Add unit test to reproduce issue

fix cross access to async events.

.github/workflows/ReleaseNotes.md

@@ -0,0 +1,5 @@
+* [Core] Fixed several dead locks by implementing a new async signal (#1552)
+* [Client] All waiting tasks will now fail when the client connection gets closed. This fixes several freezes (#1561)
+* [Server] Fixed duplicated invocation of the event _ClientAcknowledgedPublishPacketAsync_ for QoS level 2 (#1550)
+* [Server] Fixed issue in upgrading and downgrading of QoS levels for subscriptions and retained messages (#1560)
+* [Server] Fixed memory leak when old sessions are discarded (#1553)

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ name: CI
 on: [push, pull_request]
 
 env:
-  VERSION: "4.1.0.${{github.run_number}}"
+  VERSION: "4.1.3.${{github.run_number}}"
 
 jobs:
   build:

MQTTnet.sln.DotSettings

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

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.
+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.
 
 ## Features
 
@@ -50,39 +50,6 @@ 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.Shared;
+using MQTTnet.Samples.Helpers;
 
 namespace MQTTnet.Samples.Client;
 
@@ -119,7 +119,7 @@ public static class Client_Connection_Samples
                         // The used public broker sometimes has invalid certificates. This sample accepts all
                         // certificates. This should not be used in live environments.
                         o.CertificateValidationHandler = _ => true;
-                        
+
                         // The default value is determined by the OS. Set manually to force version.
                         o.SslProtocol = SslProtocols.Tls12;
                     })
@@ -134,15 +134,17 @@ public static class Client_Connection_Samples
         }
     }
 
-    public static async Task Connect_Client_Using_WebSockets()
+    public static async Task Connect_Client_Using_WebSocket4Net()
     {
         /*
          * This sample creates a simple MQTT client and connects to a public broker using a WebSocket connection.
+         * Instead of the .NET implementation of WebSockets the implementaion from WebSocket4Net is used. It provides more
+         * encryption algorithms and supports more platforms.
          * 
          * This is a modified version of the sample _Connect_Client_! See other sample for more details.
          */
 
-        var mqttFactory = new MqttFactory();
+        var mqttFactory = new MqttFactory().UseWebSocket4Net();
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
@@ -155,18 +157,16 @@ public static class Client_Connection_Samples
             response.DumpToConsole();
         }
     }
-    
-    public static async Task Connect_Client_Using_WebSocket4Net()
+
+    public static async Task Connect_Client_Using_WebSockets()
     {
         /*
          * This sample creates a simple MQTT client and connects to a public broker using a WebSocket connection.
-         * Instead of the .NET implementation of WebSockets the implementaion from WebSocket4Net is used. It provides more
-         * encryption algorithms and supports more platforms.
          * 
          * This is a modified version of the sample _Connect_Client_! See other sample for more details.
          */
 
-        var mqttFactory = new MqttFactory().UseWebSocket4Net();
+        var mqttFactory = new MqttFactory();
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
@@ -206,7 +206,7 @@ public static class Client_Connection_Samples
             using (var timeout = new CancellationTokenSource(5000))
             {
                 var response = await mqttClient.ConnectAsync(mqttClientOptions, timeout.Token);
-                
+
                 Console.WriteLine("The MQTT client is connected.");
 
                 response.DumpToConsole();
@@ -214,6 +214,49 @@ public static class Client_Connection_Samples
         }
     }
 
+    public static async Task Disconnect_Clean()
+    {
+        /*
+         * This sample disconnects from the server with sending a DISCONNECT packet.
+         * This way of disconnecting is treated as a clean disconnect which will not
+         * trigger sending the last will etc.
+         */
+
+        var mqttFactory = new MqttFactory();
+
+        using (var mqttClient = mqttFactory.CreateMqttClient())
+        {
+            var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
+
+            await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
+
+            // Calling _DisconnectAsync_ will send a DISCONNECT packet before closing the connection.
+            // Using a reason code requires MQTT version 5.0.0!
+            await mqttClient.DisconnectAsync(MqttClientDisconnectReason.ImplementationSpecificError);
+        }
+    }
+
+    public static async Task Disconnect_Non_Clean()
+    {
+        /*
+         * This sample disconnects from the server without sending a DISCONNECT packet.
+         * This way of disconnecting is treated as a non clean disconnect which will
+         * trigger sending the last will etc.
+         */
+
+        var mqttFactory = new MqttFactory();
+
+        var mqttClient = mqttFactory.CreateMqttClient();
+
+        var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
+
+        await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
+
+        // Calling _Dispose_ or use of a _using_ statement will close the transport connection
+        // without sending a DISCONNECT packet to the server.
+        mqttClient.Dispose();
+    }
+
     public static async Task Inspect_Certificate_Validation_Errors()
     {
         /*
@@ -248,7 +291,7 @@ public static class Client_Connection_Samples
             }
         }
     }
-    
+
     public static async Task Ping_Server()
     {
         /*
@@ -321,14 +364,12 @@ 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.
-                            if (!mqttClient.IsConnected)
+                            // This code will also do the very first connect! So no call to _ConnectAsync_ is required in the first place.
+                            if (!await mqttClient.TryPingAsync())
                             {
                                 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

@@ -40,6 +40,53 @@ 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

@@ -7,7 +7,9 @@
 // ReSharper disable InconsistentNaming
 
 using MQTTnet.Client;
-using MQTTnet.Samples.Shared;
+using MQTTnet.Packets;
+using MQTTnet.Protocol;
+using MQTTnet.Samples.Helpers;
 
 namespace MQTTnet.Samples.Client;
 
@@ -23,9 +25,7 @@ public static class Client_Subscribe_Samples
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
-            var mqttClientOptions = new MqttClientOptionsBuilder()
-                .WithTcpServer("broker.hivemq.com")
-                .Build();
+            var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
 
             // Setup message handling before connecting so that queued messages
             // are also handled properly. When there is no event handler attached all
@@ -41,7 +41,11 @@ public static class Client_Subscribe_Samples
             await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
 
             var mqttSubscribeOptions = mqttFactory.CreateSubscribeOptionsBuilder()
-                .WithTopicFilter(f => { f.WithTopic("mqttnet/samples/topic/2"); })
+                .WithTopicFilter(
+                    f =>
+                    {
+                        f.WithTopic("mqttnet/samples/topic/2");
+                    })
                 .Build();
 
             await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);
@@ -53,6 +57,95 @@ 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()
     {
         /*
@@ -63,14 +156,16 @@ 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(f => { f.WithTopic("mqttnet/samples/topic/1"); })
+                .WithTopicFilter(
+                    f =>
+                    {
+                        f.WithTopic("mqttnet/samples/topic/1");
+                    })
                 .Build();
 
             var response = await mqttClient.SubscribeAsync(mqttSubscribeOptions, CancellationToken.None);

Samples/Diagnostics/Logger_Samples.cs

@@ -8,7 +8,6 @@
 
 using System.Text;
 using MQTTnet.Diagnostics;
-using MQTTnet.Samples.Shared;
 
 namespace MQTTnet.Samples.Diagnostics;
 
@@ -23,7 +22,9 @@ public static class Logger_Samples
 
         var mqttFactory = new MqttFactory(new MyLogger());
 
-        var mqttClientOptions = mqttFactory.CreateClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
+        var mqttClientOptions = mqttFactory.CreateClientOptionsBuilder()
+            .WithTcpServer("broker.hivemq.com")
+            .Build();
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
@@ -31,31 +32,13 @@ public static class Logger_Samples
 
             Console.WriteLine("MQTT client is connected.");
 
-            var mqttClientDisconnectOptions = mqttFactory.CreateClientDisconnectOptionsBuilder().Build();
+            var mqttClientDisconnectOptions = mqttFactory.CreateClientDisconnectOptionsBuilder()
+                .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()
     {
         /*
@@ -81,7 +64,9 @@ public static class Logger_Samples
 
         var mqttFactory = new MqttFactory(mqttEventLogger);
 
-        var mqttClientOptions = mqttFactory.CreateClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build();
+        var mqttClientOptions = mqttFactory.CreateClientOptionsBuilder()
+            .WithTcpServer("broker.hivemq.com")
+            .Build();
 
         using (var mqttClient = mqttFactory.CreateMqttClient())
         {
@@ -89,7 +74,8 @@ public static class Logger_Samples
 
             Console.WriteLine("MQTT client is connected.");
 
-            var mqttClientDisconnectOptions = mqttFactory.CreateClientDisconnectOptionsBuilder().Build();
+            var mqttClientDisconnectOptions = mqttFactory.CreateClientDisconnectOptionsBuilder()
+                .Build();
 
             await mqttClient.DisconnectAsync(mqttClientDisconnectOptions, CancellationToken.None);
         }
@@ -97,7 +83,7 @@ public static class Logger_Samples
 
     sealed class MyLogger : IMqttNetLogger
     {
-        public bool IsEnabled { get; } = true;
+        public bool IsEnabled { get; set; } = true;
 
         public void Publish(MqttNetLogLevel logLevel, string source, string message, object[] parameters, Exception exception)
         {

Samples/Helpers/ObjectExtensions.cs

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

Samples/Program.cs

@@ -8,7 +8,6 @@ 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_Intercepting_Samples.cs

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

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;
@@ -170,4 +170,50 @@ 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

@@ -1,53 +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 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);
-            }
-        }
-    }
-}

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

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

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

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

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

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

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

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

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

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

Source/MQTTnet.AspNetCore.Tests/MqttConnectionContextTest.cs

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

Source/MQTTnet.AspNetCore.Tests/ReaderExtensionsTest.cs

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

Source/MQTTnet.AspnetCore/MqttConnectionContext.cs

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

Source/MQTTnet.Benchmarks/AsyncLockBenchmark.cs

@@ -0,0 +1,66 @@
+// 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

@@ -12,14 +12,10 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="BenchmarkDotNet" Version="0.13.1" />
+        <PackageReference Include="BenchmarkDotNet" Version="0.13.2" />
         <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

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

Source/MQTTnet.Benchmarks/Program.cs

@@ -29,6 +29,7 @@ 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)
@@ -75,6 +76,9 @@ 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(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+            public Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken)
             {
                 throw new NotSupportedException();
             }

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs

@@ -167,7 +167,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
             try
             {
-                using (await _messageQueueLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+                using (await _messageQueueLock.EnterAsync().ConfigureAwait(false))
                 {
                     if (_messageQueue.Count >= Options.MaxPendingMessages)
                     {
@@ -296,7 +296,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
             _subscriptionsQueuedSignal.Release();
 
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public Task UnsubscribeAsync(ICollection<string> topics)
@@ -319,7 +319,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
             _subscriptionsQueuedSignal.Release();
 
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         protected override void Dispose(bool disposing)
@@ -693,7 +693,7 @@ namespace MQTTnet.Extensions.ManagedClient
             {
                 await InternalClient.PublishAsync(message.ApplicationMessage).ConfigureAwait(false);
 
-                using (await _messageQueueLock.WaitAsync(CancellationToken.None).ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
+                using (await _messageQueueLock.EnterAsync().ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
                 {
                     // While publishing this message, this.PublishAsync could have booted this
                     // message off the queue to make room for another (when using a cap
@@ -724,7 +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.WaitAsync(CancellationToken.None).ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
+                    using (await _messageQueueLock.EnterAsync().ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
                     {
                         _messageQueue.RemoveFirst(i => i.Id.Equals(message.Id));
 

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClientStorageManager.cs

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

Source/MQTTnet.Extensions.Rpc/MqttRpcClient.cs

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

Source/MQTTnet.Extensions.WebSocket4Net/WebSocket4NetMqttChannel.cs

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

Source/MQTTnet.TestApp/AsyncLockTest.cs

@@ -0,0 +1,28 @@
+// 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.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.TestApp
@@ -40,7 +40,7 @@ namespace MQTTnet.TestApp
                     Console.WriteLine($"+ Retain = {e.ApplicationMessage.Retain}");
                     Console.WriteLine();
                     
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 client.ConnectedAsync += async e =>

Source/MQTTnet.TestApp/ManagedClientTest.cs

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

Source/MQTTnet.TestApp/MessageThroughputTest.cs

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

Source/MQTTnet.TestApp/Program.cs

@@ -28,6 +28,7 @@ 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')
@@ -88,6 +89,10 @@ 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.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.TestApp
@@ -144,7 +144,7 @@ namespace MQTTnet.TestApp
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.ConnectAsync(options);

Source/MQTTnet.TestApp/ServerTest.cs

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

Source/MQTTnet.Tests/Client/ManagedMqttClient_Tests.cs

@@ -11,18 +11,62 @@ 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
 {
     [TestClass]
     public sealed class ManagedMqttClient_Tests : BaseTestClass
     {
+        [TestMethod]
+        public async Task Subscribe_When_Offline_But_Existing_Session()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer(o => o.WithPersistentSessions());
+                
+                // The connected client will crate a new session.
+                var mqttClient = await testEnvironment.ConnectClient();
+                mqttClient.Dispose();
+                
+                mqttClient = testEnvironment.CreateClient();
+                var managedClient = testEnvironment.Factory.CreateManagedMqttClient(mqttClient);
+                
+                // Subscribe before starting!
+                await managedClient.SubscribeAsync("a");
+
+                MqttApplicationMessage applicationMessage = null;
+                managedClient.ApplicationMessageReceivedAsync += e =>
+                {
+                    applicationMessage = e.ApplicationMessage;
+                    return CompletedTask.Instance;
+                };
+                
+                // Now the client must send the new subscription. 
+                await managedClient.StartAsync(
+                    new ManagedMqttClientOptions
+                    {
+                        ClientOptions = testEnvironment.Factory.CreateClientOptionsBuilder().WithCleanSession(false).WithTcpServer("127.0.0.1", testEnvironment.ServerPort).Build()
+                    });
+
+                await LongTestDelay();
+                
+                await server.InjectApplicationMessage(
+                    new InjectedMqttApplicationMessage(
+                        new MqttApplicationMessage
+                        {
+                            Topic = "a"
+                        }));
+
+                await LongTestDelay();
+                
+                Assert.IsNotNull(applicationMessage);
+            }
+        }
+        
         [TestMethod]
         public async Task Connect_To_Invalid_Server()
         {
@@ -31,7 +75,6 @@ namespace MQTTnet.Tests.Client
                 testEnvironment.IgnoreClientLogErrors = true;
 
                 var mqttClient = testEnvironment.CreateClient();
-
                 var managedClient = testEnvironment.Factory.CreateManagedMqttClient(mqttClient);
 
                 ConnectingFailedEventArgs connectingFailedEventArgs = null;
@@ -39,7 +82,7 @@ namespace MQTTnet.Tests.Client
                 managedClient.ConnectingFailedAsync += args =>
                 {
                     connectingFailedEventArgs = args;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await managedClient.StartAsync(
@@ -117,7 +160,7 @@ namespace MQTTnet.Tests.Client
                 receivingClient.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await receivingClient.SubscribeAsync("My/last/will");
@@ -281,7 +324,7 @@ namespace MQTTnet.Tests.Client
                     });
 
                 // Wait a bit for the retained message to be available
-                await Task.Delay(1000);
+                await LongTestDelay();
 
                 await sendingClient.DisconnectAsync();
 
@@ -290,28 +333,29 @@ namespace MQTTnet.Tests.Client
                 var clientOptions = new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort);
 
                 var receivedManagedMessages = new List<MqttApplicationMessage>();
-                var managedClient = new ManagedMqttClient(testEnvironment.CreateClient(), new MqttNetEventLogger());
+                
+                var managedClient = testEnvironment.Factory.CreateManagedMqttClient(testEnvironment.CreateClient());
                 managedClient.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedManagedMessages.Add(e.ApplicationMessage);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await managedClient.SubscribeAsync("topic");
 
                 await managedClient.StartAsync(new ManagedMqttClientOptionsBuilder().WithClientOptions(clientOptions).WithAutoReconnectDelay(TimeSpan.FromSeconds(1)).Build());
 
-                await Task.Delay(1000);
+                await LongTestDelay();
 
                 Assert.AreEqual(1, receivedManagedMessages.Count);
 
                 await managedClient.StopAsync();
 
-                await Task.Delay(500);
+                await LongTestDelay();
 
                 await managedClient.StartAsync(new ManagedMqttClientOptionsBuilder().WithClientOptions(clientOptions).WithAutoReconnectDelay(TimeSpan.FromSeconds(1)).Build());
 
-                await Task.Delay(1000);
+                await LongTestDelay();
 
                 // After reconnect and then some delay, the retained message must not be received,
                 // showing that the subscriptions were cleared
@@ -319,7 +363,9 @@ namespace MQTTnet.Tests.Client
 
                 // Make sure that it gets received after subscribing again.
                 await managedClient.SubscribeAsync("topic");
-                await Task.Delay(500);
+                
+                await LongTestDelay();
+                
                 Assert.AreEqual(2, receivedManagedMessages.Count);
             }
         }
@@ -428,7 +474,7 @@ namespace MQTTnet.Tests.Client
             managedClient.ConnectedAsync += e =>
             {
                 connected.TrySetResult(true);
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             };
 
             return connected.Task;
@@ -457,7 +503,7 @@ namespace MQTTnet.Tests.Client
                     result.TrySetResult(receivedMessages);
                 }
 
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             };
 
             return result.Task;
@@ -486,7 +532,7 @@ namespace MQTTnet.Tests.Client
         public Task SaveQueuedMessagesAsync(IList<ManagedMqttApplicationMessage> messages)
         {
             _messages = messages;
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
     }
 }

Source/MQTTnet.Tests/Client/MqttClient_Tests.cs

@@ -14,15 +14,17 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Tests.Mockups;
 
+// ReSharper disable InconsistentNaming
+
 namespace MQTTnet.Tests.Client
 {
     [TestClass]
-    public sealed class Client_Tests : BaseTestClass
+    public sealed class MqttClient_Tests : BaseTestClass
     {
         [DataTestMethod]
         [DataRow(MqttQualityOfServiceLevel.ExactlyOnce)]
@@ -56,7 +58,7 @@ namespace MQTTnet.Tests.Client
                     }
 
                     _ = InvokeInternal();
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var publishes = Task.WhenAll(publisher.PublishStringAsync("a", null, qos), publisher.PublishStringAsync("b", null, qos));
@@ -107,7 +109,7 @@ namespace MQTTnet.Tests.Client
                     client.DisconnectedAsync += args =>
                     {
                         disconnectHandlerCalled = true;
-                        return PlatformAbstractionLayer.CompletedTask;
+                        return CompletedTask.Instance;
                     };
 
                     await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("1.2.3.4").Build());
@@ -121,7 +123,7 @@ namespace MQTTnet.Tests.Client
                     //Assert.IsInstanceOfType(exception.InnerException, typeof(SocketException));
                 }
 
-                await Task.Delay(100); // disconnected handler is called async
+                await LongTestDelay(); // disconnected handler is called async
                 Assert.IsTrue(disconnectHandlerCalled);
             }
         }
@@ -136,7 +138,7 @@ namespace MQTTnet.Tests.Client
                 client.DisconnectedAsync += e =>
                 {
                     ex = e.Exception;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 try
@@ -145,6 +147,7 @@ namespace MQTTnet.Tests.Client
                 }
                 catch
                 {
+                    // Ignore errors.
                 }
 
                 await Task.Delay(500);
@@ -167,7 +170,7 @@ namespace MQTTnet.Tests.Client
                 server.InterceptingPublishAsync += c =>
                 {
                     i++;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.SendAsync(
@@ -208,7 +211,7 @@ namespace MQTTnet.Tests.Client
                 client.DisconnectedAsync += e =>
                 {
                     handlerFired = true;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await server.StopAsync();
@@ -239,10 +242,10 @@ namespace MQTTnet.Tests.Client
 
                 for (var i = 0; i < 98; i++)
                 {
-                    Assert.IsFalse(clients[i].IsConnected, $"clients[{i}] is not connected");
+                    Assert.IsFalse(clients[i].TryPingAsync().GetAwaiter().GetResult(), $"clients[{i}] is not connected");
                 }
 
-                Assert.IsTrue(clients[99].IsConnected);
+                Assert.IsTrue(clients[99].TryPingAsync().GetAwaiter().GetResult());
 
                 Assert.AreEqual(1, clientStatus.Count);
                 Assert.AreEqual(1, sessionStatus.Count);
@@ -252,7 +255,7 @@ namespace MQTTnet.Tests.Client
                 receiveClient.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedPayload = e.ApplicationMessage.ConvertPayloadToString();
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await receiveClient.SubscribeAsync("x");
@@ -310,7 +313,7 @@ namespace MQTTnet.Tests.Client
                 receiver.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await sender.PublishAsync(message.Build(), CancellationToken.None);
@@ -332,7 +335,7 @@ namespace MQTTnet.Tests.Client
 
                 var client = await testEnvironment.ConnectClient();
 
-                Assert.IsTrue(client.IsConnected);
+                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
             }
         }
 
@@ -343,11 +346,11 @@ namespace MQTTnet.Tests.Client
             {
                 await testEnvironment.StartServer();
                 var client = await testEnvironment.ConnectClient();
-                Assert.IsTrue(client.IsConnected);
+                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
 
                 await client.DisconnectAsync();
 
-                Assert.IsFalse(client.IsConnected);
+                Assert.IsFalse(client.TryPingAsync().GetAwaiter().GetResult());
             }
         }
 
@@ -453,7 +456,7 @@ namespace MQTTnet.Tests.Client
                         receivedValues.Add(value);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 }
 
                 client1.ApplicationMessageReceivedAsync += Handler1;
@@ -531,7 +534,7 @@ namespace MQTTnet.Tests.Client
                 client2.ApplicationMessageReceivedAsync += e =>
                 {
                     client2TopicResults.Add(Encoding.UTF8.GetString(e.ApplicationMessage.Payload));
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client2.SubscribeAsync(client2Topic);
@@ -566,7 +569,7 @@ namespace MQTTnet.Tests.Client
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client1.SubscribeAsync("a");
@@ -591,17 +594,17 @@ namespace MQTTnet.Tests.Client
                 var client = await testEnvironment.ConnectClient();
 
                 await Task.Delay(500);
-                Assert.IsTrue(client.IsConnected);
+                Assert.IsTrue(await client.TryPingAsync());
 
                 await server.StopAsync();
                 await Task.Delay(500);
-                Assert.IsFalse(client.IsConnected);
+                Assert.IsFalse(await client.TryPingAsync());
 
                 await server.StartAsync();
                 await Task.Delay(500);
 
                 await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort).Build());
-                Assert.IsTrue(client.IsConnected);
+                Assert.IsTrue(await client.TryPingAsync());
             }
         }
 
@@ -637,6 +640,7 @@ namespace MQTTnet.Tests.Client
                 }
                 catch
                 {
+                    // Ignore errors.
                 }
 
                 SpinWait.SpinUntil(() => tries >= maxTries, 10000);
@@ -656,11 +660,11 @@ namespace MQTTnet.Tests.Client
                 var client = await testEnvironment.ConnectClient();
 
                 await Task.Delay(500);
-                Assert.IsTrue(client.IsConnected);
+                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
 
                 await server.StopAsync();
                 await Task.Delay(500);
-                Assert.IsFalse(client.IsConnected);
+                Assert.IsFalse(client.TryPingAsync().GetAwaiter().GetResult());
 
                 for (var i = 0; i < 5; i++)
                 {
@@ -671,6 +675,7 @@ namespace MQTTnet.Tests.Client
                     }
                     catch
                     {
+                        // Ignore errors.
                     }
                 }
 
@@ -678,7 +683,7 @@ namespace MQTTnet.Tests.Client
                 await Task.Delay(500);
 
                 await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort).Build());
-                Assert.IsTrue(client.IsConnected);
+                await client.PingAsync();
             }
         }
 
@@ -723,7 +728,7 @@ namespace MQTTnet.Tests.Client
                         replies.Add(eventArgs.ApplicationMessage.Topic);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 }
 
                 client2.ApplicationMessageReceivedAsync += Handler2;
@@ -761,7 +766,7 @@ namespace MQTTnet.Tests.Client
                         replyReceived = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 client2.ApplicationMessageReceivedAsync += async e =>
@@ -812,7 +817,7 @@ namespace MQTTnet.Tests.Client
                         replyReceived = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.PublishStringAsync("request", null, MqttQualityOfServiceLevel.AtLeastOnce);
@@ -828,14 +833,14 @@ namespace MQTTnet.Tests.Client
         {
             using (var testEnvironment = new TestEnvironment(TestContext))
             {
-                var server = await testEnvironment.StartServer();
+                await testEnvironment.StartServer();
                 var client = await testEnvironment.ConnectClient();
 
-                Assert.IsTrue(client.IsConnected);
+                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
                 client.DisconnectedAsync += e =>
                 {
                     Assert.IsTrue(e.ClientWasConnected);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.DisconnectAsync();
@@ -851,11 +856,11 @@ namespace MQTTnet.Tests.Client
                 var server = await testEnvironment.StartServer();
                 var client = await testEnvironment.ConnectClient();
 
-                Assert.IsTrue(client.IsConnected);
+                Assert.IsTrue(client.TryPingAsync().GetAwaiter().GetResult());
                 client.DisconnectedAsync += e =>
                 {
                     Assert.IsTrue(e.ClientWasConnected);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await server.StopAsync();
@@ -892,7 +897,7 @@ namespace MQTTnet.Tests.Client
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("localhost", testEnvironment.ServerPort).Build());
@@ -917,14 +922,14 @@ namespace MQTTnet.Tests.Client
                 client1.DisconnectedAsync += c =>
                 {
                     disconnectedFired = true;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var messageReceived = false;
                 client1.ApplicationMessageReceivedAsync += e =>
                 {
                     messageReceived = true;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("topic1").WithExactlyOnceQoS().Build());
@@ -941,7 +946,7 @@ namespace MQTTnet.Tests.Client
                 await Task.Delay(500);
 
                 Assert.IsTrue(messageReceived);
-                Assert.IsTrue(client1.IsConnected);
+                Assert.IsTrue(client1.TryPingAsync().GetAwaiter().GetResult());
                 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.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Diagnostics
 {
@@ -33,7 +33,7 @@ namespace MQTTnet.Tests.Diagnostics
                     mqttClient.InspectPackage += eventArgs =>
                     {
                         packets.Add(eventArgs.Direction + ":" + Convert.ToBase64String(eventArgs.Buffer));
-                        return PlatformAbstractionLayer.CompletedTask;
+                        return CompletedTask.Instance;
                     };
                 
                     await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);

Source/MQTTnet.Tests/Factory/MqttFactory_Tests.cs

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

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V3_Binary_Tests.cs

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

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V3_Tests.cs

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

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V5_Tests.cs

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

Source/MQTTnet.Tests/Internal/AsyncEvent_Tests.cs

@@ -6,7 +6,6 @@ using System;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Internal
@@ -16,6 +15,28 @@ namespace MQTTnet.Tests.Internal
     {
         int _testEventAsyncCount;
 
+        [TestMethod]
+        public async Task Attach_and_Detach_Sync_And_Async_EventHandler()
+        {
+            var testClass = new TestClass();
+
+            testClass.TestEventAsync += OnTestEventAsync;
+            testClass.TestEvent += OnTestEvent;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+            Assert.AreEqual(2, _testEventAsyncCount);
+
+            testClass.TestEvent -= OnTestEvent;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+            Assert.AreEqual(3, _testEventAsyncCount);
+
+            testClass.TestEventAsync -= OnTestEventAsync;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+            Assert.AreEqual(3, _testEventAsyncCount);
+        }
+
         [TestMethod]
         public async Task Attach_EventHandler()
         {
@@ -28,6 +49,19 @@ namespace MQTTnet.Tests.Internal
             Assert.AreEqual(1, _testEventAsyncCount);
         }
 
+        [TestMethod]
+        public async Task Attach_Sync_And_Async_EventHandler()
+        {
+            var testClass = new TestClass();
+
+            testClass.TestEventAsync += OnTestEventAsync;
+            testClass.TestEvent += OnTestEvent;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+
+            Assert.AreEqual(2, _testEventAsyncCount);
+        }
+
         [TestMethod]
         public async Task Detach_EventHandler()
         {
@@ -41,22 +75,62 @@ 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 PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         sealed class TestClass
         {
             readonly AsyncEvent<EventArgs> _testEvent = new AsyncEvent<EventArgs>();
 
+            public event Action<EventArgs> TestEvent
+            {
+                add => _testEvent.AddHandler(value);
+                remove => _testEvent.RemoveHandler(value);
+            }
+
             public event Func<EventArgs, Task> TestEventAsync
             {
                 add => _testEvent.AddHandler(value);
                 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

@@ -3,6 +3,7 @@
 // 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;
@@ -11,8 +12,186 @@ using MQTTnet.Internal;
 namespace MQTTnet.Tests.Internal
 {
     [TestClass]
-    public class AsyncLock_Tests
+    public sealed 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()
         {
@@ -21,7 +200,7 @@ namespace MQTTnet.Tests.Internal
             var @lock = new AsyncLock();
             for (var i = 0; i < 100; i++)
             {
-                using (await @lock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+                using (await @lock.EnterAsync().ConfigureAwait(false))
                 {
                     sum++;
                 }
@@ -37,66 +216,76 @@ namespace MQTTnet.Tests.Internal
             var @lock = new AsyncLock();
 
             // This call will never "release" the lock due to missing _using_.
-            await @lock.WaitAsync(CancellationToken.None).ConfigureAwait(false);
+            await @lock.EnterAsync().ConfigureAwait(false);
 
             using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
             {
-                await @lock.WaitAsync(cts.Token).ConfigureAwait(false);
+                await @lock.EnterAsync(cts.Token).ConfigureAwait(false);
             }
         }
 
-        //[TestMethod]
-        //public async Task Test_Cancellation_With_Later_Access()
-        //{
-        //    var @lock = new AsyncLock();
-
-        //    var releaser = await @lock.WaitAsync().ConfigureAwait(false);
-
-        //    try
-        //    {
-        //        using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
-        //        {
-        //            await @lock.WaitAsync(cts.Token).ConfigureAwait(false);
-        //        }
-        //    }
-        //    catch (OperationCanceledException)
-        //    {
-        //    }
-            
-        //    releaser.Dispose();
-
-        //    using (await @lock.WaitAsync().ConfigureAwait(false))
-        //    {
-        //        // When the method finished, the thread got access.
-        //    }
-        //}
-
         [TestMethod]
-        public void Lock_10_Parallel_Tasks()
+        public async Task Test_Cancellation_With_Later_Access()
         {
-            const int ThreadsCount = 10;
+            var asyncLock = new AsyncLock();
 
-            var threads = new Task[ThreadsCount];
-            var @lock = new AsyncLock();
-            var globalI = 0;
-            for (var i = 0; i < ThreadsCount; i++)
+            var releaser = await asyncLock.EnterAsync().ConfigureAwait(false);
+
+            try
             {
-#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 (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
                 {
-                    using (var releaser = await @lock.WaitAsync(CancellationToken.None))
-                    {
-                        var localI = globalI;
-                        await Task.Delay(10); // 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
+                    await asyncLock.EnterAsync(timeout.Token).ConfigureAwait(false);
+                }
+
+                Assert.Fail("Exception should be thrown!");
+            }
+            catch (OperationCanceledException)
+            {
             }
 
-            Task.WaitAll(threads);
-            Assert.AreEqual(ThreadsCount, globalI);
+            releaser.Dispose();
+
+            using (await asyncLock.EnterAsync(CancellationToken.None).ConfigureAwait(false))
+            {
+                // When the method finished, the thread got access.
+            }
+        }
+
+        [TestMethod]
+        public async Task Use_After_Cancellation()
+        {
+            var @lock = new AsyncLock();
+
+            // This call will not yet "release" the lock due to missing _using_.
+            var releaser = await @lock.EnterAsync().ConfigureAwait(false);
+
+            try
+            {
+                using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
+                {
+                    await @lock.EnterAsync(cts.Token).ConfigureAwait(false);
+                }
+            }
+            catch (OperationCanceledException)
+            {
+                // Expected exception!
+            }
+
+            releaser.Dispose();
+
+            // Regular usage after cancellation.
+            using (await @lock.EnterAsync().ConfigureAwait(false))
+            {
+            }
+
+            using (await @lock.EnterAsync().ConfigureAwait(false))
+            {
+            }
+
+            using (await @lock.EnterAsync().ConfigureAwait(false))
+            {
+            }
         }
     }
-}
+}

Source/MQTTnet.Tests/Internal/AsyncSignal_Tests.cs

@@ -0,0 +1,114 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Diagnostics;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Internal;
+
+namespace MQTTnet.Tests.Internal
+{
+    [TestClass]
+    public sealed class AsyncSignal_Tests
+    {
+        [TestMethod]
+        [ExpectedException(typeof(TaskCanceledException))]
+        public async Task Cancel_If_No_Signal()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(2)))
+            {
+                await asyncSignal.WaitAsync(timeout.Token);
+
+                Assert.Fail("There is no signal. So we must fail here!");
+            }
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(ObjectDisposedException))]
+        public async Task Dispose_Properly()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            // The timeout will not be reached but another task will kill the async signal.
+            using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(999)))
+            {
+                _ = Task.Run(
+                    async () =>
+                    {
+                        await Task.Delay(2000, CancellationToken.None);
+                        asyncSignal.Dispose();
+                    },
+                    CancellationToken.None);
+
+                await asyncSignal.WaitAsync(timeout.Token);
+
+                Assert.Fail("There is no signal. So we must fail here!");
+            }
+        }
+
+        [TestMethod]
+        public async Task Loop_Signal()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            for (var i = 0; i < 10; i++)
+            {
+                asyncSignal.Set();
+                
+                // WaitAsync should return directly because the signal is available.
+                await asyncSignal.WaitAsync();
+            }
+        }
+
+        [TestMethod]
+        public async Task Signal()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            // The timeout will not be reached but another task will kill the async signal.
+            using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(3)))
+            {
+                var stopwatch = Stopwatch.StartNew();
+
+                _ = Task.Run(
+                    async () =>
+                    {
+                        await Task.Delay(1000, CancellationToken.None);
+                        asyncSignal.Set();
+                    },
+                    CancellationToken.None);
+
+                await asyncSignal.WaitAsync(timeout.Token);
+
+                stopwatch.Stop();
+
+                Assert.IsTrue(stopwatch.ElapsedMilliseconds > 900);
+            }
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(InvalidOperationException))]
+        public async Task Fail_For_Two_Waiters()
+        {
+            var asyncSignal = new AsyncSignal();
+
+            // This thread will wait properly because it is the first waiter.
+            _ = Task.Run(
+                async () =>
+                {
+                    await asyncSignal.WaitAsync();
+                },
+                CancellationToken.None);
+
+            await Task.Delay(1000);
+            
+            // Now the current thread must fail because there is already a waiter.
+            await asyncSignal.WaitAsync();
+        }
+    }
+}

Source/MQTTnet.Tests/Internal/MqttPacketBus_Tests.cs

@@ -15,7 +15,7 @@ namespace MQTTnet.Tests.Internal
     public sealed class MqttPacketBus_Tests
     {
         [TestMethod]
-        [ExpectedException(typeof(OperationCanceledException))]
+        [ExpectedException(typeof(TaskCanceledException))]
         public async Task Wait_With_Empty_Bus()
         {
             var bus = new MqttPacketBus();

Source/MQTTnet.Tests/MQTTnet.Tests.csproj

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

Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs

@@ -8,10 +8,9 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
-using MQTTnet.Tests.Mockups;
 
 namespace MQTTnet.Tests.MQTTv5
 {
@@ -43,7 +42,7 @@ namespace MQTTnet.Tests.MQTTv5
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.PublishAsync(new MqttApplicationMessageBuilder()
@@ -245,7 +244,7 @@ namespace MQTTnet.Tests.MQTTv5
                 receiver.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedMessage = e.ApplicationMessage;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var sender = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithProtocolVersion(MqttProtocolVersion.V500));

Source/MQTTnet.Tests/MQTTv5/Server_Tests.cs

@@ -8,7 +8,7 @@ using MQTTnet.Formatter;
 using MQTTnet.Tests.Mockups;
 using System.Threading;
 using System.Threading.Tasks;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Tests.MQTTv5
@@ -31,7 +31,7 @@ namespace MQTTnet.Tests.MQTTv5
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
@@ -171,11 +171,53 @@ namespace MQTTnet.Tests.MQTTv5
                 // Session should be present
 
                 Assert.IsTrue(!result3.IsSessionPresent, "Session is present when it should not");
-
             }
         }
 
-        MqttClientOptions CreateClientOptions(TestEnvironment testEnvironment, string clientId, bool cleanSession, uint sessionExpiryInterval)
+        [TestMethod]
+        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

@@ -2,11 +2,13 @@
 // 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
 {
@@ -32,12 +34,12 @@ namespace MQTTnet.Tests.Mockups
 
         public Task ConnectAsync(CancellationToken cancellationToken)
         {
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public Task DisconnectAsync(CancellationToken cancellationToken)
         {
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
@@ -45,9 +47,9 @@ namespace MQTTnet.Tests.Mockups
             return _stream.ReadAsync(buffer, offset, count, cancellationToken);
         }
 
-        public Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        public Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, CancellationToken cancellationToken)
         {
-            return _stream.WriteAsync(buffer, offset, count, cancellationToken);
+            return _stream.WriteAsync(buffer.Array, buffer.Offset, buffer.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.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Tests.Mockups
 {
@@ -46,7 +46,7 @@ namespace MQTTnet.Tests.Mockups
                 _receivedEventArgs.Add(eventArgs);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
     }
 }

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

Source/MQTTnet.Tests/RPC_Tests.cs

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

Source/MQTTnet.Tests/RoundtripTime_Tests.cs

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

Source/MQTTnet.Tests/Server/Assigned_Client_ID_Tests.cs

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

Source/MQTTnet.Tests/Server/Events_Tests.cs

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

Source/MQTTnet.Tests/Server/General.cs

@@ -11,8 +11,7 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Adapter;
 using MQTTnet.Client;
-using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
@@ -99,7 +98,7 @@ namespace MQTTnet.Tests.Server
                 server.ValidatingConnectionAsync += e =>
                 {
                     e.ReasonCode = MqttConnectReasonCode.NotAuthorized;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var connectingFailedException = await Assert.ThrowsExceptionAsync<MqttConnectingFailedException>(() => testEnvironment.ConnectClient());
@@ -122,7 +121,7 @@ namespace MQTTnet.Tests.Server
                         e.Response.ReasonCode = MqttSubscribeReasonCode.UnspecifiedError;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 // Prepare some retained messages.
@@ -146,7 +145,7 @@ namespace MQTTnet.Tests.Server
                         buffer.Append(e.ApplicationMessage.Topic);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client2.SubscribeAsync("y");
@@ -172,13 +171,13 @@ namespace MQTTnet.Tests.Server
                 server.ClientConnectedAsync += e =>
                 {
                     Interlocked.Increment(ref clientConnectedCalled);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.ClientDisconnectedAsync += e =>
                 {
                     Interlocked.Increment(ref clientDisconnectedCalled);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var c1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder());
@@ -225,7 +224,7 @@ namespace MQTTnet.Tests.Server
                                         // Clear retained message.
                                         await client.PublishAsync(
                                             new MqttApplicationMessageBuilder().WithTopic("r" + i2)
-                                                .WithPayload(PlatformAbstractionLayer.EmptyByteArray)
+                                                .WithPayload(EmptyBuffer.Array)
                                                 .WithRetainFlag()
                                                 .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce)
                                                 .Build());
@@ -276,7 +275,7 @@ namespace MQTTnet.Tests.Server
                 {
                     e.ApplicationMessage = new MqttApplicationMessage { Topic = "new_topic" };
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 string receivedTopic = null;
@@ -286,7 +285,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += a =>
                 {
                     receivedTopic = a.ApplicationMessage.Topic;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("original_topic").Build());
@@ -305,7 +304,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     e.ApplicationMessage.Payload = Encoding.ASCII.GetBytes("extended");
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var c1 = await testEnvironment.ConnectClient();
@@ -316,7 +315,7 @@ namespace MQTTnet.Tests.Server
                 c2.ApplicationMessageReceivedAsync += e =>
                 {
                     isIntercepted = string.Compare("extended", Encoding.UTF8.GetString(e.ApplicationMessage.Payload), StringComparison.Ordinal) == 0;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("test").Build());
@@ -339,7 +338,7 @@ namespace MQTTnet.Tests.Server
                 server.ApplicationMessageNotConsumedAsync += e =>
                 {
                     undeliverd = e.ApplicationMessage.Topic;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -376,7 +375,7 @@ namespace MQTTnet.Tests.Server
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await Task.Delay(500);
@@ -400,7 +399,7 @@ namespace MQTTnet.Tests.Server
                 s.RetainedMessageChangedAsync += e =>
                 {
                     savedRetainedMessages = e.StoredRetainedMessages;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var c1 = await testEnvironment.ConnectClient();
@@ -440,7 +439,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedTopic = e.ApplicationMessage.Topic;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync("#");
@@ -487,7 +486,7 @@ namespace MQTTnet.Tests.Server
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var message = new MqttApplicationMessageBuilder().WithTopic("a").WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce).Build();
@@ -521,13 +520,13 @@ namespace MQTTnet.Tests.Server
                 c2.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 c3.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c2.SubscribeAsync(new MqttTopicFilter { Topic = "a", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce }).ConfigureAwait(false);
@@ -581,7 +580,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("c");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.ClientDisconnectedAsync += e =>
@@ -591,7 +590,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("d");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var clientOptionsBuilder = new MqttClientOptionsBuilder().WithClientId(Guid.NewGuid().ToString());
@@ -621,7 +620,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("r");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c2.SubscribeAsync("topic");
@@ -669,7 +668,7 @@ namespace MQTTnet.Tests.Server
                 server.ValidatingConnectionAsync += e =>
                 {
                     ConnectionValidationHandler(e);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var events = new List<string>();
@@ -681,7 +680,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("c");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.ClientDisconnectedAsync += e =>
@@ -691,7 +690,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("d");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var clientOptions = new MqttClientOptionsBuilder().WithClientId("same_id");
@@ -706,7 +705,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("x");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 c1.ApplicationMessageReceivedAsync += e =>
@@ -716,7 +715,7 @@ namespace MQTTnet.Tests.Server
                         events.Add("r");
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 c1.SubscribeAsync("topic").Wait();
@@ -785,7 +784,7 @@ namespace MQTTnet.Tests.Server
                 client1.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedBody = e.ApplicationMessage.Payload;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client1.SubscribeAsync("string");
@@ -822,7 +821,7 @@ namespace MQTTnet.Tests.Server
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await Task.Delay(500);
@@ -850,7 +849,7 @@ namespace MQTTnet.Tests.Server
                 c1.DisconnectedAsync += e =>
                 {
                     disconnectCalled++;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await Task.Delay(100);
@@ -935,7 +934,62 @@ namespace MQTTnet.Tests.Server
                 Assert.IsTrue(client.IsConnected);
             }
         }
-        
+
+        [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.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
@@ -26,7 +26,7 @@ namespace MQTTnet.Tests.Server
                     receiverClient.ApplicationMessageReceivedAsync += e =>
                     {
                         Interlocked.Increment(ref receivedMessages);
-                        return PlatformAbstractionLayer.CompletedTask;
+                        return CompletedTask.Instance;
                     };
 
                     await receiverClient.SubscribeAsync("t/+");
@@ -55,7 +55,7 @@ namespace MQTTnet.Tests.Server
                             });
                     }
 
-                    SpinWait.SpinUntil(() => receivedMessages == 100000, TimeSpan.FromSeconds(60));
+                    SpinWait.SpinUntil(() => receivedMessages == 100000, TimeSpan.FromSeconds(90));
 
                     Assert.AreEqual(100000, receivedMessages);
                 }
@@ -74,7 +74,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessages);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 for (var i = 0; i < 100; i++)
@@ -137,7 +137,7 @@ namespace MQTTnet.Tests.Server
                 server.InterceptingPublishAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessages);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 for (var i = 0; i < 100; i++)

Source/MQTTnet.Tests/Server/MqttSubscriptionsManager_Tests.cs

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

Source/MQTTnet.Tests/Server/Publishing_Tests.cs

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

Source/MQTTnet.Tests/Server/QoS_Tests.cs

@@ -6,7 +6,7 @@ using System.Collections.Generic;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 
@@ -26,7 +26,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientAcknowledgedPublishPacketAsync += args =>
                 {
                     eventArgs = args;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client1 = await testEnvironment.ConnectClient();
@@ -53,7 +53,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientAcknowledgedPublishPacketAsync += args =>
                 {
                     eventArgs = args;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client1 = await testEnvironment.ConnectClient();
@@ -88,7 +88,7 @@ namespace MQTTnet.Tests.Server
                         eventArgs.Add(args);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client1 = await testEnvironment.ConnectClient();
@@ -99,25 +99,16 @@ namespace MQTTnet.Tests.Server
 
                 await LongTestDelay();
 
-                Assert.AreEqual(2, eventArgs.Count);
+                Assert.AreEqual(1, eventArgs.Count);
 
                 var firstEvent = eventArgs[0];
 
                 Assert.IsNotNull(firstEvent);
                 Assert.IsNotNull(firstEvent.PublishPacket);
                 Assert.IsNotNull(firstEvent.AcknowledgePacket);
-                Assert.IsFalse(firstEvent.IsCompleted);
+                Assert.IsTrue(firstEvent.IsCompleted);
 
                 Assert.AreEqual("A", firstEvent.PublishPacket.Topic);
-
-                var secondEvent = eventArgs[1];
-
-                Assert.IsNotNull(secondEvent);
-                Assert.IsNotNull(secondEvent.PublishPacket);
-                Assert.IsNotNull(secondEvent.AcknowledgePacket);
-                Assert.IsTrue(secondEvent.IsCompleted);
-
-                Assert.AreEqual("A", secondEvent.PublishPacket.Topic);
             }
         }
     }

Source/MQTTnet.Tests/Server/Retained_Messages_Tests.cs

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

Source/MQTTnet.Tests/Server/Security_Tests.cs

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

Source/MQTTnet.Tests/Server/Server_Reference_Tests.cs

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

Source/MQTTnet.Tests/Server/Session_Tests.cs

@@ -10,9 +10,8 @@ using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Protocol;
-using MQTTnet.Server;
 using MQTTnet.Tests.Mockups;
 
 namespace MQTTnet.Tests.Server
@@ -29,7 +28,7 @@ namespace MQTTnet.Tests.Server
 
                 await testEnvironment.StartServer(o => o.WithPersistentSessions());
 
-                const string ClientId = "Client1";
+                const string clientId = "Client1";
 
                 // Create client with clean session and long session expiry interval
 
@@ -38,7 +37,7 @@ namespace MQTTnet.Tests.Server
                         .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
                         .WithSessionExpiryInterval(9999) // not relevant for v311 but testing impact
                         .WithCleanSession() // start and end with clean session
-                        .WithClientId(ClientId)
+                        .WithClientId(clientId)
                         .Build());
 
                 // Disconnect; empty session should be removed from server
@@ -57,7 +56,7 @@ namespace MQTTnet.Tests.Server
                     .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
                     .WithSessionExpiryInterval(9999) // not relevant for v311 but testing impact
                     .WithCleanSession(false) // see if there is a session
-                    .WithClientId(ClientId)
+                    .WithClientId(clientId)
                     .Build();
 
 
@@ -83,7 +82,7 @@ namespace MQTTnet.Tests.Server
                 server.SessionDeletedAsync += e =>
                 {
                     deletedEventFired = true;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -111,7 +110,7 @@ namespace MQTTnet.Tests.Server
                     e.SessionItems["can_subscribe_x"] = true;
                     e.SessionItems["default_payload"] = "Hello World";
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await testEnvironment.ConnectClient();
@@ -123,8 +122,12 @@ namespace MQTTnet.Tests.Server
             }
         }
 
+        #if !NET452
+        
         [TestMethod]
-        public async Task Handle_Parallel_Connection_Attempts()
+        [DataRow(MqttProtocolVersion.V311)]
+        [DataRow(MqttProtocolVersion.V500)]
+        public async Task Handle_Parallel_Connection_Attempts(MqttProtocolVersion protocolVersion)
         {
             using (var testEnvironment = CreateTestEnvironment())
             {
@@ -132,7 +135,7 @@ namespace MQTTnet.Tests.Server
 
                 await testEnvironment.StartServer();
 
-                var options = new MqttClientOptionsBuilder().WithClientId("1").WithKeepAlivePeriod(TimeSpan.FromSeconds(5));
+                var options = new MqttClientOptionsBuilder().WithClientId("1").WithTimeout(TimeSpan.FromSeconds(1)).WithProtocolVersion(protocolVersion);
 
                 var hasReceive = false;
 
@@ -146,11 +149,14 @@ 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();
-
-                Assert.AreEqual(1, connectedClients.Count);
 
                 await Task.Delay(5000);
+                
+                var connectedClients = clients.Where(c => c?.TryPingAsync().GetAwaiter().GetResult() == true).ToList();
+
+                await LongTestDelay();
+                
+                Assert.AreEqual(1, connectedClients.Count);
 
                 var option2 = new MqttClientOptionsBuilder().WithClientId("2").WithKeepAlivePeriod(TimeSpan.FromSeconds(10));
                 var sendClient = await testEnvironment.ConnectClient(option2);
@@ -161,26 +167,8 @@ namespace MQTTnet.Tests.Server
                 Assert.AreEqual(true, hasReceive);
             }
         }
-
-        [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);
-            }
-        }
-
+#endif
+        
         [DataTestMethod]
         [DataRow(MqttQualityOfServiceLevel.ExactlyOnce)]
         [DataRow(MqttQualityOfServiceLevel.AtLeastOnce)]
@@ -200,7 +188,7 @@ namespace MQTTnet.Tests.Server
                     c.AutoAcknowledge = false;
                     ++count;
                     Console.WriteLine("process");
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await subscriber.SubscribeAsync("#", qos);
@@ -237,7 +225,7 @@ namespace MQTTnet.Tests.Server
                 client1.DisconnectedAsync += c =>
                 {
                     disconnectReason = c.Reason;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client2 = await testEnvironment.ConnectClient(options);
@@ -263,7 +251,7 @@ namespace MQTTnet.Tests.Server
                     e.SessionItems["can_subscribe_x"] = true;
                     e.SessionItems["default_payload"] = "Hello World";
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.InterceptingSubscriptionAsync += e =>
@@ -276,13 +264,13 @@ namespace MQTTnet.Tests.Server
                         }
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 server.InterceptingPublishAsync += e =>
                 {
                     e.ApplicationMessage.Payload = Encoding.UTF8.GetBytes(e.SessionItems["default_payload"] as string ?? string.Empty);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 string receivedPayload = null;
@@ -291,7 +279,7 @@ namespace MQTTnet.Tests.Server
                 client.ApplicationMessageReceivedAsync += e =>
                 {
                     receivedPayload = e.ApplicationMessage.ConvertPayloadToString();
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var subscribeResult = await client.SubscribeAsync("x");
@@ -341,7 +329,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
@@ -349,10 +337,10 @@ namespace MQTTnet.Tests.Server
                 // C2 has the last will defined.
                 var clientOptions = new MqttClientOptionsBuilder().WithWillTopic("My/last/will").WithClientId("WillOwner");
 
-                var c2 = await testEnvironment.ConnectClient(clientOptions);
+                await testEnvironment.ConnectClient(clientOptions);
 
                 // C3 will do the connection takeover.
-                var c3 = await testEnvironment.ConnectClient(clientOptions);
+                await testEnvironment.ConnectClient(clientOptions);
 
                 await Task.Delay(1000);
 
@@ -360,41 +348,29 @@ namespace MQTTnet.Tests.Server
             }
         }
 
-        async Task<IMqttClient> ConnectAndSubscribe(TestEnvironment testEnvironment, MqttClientOptionsBuilder options, Action onReceive)
+        static async Task<IMqttClient> ConnectAndSubscribe(TestEnvironment testEnvironment, MqttClientOptionsBuilder options, Action onReceive)
         {
             try
             {
-                var sendClient = await testEnvironment.ConnectClient(options);
+                var client = await testEnvironment.ConnectClient(options).ConfigureAwait(false);
 
-                sendClient.ApplicationMessageReceivedAsync += e =>
+                client.ApplicationMessageReceivedAsync += e =>
                 {
                     onReceive();
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
-
-                using (var subscribeTimeout = new CancellationTokenSource(TimeSpan.FromSeconds(5)))
+                
+                using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5)))
                 {
-                    await sendClient.SubscribeAsync("aaa", MqttQualityOfServiceLevel.AtMostOnce, subscribeTimeout.Token).ConfigureAwait(false);
+                    await client.SubscribeAsync("aaa", MqttQualityOfServiceLevel.AtMostOnce, timeout.Token).ConfigureAwait(false);
                 }
 
-                return sendClient;
+                return client;
             }
             catch (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.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
@@ -33,7 +33,7 @@ namespace MQTTnet.Tests.Server
                         e.Response.ReasonCode = MqttSubscribeReasonCode.TopicFilterInvalid;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -46,6 +46,22 @@ namespace MQTTnet.Tests.Server
             }
         }
 
+        [TestMethod]
+        [ExpectedException(typeof(MqttClientDisconnectedException))]
+        public async Task Disconnect_While_Subscribing()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                // The client will be disconnect directly after subscribing!
+                server.ClientSubscribedTopicAsync += ev => server.DisconnectClientAsync(ev.ClientId, MqttDisconnectReasonCode.NormalDisconnection);
+
+                var client = await testEnvironment.ConnectClient();
+                await client.SubscribeAsync("#");
+            }
+        }
+        
         [TestMethod]
         public async Task Enqueue_Message_After_Subscription()
         {
@@ -56,7 +72,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientSubscribedTopicAsync += e =>
                 {
                     server.InjectApplicationMessage(new InjectedMqttApplicationMessage(new MqttApplicationMessageBuilder().WithTopic("test_topic").Build()));
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -81,7 +97,7 @@ namespace MQTTnet.Tests.Server
                 {
                     // Set the topic to "a" regards what the client wants to subscribe.
                     e.TopicFilter.Topic = "a";
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var topicAReceived = false;
@@ -99,7 +115,7 @@ namespace MQTTnet.Tests.Server
                         topicBReceived = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.SubscribeAsync("b");
@@ -146,7 +162,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 for (var i = 0; i < 500; i++)
@@ -189,7 +205,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var optionsBuilder = new MqttClientSubscribeOptionsBuilder();
@@ -229,7 +245,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttClientSubscribeOptionsBuilder().WithTopicFilter("a").Build());
@@ -267,7 +283,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttClientSubscribeOptionsBuilder().WithTopicFilter("a").WithTopicFilter("b").WithTopicFilter("c").Build());
@@ -301,7 +317,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var c2 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("c2"));
@@ -316,7 +332,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientSubscribedTopicAsync += e =>
                 {
                     subscribeEventCalled = e.TopicFilter.Topic == "a" && e.ClientId == c1.Options.ClientId;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.SubscribeAsync(new MqttTopicFilter { Topic = "a", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce });
@@ -331,7 +347,7 @@ namespace MQTTnet.Tests.Server
                 server.ClientUnsubscribedTopicAsync += e =>
                 {
                     unsubscribeEventCalled = e.TopicFilter == "a" && e.ClientId == c1.Options.ClientId;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await c1.UnsubscribeAsync("a");

Source/MQTTnet.Tests/Server/Subscription_TopicHash_Tests.cs

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

Source/MQTTnet.Tests/Server/Topic_Alias_Tests.cs

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

Source/MQTTnet.Tests/Server/Unsubscribe_Tests.cs

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

Source/MQTTnet.Tests/Server/User_Properties_Tests.cs

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

Source/MQTTnet.Tests/Server/Will_Tests.cs

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

Source/MQTTnet.Tests/TopicGenerator.cs

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

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -13,7 +13,6 @@ using MQTTnet.Channel;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 
@@ -23,7 +22,7 @@ namespace MQTTnet.Adapter
     {
         const uint ErrorOperationAborted = 0x800703E3;
         const int ReadBufferSize = 4096;
-        
+
         readonly IMqttChannel _channel;
         readonly byte[] _fixedHeaderBuffer = new byte[2];
         readonly MqttNetSourceLogger _logger;
@@ -73,7 +72,26 @@ namespace MQTTnet.Adapter
 
             try
             {
-                await _channel.ConnectAsync(cancellationToken).ConfigureAwait(false);
+                /*
+                 * We have to implement a small workaround here to support connecting in Xamarin
+                 * with a disabled WiFi network. If the WiFi is disabled the connect method will
+                 * block forever. Even a cancellation token is not supported properly.
+                 */
+
+                var connectTask = _channel.ConnectAsync(cancellationToken);
+
+                var timeout = new TaskCompletionSource<object>();
+                using (cancellationToken.Register(() => timeout.TrySetResult(null)))
+                {
+                    await Task.WhenAny(connectTask, timeout.Task).ConfigureAwait(false);
+                    if (timeout.Task.IsCompleted && !connectTask.IsCompleted)
+                    {
+                        throw new OperationCanceledException("MQTT connect cancelled.", cancellationToken);
+                    }
+
+                    // Make sure that the exception from the connect task gets thrown.
+                    await connectTask.ConfigureAwait(false);
+                }
             }
             catch (Exception exception)
             {
@@ -176,7 +194,7 @@ namespace MQTTnet.Adapter
             // This lock makes sure that multiple threads can send packets at the same time.
             // This is required when a disconnect is sent from another thread while the 
             // worker thread is still sending publish packets etc.
-            using (await _syncRoot.WaitAsync(cancellationToken).ConfigureAwait(false))
+            using (await _syncRoot.EnterAsync(cancellationToken).ConfigureAwait(false))
             {
                 // Check for cancellation here again because "WaitAsync" might take some time.
                 cancellationToken.ThrowIfCancellationRequested();
@@ -188,11 +206,14 @@ 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.Payload.Array, packetBuffer.Payload.Offset, packetBuffer.Payload.Count, cancellationToken).ConfigureAwait(false);
+                        await _channel.WriteAsync(packetBuffer.Packet, false, 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);
@@ -274,8 +295,26 @@ namespace MQTTnet.Adapter
 
             while (totalBytesRead < buffer.Length)
             {
-                var bytesRead = await _channel.ReadAsync(buffer, totalBytesRead, buffer.Length - totalBytesRead, cancellationToken).ConfigureAwait(false);
+                // Check two times for cancellation because the call to _ReadAsync_ might block for some time.
+                if (cancellationToken.IsCancellationRequested)
+                {
+                    return ReadFixedHeaderResult.Cancelled;
+                }
 
+                int bytesRead;
+                try
+                {
+                    bytesRead = await _channel.ReadAsync(buffer, totalBytesRead, buffer.Length - totalBytesRead, cancellationToken).ConfigureAwait(false);
+                }
+                catch (OperationCanceledException)
+                {
+                    return ReadFixedHeaderResult.Cancelled;
+                }
+                catch (SocketException)
+                {
+                    return ReadFixedHeaderResult.ConnectionClosed;
+                }
+               
                 if (cancellationToken.IsCancellationRequested)
                 {
                     return ReadFixedHeaderResult.Cancelled;
@@ -341,7 +380,7 @@ namespace MQTTnet.Adapter
                 var fixedHeader = readFixedHeaderResult.FixedHeader;
                 if (fixedHeader.RemainingLength == 0)
                 {
-                    return new ReceivedMqttPacket(fixedHeader.Flags, PlatformAbstractionLayer.EmptyByteArraySegment, 2);
+                    return new ReceivedMqttPacket(fixedHeader.Flags, EmptyBuffer.ArraySegment, 2);
                 }
 
                 var bodyLength = fixedHeader.RemainingLength;

Source/MQTTnet/Channel/IMqttChannel.cs

@@ -12,13 +12,17 @@ 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/Exceptions/MqttClientDisconnectedException.cs

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

Source/MQTTnet/Client/Exceptions/MqttClientUnexpectedDisconnectReceivedException.cs

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

Source/MQTTnet/Client/IMqttClient.cs

@@ -7,32 +7,32 @@ namespace MQTTnet.Client
 {
     public interface IMqttClient : IDisposable
     {
-        event Func<MqttClientConnectedEventArgs, Task> ConnectedAsync;
-        
-        event Func<MqttClientConnectingEventArgs, Task> ConnectingAsync;
-        
-        event Func<MqttClientDisconnectedEventArgs, Task> DisconnectedAsync;
-        
         event Func<MqttApplicationMessageReceivedEventArgs, Task> ApplicationMessageReceivedAsync;
         
+        event Func<MqttClientConnectedEventArgs, Task> ConnectedAsync;
+
+        event Func<MqttClientConnectingEventArgs, Task> ConnectingAsync;
+
+        event Func<MqttClientDisconnectedEventArgs, Task> DisconnectedAsync;
+
         event Func<InspectMqttPacketEventArgs, Task> InspectPackage;
-        
+
         bool IsConnected { get; }
-        
+
         MqttClientOptions Options { get; }
-        
+
         Task<MqttClientConnectResult> ConnectAsync(MqttClientOptions options, CancellationToken cancellationToken = default);
-        
+
         Task DisconnectAsync(MqttClientDisconnectOptions options, CancellationToken cancellationToken = default);
-        
+
         Task PingAsync(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);
+
+        Task SendExtendedAuthenticationExchangeDataAsync(MqttExtendedAuthenticationExchangeData data, CancellationToken cancellationToken = default);
+
+        Task<MqttClientSubscribeResult> SubscribeAsync(MqttClientSubscribeOptions options, CancellationToken cancellationToken = default);
+
+        Task<MqttClientUnsubscribeResult> UnsubscribeAsync(MqttClientUnsubscribeOptions options, CancellationToken cancellationToken = default);
     }
 }

Source/MQTTnet/Client/MqttClientExtensions.cs

@@ -14,6 +14,26 @@ 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

@@ -13,80 +13,83 @@ namespace MQTTnet.Client
     public sealed class MqttApplicationMessageReceivedEventArgs : EventArgs
     {
         readonly Func<MqttApplicationMessageReceivedEventArgs, CancellationToken, Task> _acknowledgeHandler;
-        
+
         int _isAcknowledged;
-        
+
         public MqttApplicationMessageReceivedEventArgs(
-            string clientId, 
+            string clientId,
             MqttApplicationMessage applicationMessage,
             MqttPublishPacket publishPacket,
             Func<MqttApplicationMessageReceivedEventArgs, CancellationToken, Task> acknowledgeHandler)
         {
             ClientId = clientId;
             ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
-            PublishPacket = publishPacket;
-            PacketIdentifier = publishPacket.PacketIdentifier;
+            PublishPacket = publishPacket ?? throw new ArgumentNullException(nameof(publishPacket));
             _acknowledgeHandler = acknowledgeHandler;
         }
 
-        internal MqttPublishPacket PublishPacket { get; set; }
-        
+        public MqttApplicationMessage ApplicationMessage { get; }
+
         /// <summary>
-        /// Gets the client identifier.
-        /// Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
+        ///     Gets or sets whether the library should send MQTT ACK packets automatically if required.
+        /// </summary>
+        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; }
 
-        public MqttApplicationMessage ApplicationMessage { get; }
-
-        public bool ProcessingFailed { get; set; }
-
         /// <summary>
-        /// 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 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 whether this message was handled.
-        /// This value can be used in user code for custom control flow.
+        ///     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 or sets whether the library should send MQTT ACK packets automatically if required.
+        ///     Gets the identifier of the MQTT packet
         /// </summary>
-        public bool AutoAcknowledge { get; set; } = true;
-        
-        public object Tag { get; set; }
+        public ushort PacketIdentifier => PublishPacket.PacketIdentifier;
 
         /// <summary>
-        /// Gets the indentifier of MQTT packet
+        ///     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 ushort PacketIdentifier { get; }
+        public bool ProcessingFailed { get; set; }
 
-        public Task AcknowledgeAsync(CancellationToken cancellationToken) 
+        /// <summary>
+        ///     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.
+        /// </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.
+        /// </summary>
+        public List<MqttUserProperty> ResponseUserProperties { get; } = new List<MqttUserProperty>();
+
+        public object Tag { get; set; }
+
+        internal MqttPublishPacket PublishPacket { get; set; }
+
+        public Task AcknowledgeAsync(CancellationToken cancellationToken)
         {
             if (_acknowledgeHandler == null)
             {
                 throw new NotSupportedException("Deferred acknowledgement of application message is not yet supported in MQTTnet server.");
             }
-            
+
             if (Interlocked.CompareExchange(ref _isAcknowledged, 1, 0) == 0)
             {
                 return _acknowledgeHandler(this, cancellationToken);
             }
-            
+
             throw new InvalidOperationException("The application message is already acknowledged.");
         }
     }
-}
+}

Source/MQTTnet/Exceptions/MqttCommunicationException.cs

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

Source/MQTTnet/Formatter/MqttBufferReader.cs

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

Source/MQTTnet/Formatter/MqttPacketBuffer.cs

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

Source/MQTTnet/Formatter/MqttPacketFactories.cs

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

Source/MQTTnet/Formatter/MqttPublishPacketFactory.cs

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

Source/MQTTnet/Formatter/MqttUnsubAckPacketFactory.cs

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

Source/MQTTnet/Implementations/CrossPlatformSocket.cs

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

Source/MQTTnet/Implementations/MqttTcpChannel.Uwp.cs

@@ -108,12 +108,12 @@ namespace MQTTnet.Implementations
             return _readStream.ReadAsync(buffer, offset, count, cancellationToken);
         }
 
-        public Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        public Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, 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, offset, count);
+            _writeStream.Write(buffer.Array, buffer.Offset, buffer.Count);
             return _writeStream.FlushAsync(cancellationToken);
         }
 

Source/MQTTnet/Implementations/MqttTcpChannel.cs

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

Source/MQTTnet/Implementations/MqttTcpServerAdapter.cs

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

Source/MQTTnet/Implementations/MqttTcpServerListener.cs

@@ -134,7 +134,7 @@ namespace MQTTnet.Implementations
                         continue;
                     }
 
-                    Task.Run(() => TryHandleClientConnectionAsync(clientSocket), cancellationToken).RunInBackground(_logger);
+                    _ = Task.Factory.StartNew(() => TryHandleClientConnectionAsync(clientSocket), cancellationToken, TaskCreationOptions.PreferFairness, TaskScheduler.Default).ConfigureAwait(false);
                 }
                 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;
@@ -89,14 +89,26 @@ 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(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        public async Task WriteAsync(ArraySegment<byte> buffer, bool isEndOfPacket, 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.
@@ -105,88 +117,11 @@ namespace MQTTnet.Implementations
                 return;
             }
 
-            using (await _sendLock.WaitAsync(cancellationToken).ConfigureAwait(false))
+            using (await _sendLock.EnterAsync(cancellationToken).ConfigureAwait(false))
             {
-                await _webSocket.SendAsync(new ArraySegment<byte>(buffer, offset, count), WebSocketMessageType.Binary, true, cancellationToken).ConfigureAwait(false);
+                await _webSocket.SendAsync(buffer, WebSocketMessageType.Binary, isEndOfPacket, 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()
@@ -230,5 +165,77 @@ 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

@@ -1,43 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-// See the LICENSE file in the project root for more information.
-
-using System;
-using System.Threading;
-using System.Threading.Tasks;
-
-namespace MQTTnet.Implementations
-{
-    public static class PlatformAbstractionLayer
-    {
-#if NET452
-        public static Task CompletedTask => 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

@@ -13,7 +13,16 @@ namespace MQTTnet.Internal
     {
         readonly List<AsyncEventInvocator<TEventArgs>> _handlers = new List<AsyncEventInvocator<TEventArgs>>();
 
-        public bool HasHandlers => _handlers.Count > 0;
+        ICollection<AsyncEventInvocator<TEventArgs>> _handlersForInvoke;
+
+        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)
         {
@@ -22,7 +31,13 @@ namespace MQTTnet.Internal
                 throw new ArgumentNullException(nameof(handler));
             }
 
-            _handlers.Add(new AsyncEventInvocator<TEventArgs>(null, handler));
+            lock (_handlers)
+            {
+                _handlers.Add(new AsyncEventInvocator<TEventArgs>(null, handler));
+
+                HasHandlers = true;
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
+            }
         }
 
         public void AddHandler(Action<TEventArgs> handler)
@@ -32,17 +47,32 @@ namespace MQTTnet.Internal
                 throw new ArgumentNullException(nameof(handler));
             }
 
-            _handlers.Add(new AsyncEventInvocator<TEventArgs>(handler, null));
+            lock (_handlers)
+            {
+                _handlers.Add(new AsyncEventInvocator<TEventArgs>(handler, null));
+
+                HasHandlers = true;
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
+            }
         }
 
         public async Task InvokeAsync(TEventArgs eventArgs)
         {
-            foreach (var handler in _handlers)
+            if (!HasHandlers)
+            {
+                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);
             }
         }
-        
+
         public void RemoveHandler(Func<TEventArgs, Task> handler)
         {
             if (handler == null)
@@ -50,7 +80,13 @@ namespace MQTTnet.Internal
                 throw new ArgumentNullException(nameof(handler));
             }
 
-            _handlers.RemoveAll(h => h.WrapsHandler(handler));
+            lock (_handlers)
+            {
+                _handlers.RemoveAll(h => h.WrapsHandler(handler));
+
+                HasHandlers = _handlers.Count > 0;
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
+            }
         }
 
         public void RemoveHandler(Action<TEventArgs> handler)
@@ -60,7 +96,13 @@ namespace MQTTnet.Internal
                 throw new ArgumentNullException(nameof(handler));
             }
 
-            _handlers.RemoveAll(h => h.WrapsHandler(handler));
+            lock (_handlers)
+            {
+                _handlers.RemoveAll(h => h.WrapsHandler(handler));
+
+                HasHandlers = _handlers.Count > 0;
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
+            }
         }
 
         public async Task TryInvokeAsync(TEventArgs eventArgs, MqttNetSourceLogger logger)
@@ -81,7 +123,7 @@ namespace MQTTnet.Internal
             }
             catch (Exception exception)
             {
-                logger.Warning(exception, $"Error while invoking event ({typeof(TEventArgs)}).");
+                logger.Warning(exception, $"Error while invoking event with arguments of type {typeof(TEventArgs)}.");
             }
         }
     }

Source/MQTTnet/Internal/AsyncEventInvocator.cs

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

Source/MQTTnet/Internal/AsyncLock.cs

@@ -3,6 +3,8 @@
 // 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;
 
@@ -10,77 +12,152 @@ namespace MQTTnet.Internal
 {
     public sealed class AsyncLock : IDisposable
     {
+        readonly Task<IDisposable> _completedTask;
+        readonly IDisposable _releaser;
         readonly object _syncRoot = new object();
-        readonly Task<IDisposable> _releaser;
+        readonly Queue<AsyncLockWaiter> _waiters = new Queue<AsyncLockWaiter>(64);
 
-        SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
+        volatile bool _isDisposed;
+        bool _isLocked;
 
         public AsyncLock()
         {
-            _releaser = Task.FromResult((IDisposable)new Releaser(this));
-        }
-
-        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);
+            _releaser = new Releaser(this);
+            _completedTask = Task.FromResult(_releaser);
         }
 
         public void Dispose()
         {
             lock (_syncRoot)
             {
-                _semaphore?.Dispose();
-                _semaphore = null;
+                _isDisposed = true;
+
+                while (_waiters.Any())
+                {
+                    _waiters.Dequeue().Dispose();
+                }
             }
         }
 
-        internal void Release()
+        public Task<IDisposable> EnterAsync(CancellationToken cancellationToken = default)
+        {
+            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)
             {
-                _semaphore?.Release();
+                if (_isDisposed)
+                {
+                    // 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 Releaser : IDisposable
+        sealed class AsyncLockWaiter : IDisposable
         {
-            readonly AsyncLock _lock;
+            readonly CancellationTokenRegistration _cancellationRegistration;
+            readonly bool _hasCancellationRegistration;
+            readonly AsyncTaskCompletionSource<IDisposable> _promise = new AsyncTaskCompletionSource<IDisposable>();
 
-            internal Releaser(AsyncLock @lock)
+            public AsyncLockWaiter(CancellationToken cancellationToken)
             {
-                _lock = @lock;
+                cancellationToken.ThrowIfCancellationRequested();
+                
+                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()
             {
-                _lock.Release();
+                if (_hasCancellationRegistration)
+                {
+                    _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/AsyncQueue.cs

@@ -11,64 +11,39 @@ namespace MQTTnet.Internal
 {
     public sealed class AsyncQueue<TItem> : IDisposable
     {
+        readonly AsyncSignal _signal = new AsyncSignal();
         readonly object _syncRoot = new object();
-        SemaphoreSlim _semaphore = new SemaphoreSlim(0);
+
         ConcurrentQueue<TItem> _queue = new ConcurrentQueue<TItem>();
 
+        bool _isDisposed;
+        
         public int Count => _queue.Count;
 
+        public void Clear()
+        {
+            Interlocked.Exchange(ref _queue, new ConcurrentQueue<TItem>());
+        }
+
+        public void Dispose()
+        {
+            lock (_syncRoot)
+            {
+                _signal?.Dispose();
+
+                _isDisposed = true;
+            }
+        }
+
         public void Enqueue(TItem item)
         {
             lock (_syncRoot)
             {
                 _queue.Enqueue(item);
-                _semaphore?.Release();
+                _signal.Set();
             }
         }
 
-        public async Task<AsyncQueueDequeueResult<TItem>> TryDequeueAsync(CancellationToken cancellationToken)
-        {
-            while (!cancellationToken.IsCancellationRequested)
-            {
-                try
-                {
-                    Task task;
-                    lock (_syncRoot)
-                    {
-                        if (_semaphore == null)
-                        {
-                            return new AsyncQueueDequeueResult<TItem>(false, default);
-                        }
-
-                        task = _semaphore.WaitAsync(cancellationToken);
-                    }
-                    
-                    await task.ConfigureAwait(false);
-
-                    if (cancellationToken.IsCancellationRequested)
-                    {
-                        return new AsyncQueueDequeueResult<TItem>(false, default);
-                    }
-
-                    if (_queue.TryDequeue(out var item))
-                    {
-                        return new AsyncQueueDequeueResult<TItem>(true, item);
-                    }
-                }
-                catch (ArgumentNullException)
-                {
-                    // The semaphore throws this internally sometimes.
-                    return new AsyncQueueDequeueResult<TItem>(false, default);
-                }
-                catch (OperationCanceledException)
-                {
-                    return new AsyncQueueDequeueResult<TItem>(false, default);
-                }
-            }
-
-            return new AsyncQueueDequeueResult<TItem>(false, default);
-        }
-
         public AsyncQueueDequeueResult<TItem> TryDequeue()
         {
             if (_queue.TryDequeue(out var item))
@@ -79,18 +54,42 @@ namespace MQTTnet.Internal
             return new AsyncQueueDequeueResult<TItem>(false, default);
         }
 
-        public void Clear()
+        public async Task<AsyncQueueDequeueResult<TItem>> TryDequeueAsync(CancellationToken cancellationToken)
         {
-            Interlocked.Exchange(ref _queue, new ConcurrentQueue<TItem>());
-        }
-
-        public void Dispose()
-        {
-            lock (_syncRoot)
+            while (!cancellationToken.IsCancellationRequested)
             {
-                _semaphore?.Dispose();
-                _semaphore = null;
+                try
+                {
+                    Task task;
+                    lock (_syncRoot)
+                    {
+                        if (_isDisposed)
+                        {
+                            return new AsyncQueueDequeueResult<TItem>(false, default);
+                        }
+
+                        task = _signal.WaitAsync(cancellationToken);
+                    }
+
+                    await task.ConfigureAwait(false);
+
+                    if (cancellationToken.IsCancellationRequested)
+                    {
+                        return new AsyncQueueDequeueResult<TItem>(false, default);
+                    }
+
+                    if (_queue.TryDequeue(out var item))
+                    {
+                        return new AsyncQueueDequeueResult<TItem>(true, item);
+                    }
+                }
+                catch (OperationCanceledException)
+                {
+                    return new AsyncQueueDequeueResult<TItem>(false, default);
+                }
             }
+
+            return new AsyncQueueDequeueResult<TItem>(false, default);
         }
     }
-}
+}

Source/MQTTnet/Internal/AsyncSignal.cs

@@ -0,0 +1,113 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Internal
+{
+    public sealed class AsyncSignal : IDisposable
+    {
+        readonly object _syncRoot = new object();
+
+        bool _isDisposed;
+        bool _isSignaled;
+        AsyncSignalWaiter _waiter;
+
+        public void Dispose()
+        {
+            lock (_syncRoot)
+            {
+                _waiter?.Dispose();
+                _waiter = null;
+
+                _isDisposed = true;
+            }
+        }
+
+        public void Set()
+        {
+            lock (_syncRoot)
+            {
+                _isSignaled = true;
+
+                // If there is already a waiting task let it run.
+                _waiter?.Approve();
+                _waiter = null;
+            }
+        }
+
+        public Task WaitAsync(CancellationToken cancellationToken = default)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            lock (_syncRoot)
+            {
+                ThrowIfDisposed();
+
+                if (_isSignaled)
+                {
+                    return CompletedTask.Instance;
+                }
+
+                if (_waiter != null)
+                {
+                    if (!_waiter.Task.IsCompleted)
+                    {
+                        throw new InvalidOperationException("Only one waiting task is permitted per async signal.");
+                    }
+                    
+                    _waiter.Dispose();
+                }
+
+                _waiter = new AsyncSignalWaiter(cancellationToken);
+                return _waiter.Task;
+            }
+        }
+
+        void ThrowIfDisposed()
+        {
+            if (_isDisposed)
+            {
+                throw new ObjectDisposedException(nameof(AsyncSignal));
+            }
+        }
+
+        sealed class AsyncSignalWaiter : IDisposable
+        {
+            readonly AsyncTaskCompletionSource<bool> _promise = new AsyncTaskCompletionSource<bool>();
+
+            // ReSharper disable once FieldCanBeMadeReadOnly.Local
+            CancellationTokenRegistration _cancellationTokenRegistration;
+
+            public AsyncSignalWaiter(CancellationToken cancellationToken)
+            {
+                if (cancellationToken.CanBeCanceled)
+                {
+                    _cancellationTokenRegistration = cancellationToken.Register(Cancel);
+                }
+            }
+
+            public Task Task => _promise.Task;
+
+            public void Approve()
+            {
+                _promise.TrySetResult(true);
+            }
+
+            public void Dispose()
+            {
+                _cancellationTokenRegistration.Dispose();
+
+                _promise.TrySetException(new ObjectDisposedException(nameof(AsyncSignalWaiter)));
+            }
+
+            void Cancel()
+            {
+                _promise.TrySetCanceled();
+            }
+        }
+    }
+}