposed.");
+            }
+
+            if (task.Status == TaskStatus.RanToCompletion)
+            {
+                return _releaser;
+            }
+
+            // Wait for the _WaitAsync_ method and return the releaser afterwards.
+            return task.ContinueWith(
+                (_, state) => (IDisposable)state, 
+                _releaser.Result, 
+                cancellationToken, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
         }
 
         public void Dispose()
         {
             lock (_syncRoot)
             {
-                _isDisposed = true;
+                _semaphore?.Dispose();
-
+                _semaphore = null;
-                while (_waiters.Any())
-                {
-                    _waiters.Dequeue().Dispose();
-                }
             }
         }
 
-        public Task<IDisposable> EnterAsync(CancellationToken cancellationToken = default)
+        internal void Release()
-        {
-            cancellationToken.ThrowIfCancellationRequested();
-            
-            if (_isDisposed)
-            {
-                throw new ObjectDisposedException(nameof(AsyncLock));
-            }
-
-            lock (_syncRoot)
-            {
-                if (!_isLocked)
-                {
-                    _isLocked = true;
-                    return _completedTask;
-                }
-
-                var waiter = new AsyncLockWaiter(cancellationToken);
-                _waiters.Enqueue(waiter);
-
-                return waiter.Task;
-            }
-        }
-
-        void Release()
         {
             lock (_syncRoot)
             {
-                if (_isDisposed)
+                _semaphore?.Release();
-                {
-                    // All waiters have been canceled with a ObjectDisposedException.
-                    // So there is nothing left to do.
-                    return;
-                }
-
-                // Assume that there is no waiter left first.
-                _isLocked = false;
-
-                // 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 waiter = _waiters.Dequeue();
-                    var isApproved = waiter.Approve(_releaser);
-                    waiter.Dispose();
-
-                    if (isApproved)
-                    {
-                        _isLocked = true;
-                        return;
-                    }
-                }
             }
         }
 
-        sealed class AsyncLockWaiter : IDisposable
+        sealed class Releaser : IDisposable
         {
-            readonly CancellationTokenRegistration _cancellationRegistration;
+            readonly AsyncLock _lock;
-            readonly bool _hasCancellationRegistration;
-            readonly AsyncTaskCompletionSource<IDisposable> _promise = new AsyncTaskCompletionSource<IDisposable>();
 
-            public AsyncLockWaiter(CancellationToken cancellationToken)
+            internal Releaser(AsyncLock @lock)
             {
-                cancellationToken.ThrowIfCancellationRequested();
+                _lock = @lock;
-                
-                if (cancellationToken.CanBeCanceled)
-                {
-                    _cancellationRegistration = cancellationToken.Register(Cancel);
-                    _hasCancellationRegistration = true;
-                }
-            }
-
-            public Task<IDisposable> Task => _promise.Task;
-
-            public bool Approve(IDisposable scope)
-            {
-                if (scope == null)
-                {
-                    throw new ArgumentNullException(nameof(scope));
-                }
-
-                if (_promise.Task.IsCompleted)
-                {
-                    return false;
-                }
-
-                return _promise.TrySetResult(scope);
             }
 
             public void Dispose()
             {
-                if (_hasCancellationRegistration)
+                _lock.Release();
-                {
-                    _cancellationRegistration.Dispose();
-                }
-
-                _promise.TrySetException(new ObjectDisposedException(nameof(AsyncLockWaiter)));
-            }
-
-            void Cancel()
-            {
-                _promise.TrySetCanceled();
-            }
-        }
-
-        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/AsyncTaskCompletionSource.cs

@@ -3,7 +3,6 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Threading;
 using System.Threading.Tasks;
 
 namespace MQTTnet.Internal
@@ -23,7 +22,7 @@ namespace MQTTnet.Internal
 
         public Task<TResult> Task => _taskCompletionSource.Task;
 
-        public void TrySetCanceled()
+        public bool TrySetCanceled()
         {
 #if NET452
             // To prevent deadlocks it is required to call the _TrySetCanceled_ method
@@ -32,14 +31,19 @@ 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());
-            SpinWait.SpinUntil(() => _taskCompletionSource.Task.IsCompleted);
+            return true;
 #else
-            _taskCompletionSource.TrySetCanceled();
+            return _taskCompletionSource.TrySetCanceled();
 #endif
         }
 
-        public void TrySetException(Exception exception)
+        public bool 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
@@ -47,9 +51,9 @@ 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));
-            SpinWait.SpinUntil(() => _taskCompletionSource.Task.IsCompleted);
+            return true;
 #else
-            _taskCompletionSource.TrySetException(exception);
+            return _taskCompletionSource.TrySetException(exception);
 #endif
         }
 
@@ -61,14 +65,7 @@ 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

@@ -1,17 +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.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

@@ -1,19 +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;
-
-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);
-    }
-}

Source/MQTTnet/MQTTnet.csproj

@@ -14,6 +14,7 @@
         <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">$(TargetFrameworks);net452;net461</TargetFrameworks>
         <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' AND '$(MSBuildRuntimeType)' != 'Core' ">$(TargetFrameworks);uap10.0</TargetFrameworks>
         <LangVersion>7.3</LangVersion>
+        
         <AssemblyName>MQTTnet</AssemblyName>
         <RootNamespace>MQTTnet</RootNamespace>
         <GeneratePackageOnBuild>True</GeneratePackageOnBuild>

Source/MQTTnet/Server/Events/ValidatingConnectionEventArgs.cs

@@ -10,7 +10,6 @@ using System.Text;
 using MQTTnet.Adapter;
 using MQTTnet.Formatter;
 using MQTTnet.Implementations;
-using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
@@ -89,7 +88,7 @@ namespace MQTTnet.Server
         /// </summary>
         public uint MaximumPacketSize => _connectPacket.MaximumPacketSize;
 
-        public string Password => Encoding.UTF8.GetString(RawPassword ?? EmptyBuffer.Array);
+        public string Password => Encoding.UTF8.GetString(RawPassword ?? PlatformAbstractionLayer.EmptyByteArray);
 
         public MqttProtocolVersion ProtocolVersion => ChannelAdapter.PacketFormatterAdapter.ProtocolVersion;
 

Source/MQTTnet/Server/Internal/MqttClient.cs

@@ -98,7 +98,7 @@ namespace MQTTnet.Server
 
                 try
                 {
-                    _ = Task.Factory.StartNew(() => SendPacketsLoop(cancellationToken), cancellationToken, TaskCreationOptions.PreferFairness, TaskScheduler.Default).ConfigureAwait(false);
+                    Task.Run(() => SendPacketsLoop(cancellationToken), cancellationToken).RunInBackground(_logger);
 
                     IsRunning = true;
 
@@ -147,20 +147,13 @@ namespace MQTTnet.Server
         {
             IsRunning = false;
 
-            // Sending DISCONNECT packets from the server to the client is only supported when using MQTTv5+.
+            if (reason == MqttDisconnectReasonCode.SessionTakenOver || reason == MqttDisconnectReasonCode.KeepAliveTimeout)
-            if (ChannelAdapter.PacketFormatterAdapter.ProtocolVersion == MqttProtocolVersion.V500)
             {
-                // The Client or Server MAY send a DISCONNECT packet before closing the Network Connection.
+                // Is is very important to send the DISCONNECT packet here BEFORE cancelling the
-                // This library does not sent a DISCONNECT packet for a normal disconnection. Maybe adding
+                // token because the entire connection is closed (disposed) as soon as the cancellation
-                // a configuration option is requested in the future.
+                // token is cancelled. To there is no chance that the DISCONNECT packet will ever arrive
-                if (reason != MqttDisconnectReasonCode.NormalDisconnection)
+                // at the client!
-                {
+                await TrySendDisconnectPacket(reason).ConfigureAwait(false);
-                    // Is is very important to send the DISCONNECT packet here BEFORE cancelling the
-                    // token because the entire connection is closed (disposed) as soon as the cancellation
-                    // token is cancelled. To there is no chance that the DISCONNECT packet will ever arrive
-                    // at the client!
-                    await TrySendDisconnectPacket(reason).ConfigureAwait(false);
-                }
             }
 
             StopInternal();
@@ -181,7 +174,7 @@ namespace MQTTnet.Server
                 return _eventContainer.ClientAcknowledgedPublishPacketEvent.TryInvokeAsync(eventArgs, _logger);
             }
 
-            return CompletedTask.Instance;
+            return PlatformAbstractionLayer.CompletedTask;
         }
 
         Task HandleIncomingPubAckPacket(MqttPubAckPacket pubAckPacket)
@@ -193,7 +186,7 @@ namespace MQTTnet.Server
                 return ClientAcknowledgedPublishPacket(acknowledgedPublishPacket, pubAckPacket);
             }
 
-            return CompletedTask.Instance;
+            return PlatformAbstractionLayer.CompletedTask;
         }
 
         Task HandleIncomingPubCompPacket(MqttPubCompPacket pubCompPacket)
@@ -205,7 +198,7 @@ namespace MQTTnet.Server
                 return ClientAcknowledgedPublishPacket(acknowledgedPublishPacket, pubCompPacket);
             }
 
-            return CompletedTask.Instance;
+            return PlatformAbstractionLayer.CompletedTask;
         }
 
         async Task HandleIncomingPublishPacket(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
@@ -392,19 +385,6 @@ namespace MQTTnet.Server
                         return;
                     }
 
-                    // Check for cancellation again because receive packet might block some time.
-                    if (cancellationToken.IsCancellationRequested)
-                    {
-                        return;
-                    }
-
-                    // The TCP connection of this client may be still open but the client has already been taken over by
-                    // a new TCP connection. So we must exit here to make sure to no longer process any message.
-                    if (IsTakenOver || !IsRunning)
-                    {
-                        return;
-                    }
-                    
                     var processPacket = true;
 
                     if (_eventContainer.InterceptingInboundPacketEvent.HasHandlers)
@@ -506,11 +486,6 @@ namespace MQTTnet.Server
                         return;
                     }
 
-                    if (IsTakenOver || !IsRunning)
-                    {
-                        return;
-                    }
-
                     try
                     {
                         await SendPacketAsync(packetBusItem.Packet, cancellationToken).ConfigureAwait(false);

Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs

@@ -24,7 +24,6 @@ namespace MQTTnet.Server
         readonly Dictionary<string, MqttClient> _clients = new Dictionary<string, MqttClient>(4096);
 
         readonly AsyncLock _createConnectionSyncRoot = new AsyncLock();
-
         readonly MqttServerEventContainer _eventContainer;
         readonly MqttNetSourceLogger _logger;
         readonly MqttServerOptions _options;
@@ -76,8 +75,6 @@ namespace MQTTnet.Server
 
         public async Task DeleteSessionAsync(string clientId)
         {
-            _logger.Verbose("Deleting session for client '{0}'.", clientId);
-
             MqttClient connection;
 
             lock (_clients)
@@ -196,7 +193,7 @@ namespace MQTTnet.Server
 
         public void Dispose()
         {
-            _createConnectionSyncRoot.Dispose();
+            _createConnectionSyncRoot?.Dispose();
 
             lock (_sessionsManagementLock)
             {
@@ -306,9 +303,6 @@ namespace MQTTnet.Server
 
                 await client.RunAsync().ConfigureAwait(false);
             }
-            catch (ObjectDisposedException)
-            {
-            }
             catch (OperationCanceledException)
             {
             }
@@ -444,18 +438,13 @@ namespace MQTTnet.Server
             return GetClientSession(clientId).SubscriptionsManager.Unsubscribe(fakeUnsubscribePacket, CancellationToken.None);
         }
 
-        MqttClient CreateClient(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter, MqttSession session)
-        {
-            return new MqttClient(connectPacket, channelAdapter, session, _options, _eventContainer, this, _rootLogger);
-        }
-
         async Task<MqttClient> CreateClientConnection(
             MqttConnectPacket connectPacket,
             MqttConnAckPacket connAckPacket,
             IMqttChannelAdapter channelAdapter,
             ValidatingConnectionEventArgs validatingConnectionEventArgs)
         {
-            MqttClient client;
+            MqttClient connection;
 
             bool sessionShouldPersist;
 
@@ -482,7 +471,7 @@ namespace MQTTnet.Server
                 sessionShouldPersist = !connectPacket.CleanSession;
             }
 
-            using (await _createConnectionSyncRoot.EnterAsync().ConfigureAwait(false))
+            using (await _createConnectionSyncRoot.WaitAsync(CancellationToken.None).ConfigureAwait(false))
             {
                 MqttSession session;
                 lock (_sessionsManagementLock)
@@ -495,7 +484,7 @@ namespace MQTTnet.Server
                     {
                         if (connectPacket.CleanSession)
                         {
-                            _logger.Verbose("Deleting existing session of client '{0}' due to clean start.", connectPacket.ClientId);
+                            _logger.Verbose("Deleting existing session of client '{0}'.", connectPacket.ClientId);
                             session = CreateSession(connectPacket.ClientId, validatingConnectionEventArgs.SessionItems, sessionShouldPersist);
                         }
                         else
@@ -518,40 +507,40 @@ namespace MQTTnet.Server
                     await _eventContainer.PreparingSessionEvent.InvokeAsync(preparingSessionEventArgs).ConfigureAwait(false);
                 }
 
-                MqttClient existingClient;
+                MqttClient existing;
 
                 lock (_clients)
                 {
-                    _clients.TryGetValue(connectPacket.ClientId, out existingClient);
+                    _clients.TryGetValue(connectPacket.ClientId, out existing);
-                    client = CreateClient(connectPacket, channelAdapter, session);
+                    connection = CreateConnection(connectPacket, channelAdapter, session);
 
-                    _clients[connectPacket.ClientId] = client;
+                    _clients[connectPacket.ClientId] = connection;
                 }
 
-                if (existingClient != null)
+                if (existing != null)
                 {
-                    existingClient.IsTakenOver = true;
+                    existing.IsTakenOver = true;
-                    await existingClient.StopAsync(MqttDisconnectReasonCode.SessionTakenOver).ConfigureAwait(false);
+                    await existing.StopAsync(MqttDisconnectReasonCode.SessionTakenOver).ConfigureAwait(false);
 
                     if (_eventContainer.ClientConnectedEvent.HasHandlers)
                     {
-                        var eventArgs = new ClientDisconnectedEventArgs(
+                        var eventArgs = new ClientDisconnectedEventArgs(existing.Id, MqttClientDisconnectType.Takeover, existing.Endpoint, existing.Session.Items);
-                            existingClient.Id,
-                            MqttClientDisconnectType.Takeover,
-                            existingClient.Endpoint,
-                            existingClient.Session.Items);
-                        
                         await _eventContainer.ClientDisconnectedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
                     }
                 }
             }
 
-            return client;
+            return connection;
+        }
+
+        MqttClient CreateConnection(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter, MqttSession session)
+        {
+            return new MqttClient(connectPacket, channelAdapter, session, _options, _eventContainer, this, _rootLogger);
         }
 
         MqttSession CreateSession(string clientId, IDictionary sessionItems, bool isPersistent)
         {
-            _logger.Verbose("Created new session for client '{0}'.", clientId);
+            _logger.Verbose("Created a new session for client '{0}'.", clientId);
 
             return new MqttSession(clientId, isPersistent, sessionItems, _options, _eventContainer, _retainedMessagesManager, this);
         }

Source/MQTTnet/Server/Internal/MqttRetainedMessagesManager.cs

@@ -5,8 +5,10 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
+using MQTTnet.Implementations;
 using MQTTnet.Internal;
 
 namespace MQTTnet.Server
@@ -83,7 +85,7 @@ namespace MQTTnet.Server
                         }
                         else
                         {
-                            if (existingMessage.QualityOfServiceLevel != applicationMessage.QualityOfServiceLevel || !existingMessage.Payload.SequenceEqual(applicationMessage.Payload ?? EmptyBuffer.Array))
+                            if (existingMessage.QualityOfServiceLevel != applicationMessage.QualityOfServiceLevel || !existingMessage.Payload.SequenceEqual(applicationMessage.Payload ?? PlatformAbstractionLayer.EmptyByteArray))
                             {
                                 _messages[applicationMessage.Topic] = applicationMessage;
                                 saveIsRequired = true;
@@ -101,7 +103,7 @@ namespace MQTTnet.Server
 
                 if (saveIsRequired)
                 {
-                    using (await _storageAccessLock.EnterAsync().ConfigureAwait(false))
+                    using (await _storageAccessLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
                     {
                         var eventArgs = new RetainedMessageChangedEventArgs(clientId, applicationMessage, messagesForSave);
                         await _eventContainer.RetainedMessageChangedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
@@ -130,7 +132,7 @@ namespace MQTTnet.Server
                 _messages.Clear();
             }
 
-            using (await _storageAccessLock.EnterAsync().ConfigureAwait(false))
+            using (await _storageAccessLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
             {
                 await _eventContainer.RetainedMessagesClearedEvent.InvokeAsync(EventArgs.Empty).ConfigureAwait(false);
             }

Source/MQTTnet/Server/Internal/MqttServerKeepAliveMonitor.cs

@@ -6,6 +6,7 @@ using System;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
+using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
@@ -13,20 +14,16 @@ namespace MQTTnet.Server
 {
     public sealed class MqttServerKeepAliveMonitor
     {
-        readonly MqttNetSourceLogger _logger;
         readonly MqttServerOptions _options;
         readonly MqttClientSessionsManager _sessionsManager;
+        readonly MqttNetSourceLogger _logger;
 
         public MqttServerKeepAliveMonitor(MqttServerOptions options, MqttClientSessionsManager sessionsManager, IMqttNetLogger logger)
         {
             _options = options ?? throw new ArgumentNullException(nameof(options));
             _sessionsManager = sessionsManager ?? throw new ArgumentNullException(nameof(sessionsManager));
             
-            if (logger == null)
+            if (logger == null) throw new ArgumentNullException(nameof(logger));
-            {
-                throw new ArgumentNullException(nameof(logger));
-            }
-
             _logger = logger.WithSource(nameof(MqttServerKeepAliveMonitor));
         }
 
@@ -48,7 +45,7 @@ namespace MQTTnet.Server
                 while (!cancellationToken.IsCancellationRequested)
                 {
                     TryProcessClients();
-                    Sleep(_options.KeepAliveMonitorInterval);
+                    PlatformAbstractionLayer.Sleep(_options.KeepAliveMonitorInterval);
                 }
             }
             catch (OperationCanceledException)
@@ -64,22 +61,13 @@ namespace MQTTnet.Server
             }
         }
 
-        static void Sleep(TimeSpan timeout)
+        void TryProcessClients()
         {
-#if !NETSTANDARD1_3 && !WINDOWS_UWP
+            var now = DateTime.UtcNow;
-            try
+            foreach (var client in _sessionsManager.GetClients())
             {
-                Thread.Sleep(timeout);
+                TryProcessClient(client, now);
             }
-            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
         }
 
         void TryProcessClient(MqttClient connection, DateTime now)
@@ -131,14 +119,5 @@ namespace MQTTnet.Server
                 _logger.Error(exception, "Client {0}: Unhandled exception while checking keep alive timeouts.", connection.Id);
             }
         }
-
-        void TryProcessClients()
-        {
-            var now = DateTime.UtcNow;
-            foreach (var client in _sessionsManager.GetClients())
-            {
-                TryProcessClient(client, now);
-            }
-        }
     }
 }

Source/MQTTnet/Server/MqttServer.cs

@@ -11,6 +11,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
+using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
@@ -171,7 +172,7 @@ namespace MQTTnet.Server
         {
             ThrowIfNotStarted();
 
-            return _retainedMessagesManager?.ClearMessages() ?? CompletedTask.Instance;
+            return _retainedMessagesManager?.ClearMessages() ?? PlatformAbstractionLayer.CompletedTask;
         }
 
         public Task DisconnectClientAsync(string id, MqttDisconnectReasonCode reasonCode)

Source/MQTTnet/Server/Status/MqttSessionStatus.cs

@@ -6,6 +6,7 @@ using System;
 using System.Collections;
 using System.Threading.Tasks;
 using MQTTnet.Formatter;
+using MQTTnet.Implementations;
 using MQTTnet.Internal;
 
 namespace MQTTnet.Server
@@ -34,7 +35,7 @@ namespace MQTTnet.Server
             var publishPacketFactory = new MqttPublishPacketFactory();
             _session.EnqueueDataPacket(new MqttPacketBusItem(publishPacketFactory.Create(applicationMessage)));
             
-            return CompletedTask.Instance;
+            return PlatformAbstractionLayer.CompletedTask;
         }
 
         public Task DeliverApplicationMessageAsync(MqttApplicationMessage applicationMessage)