Improve unit tests

fix cross access to async events.

.github/ISSUE_TEMPLATE/custom.md

@@ -14,6 +14,5 @@ A clear and concise description of what you want to know.
 <!-- Remove the items which don't apply from the following list -->
 - Client
 - ManagedClient
-- MQTTnet.Server standalone
 - Server
 - Generic

.github/ISSUE_TEMPLATE/feature_request.md

@@ -15,7 +15,6 @@ A clear and concise description of what the problem is. Example. I'm am trying t
 <!-- Remove the items which don't apply from the following list -->
 - Client
 - ManagedClient
-- MQTTnet.Server standalone
 - Server
 - Generic
 

.github/workflows/ReleaseNotes.md

@@ -1,6 +1,2 @@
-* [Client] Added the flag _try_private_ (MQTT 3.1.1 Bridge) to client options (#1413).
-* [Client] Fixed MQTTv5 protocol violation in PUBLISH packets (#1423).
-* [Client] Added missing "WithWill..." methods in _MqttClientOptionsBuilder_.
-* [ManagedClient] Fixed wrong event args type for connected and disconnected events (#1432).
-* [Server] Fixed wrong handling of retained messages with QoS > 0 (#1434).
-* [Server] Connections with _try_private_ flag (MQTT 3.1.1 Bridge) are now accepted (#1413).
+* [Core] Fixed dead lock and race conditions in new _AsyncLock_ implementation (#1542).
+* [Client] Fixed connection freeze when using Xamarin etc.

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ name: CI
 on: [push, pull_request]
 
 env:
-  VERSION: "4.0.1.${{github.run_number}}"
+  VERSION: "4.1.2.${{github.run_number}}"
 
 jobs:
   build:

MQTTnet.sln.DotSettings

@@ -29,7 +29,7 @@
 	<s:String x:Key="/Default/CodeStyle/CodeFormatting/XmlDocFormatter/ProcessingInstructionAttributesFormat/@EntryValue">OnDifferentLines</s:String>
 	<s:String x:Key="/Default/CodeStyle/CodeFormatting/XmlDocFormatter/TagAttributesFormat/@EntryValue">OnDifferentLines</s:String>
 	<s:String x:Key="/Default/CodeStyle/CSharpFileLayoutPatterns/Pattern/@EntryValue">&lt;?xml version="1.0" encoding="utf-16"?&gt;
-&lt;Patterns xmlns="urn:schemas-jetbrains-com:member-reordering-patterns"&gt;
+&lt;Patterns xmlns="urn:schemas-jetbrains-com:member-reordering-patterns"&gt;    
   &lt;TypePattern DisplayName="Non-reorderable types"&gt;
     &lt;TypePattern.Match&gt;
       &lt;Or&gt;
@@ -210,6 +210,7 @@
         &lt;/Or&gt;
       &lt;/Entry.Match&gt;
         &lt;Entry.SortBy&gt;
+            &lt;Access Order="Public Internal Protected Private" /&gt;
             &lt;Name /&gt;
         &lt;/Entry.SortBy&gt;
     &lt;/Entry&gt;
@@ -226,6 +227,9 @@
     &lt;/Entry&gt;
   &lt;/TypePattern&gt;
 &lt;/Patterns&gt;</s:String>
+	<s:String x:Key="/Default/CodeStyle/FileHeader/FileHeaderText/@EntryValue">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.</s:String>
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ECSharpAttributeForSingleLineMethodUpgrade/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ECSharpKeepExistingMigration/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ECSharpPlaceEmbeddedOnSameLineMigration/@EntryIndexedValue">True</s:Boolean>
@@ -237,5 +241,6 @@
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ESettingsUpgrade_002EMigrateBlankLinesAroundFieldToBlankLinesAroundProperty/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ESettingsUpgrade_002EMigrateThisQualifierSettings/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=PINGREQ/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/UserDictionary/Words/=PINGRESP/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=Tnet/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=unsub/@EntryIndexedValue">True</s:Boolean></wpf:ResourceDictionary>

README.md

@@ -12,7 +12,7 @@
 
 # MQTTnet
 
-MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker) and supports the MQTT protocol up to version 5.
+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

@@ -2,8 +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.
 
+// ReSharper disable UnusedType.Global
+// ReSharper disable UnusedMember.Global
+// ReSharper disable InconsistentNaming
+
 using System.Security.Authentication;
 using MQTTnet.Client;
+using MQTTnet.Extensions.WebSocket4Net;
 using MQTTnet.Formatter;
 using MQTTnet.Samples.Helpers;
 
@@ -111,13 +116,45 @@ public static class Client_Connection_Samples
                 .WithTls(
                     o =>
                     {
+                        // 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;
                     })
                 .Build();
 
-            await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
+            using (var timeout = new CancellationTokenSource(5000))
+            {
+                await mqttClient.ConnectAsync(mqttClientOptions, timeout.Token);
+
+                Console.WriteLine("The MQTT client is connected.");
+            }
+        }
+    }
+
+    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().UseWebSocket4Net();
+
+        using (var mqttClient = mqttFactory.CreateMqttClient())
+        {
+            var mqttClientOptions = new MqttClientOptionsBuilder().WithWebSocketServer("broker.hivemq.com:8000/mqtt").Build();
+
+            var response = await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
 
             Console.WriteLine("The MQTT client is connected.");
+
+            response.DumpToConsole();
         }
     }
 
@@ -159,16 +196,99 @@ public static class Client_Connection_Samples
                 .WithTls(
                     o =>
                     {
-                        o.SslProtocol = SslProtocols.Tls12; // The default value is determined by the OS. Set manually to force version.
+                        // The used public broker sometimes has invalid certificates. This sample accepts all
+                        // certificates. This should not be used in live environments.
+                        o.CertificateValidationHandler = _ => true;
                     })
                 .Build();
 
             // In MQTTv5 the response contains much more information.
-            var response = await mqttClient.ConnectAsync(mqttClientOptions, CancellationToken.None);
+            using (var timeout = new CancellationTokenSource(5000))
+            {
+                var response = await mqttClient.ConnectAsync(mqttClientOptions, timeout.Token);
 
-            Console.WriteLine("The MQTT client is connected.");
+                Console.WriteLine("The MQTT client is connected.");
 
-            response.DumpToConsole();
+                response.DumpToConsole();
+            }
+        }
+    }
+
+    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()
+    {
+        /*
+         * This sample prints the certificate information while connection. This data can be used to decide whether a connection is secure or not
+         * including the reason for that status.
+         */
+
+        var mqttFactory = new MqttFactory();
+
+        using (var mqttClient = mqttFactory.CreateMqttClient())
+        {
+            var mqttClientOptions = new MqttClientOptionsBuilder().WithTcpServer("mqtt.fluux.io", 8883)
+                .WithTls(
+                    o =>
+                    {
+                        o.CertificateValidationHandler = eventArgs =>
+                        {
+                            eventArgs.Certificate.Subject.DumpToConsole();
+                            eventArgs.Certificate.GetExpirationDateString().DumpToConsole();
+                            eventArgs.Chain.ChainPolicy.RevocationMode.DumpToConsole();
+                            eventArgs.Chain.ChainStatus.DumpToConsole();
+                            eventArgs.SslPolicyErrors.DumpToConsole();
+                            return true;
+                        };
+                    })
+                .Build();
+
+            // In MQTTv5 the response contains much more information.
+            using (var timeout = new CancellationTokenSource(5000))
+            {
+                await mqttClient.ConnectAsync(mqttClientOptions, timeout.Token);
+            }
         }
     }
 
@@ -244,14 +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

@@ -2,6 +2,10 @@
 // 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
+
 using MQTTnet.Client;
 
 namespace MQTTnet.Samples.Client;
@@ -36,6 +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

@@ -2,7 +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.
 
+// ReSharper disable UnusedType.Global
+// ReSharper disable UnusedMember.Global
+// ReSharper disable InconsistentNaming
+
 using MQTTnet.Client;
+using MQTTnet.Packets;
+using MQTTnet.Protocol;
 using MQTTnet.Samples.Helpers;
 
 namespace MQTTnet.Samples.Client;
@@ -19,9 +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
@@ -37,7 +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);
@@ -49,6 +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()
     {
         /*
@@ -59,14 +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

@@ -2,6 +2,10 @@
 // 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
+
 using System.Text;
 using MQTTnet.Diagnostics;
 

Samples/Diagnostics/PackageInspection_Samples.cs

@@ -2,6 +2,10 @@
 // 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
+
 using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Samples.Diagnostics;

Samples/MQTTnet.Samples.csproj

@@ -15,8 +15,7 @@
     <ItemGroup>
         <PackageReference Include="Microsoft.CodeAnalysis.Compilers" Version="4.0.1" />
         <PackageReference Include="Microsoft.Net.Compilers" Version="4.0.1" />
-        <PackageReference Include="MSTest.TestFramework" Version="2.1.2" />
-        
+
         <ProjectReference Include="..\Source\MQTTnet.AspnetCore\MQTTnet.AspNetCore.csproj" />
         <ProjectReference Include="..\Source\MQTTnet.Extensions.ManagedClient\MQTTnet.Extensions.ManagedClient.csproj" />
         <ProjectReference Include="..\Source\MQTTnet.Extensions.Rpc\MQTTnet.Extensions.Rpc.csproj" />

Samples/ManagedClient/Managed_Client_Simple_Samples.cs

@@ -1,3 +1,11 @@
+// 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
+
 using MQTTnet.Client;
 using MQTTnet.Extensions.ManagedClient;
 

Samples/Program.cs

@@ -39,10 +39,16 @@ Console.WriteLine();
 
 try
 {
-    var task = selectedSampleMethod.Invoke(null, null) as Task;
-    task?.Wait();
+    if (selectedSampleMethod.Invoke(null, null) is Task task)
+    {
+        await task;    
+    }
 }
 catch (Exception exception)
 {
     Console.WriteLine(exception.ToString());
-}
+}
+
+Console.WriteLine();
+Console.WriteLine("Press Enter to exit.");
+Console.ReadLine();

Samples/RpcClient/RpcClient_Samples.cs

@@ -2,6 +2,10 @@
 // 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
+
 using MQTTnet.Client;
 using MQTTnet.Extensions.Rpc;
 using MQTTnet.Protocol;

Samples/Server/Server_Asp_Net_Samples.cs

@@ -0,0 +1,110 @@
+// 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 Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using MQTTnet.AspNetCore;
+using MQTTnet.Server;
+
+namespace MQTTnet.Samples.Server;
+
+public static class Server_Asp_Net_Samples
+{
+    public static Task Start_Server_With_WebSockets_Support()
+    {
+        /*
+         * This sample starts a minimal ASP.NET Webserver including a hosted MQTT server.
+         */
+        var host = Host.CreateDefaultBuilder(Array.Empty<string>())
+            .ConfigureWebHostDefaults(
+                webBuilder =>
+                {
+                    webBuilder.UseKestrel(
+                        o =>
+                        {
+                            // This will allow MQTT connections based on TCP port 1883.
+                            o.ListenAnyIP(1883, l => l.UseMqtt());
+
+                            // This will allow MQTT connections based on HTTP WebSockets with URI "localhost:5000/mqtt"
+                            // See code below for URI configuration.
+                            o.ListenAnyIP(5000); // Default HTTP pipeline
+                        });
+
+                    webBuilder.UseStartup<Startup>();
+                });
+
+        return host.RunConsoleAsync();
+    }
+
+    sealed class MqttController
+    {
+        public MqttController()
+        {
+            // Inject other services via constructor.
+        }
+        
+        public Task OnClientConnected(ClientConnectedEventArgs eventArgs)
+        {
+            Console.WriteLine($"Client '{eventArgs.ClientId}' connected.");
+            return Task.CompletedTask;
+        }
+
+
+        public Task ValidateConnection(ValidatingConnectionEventArgs eventArgs)
+        {
+            Console.WriteLine($"Client '{eventArgs.ClientId}' wants to connect. Accepting!");
+            return Task.CompletedTask;
+        }
+    }
+
+    sealed class Startup
+    {
+        public void Configure(IApplicationBuilder app, IWebHostEnvironment environment, MqttController mqttController)
+        {
+            app.UseRouting();
+
+            app.UseEndpoints(
+                endpoints =>
+                {
+                    endpoints.MapConnectionHandler<MqttConnectionHandler>(
+                        "/mqtt",
+                        httpConnectionDispatcherOptions => httpConnectionDispatcherOptions.WebSockets.SubProtocolSelector =
+                            protocolList => protocolList.FirstOrDefault() ?? string.Empty);
+                });
+
+            app.UseMqttServer(
+                server =>
+                {
+                    /*
+                     * Attach event handlers etc. if required.
+                     */
+
+                    server.ValidatingConnectionAsync += mqttController.ValidateConnection;
+                    server.ClientConnectedAsync += mqttController.OnClientConnected;
+                });
+        }
+
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddHostedMqttServer(
+                optionsBuilder =>
+                {
+                    optionsBuilder.WithDefaultEndpoint();
+                });
+
+            services.AddMqttConnectionHandler();
+            services.AddConnections();
+
+            services.AddSingleton<MqttController>();
+        }
+    }
+}

Samples/Server/Server_Retained_Messages_Samples.cs

@@ -0,0 +1,82 @@
+// 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
+
+using System.Text.Json;
+
+namespace MQTTnet.Samples.Server;
+
+public static class Server_Retained_Messages_Samples
+{
+    public static async Task Persist_Retained_Messages()
+    {
+        /*
+         * This sample starts a simple MQTT server which will store all retained messages in a file.
+         */
+
+        var storePath = Path.Combine(Path.GetTempPath(), "RetainedMessages.json");
+
+        var mqttFactory = new MqttFactory();
+
+        // Due to security reasons the "default" endpoint (which is unencrypted) is not enabled by default!
+        var mqttServerOptions = mqttFactory.CreateServerOptionsBuilder().WithDefaultEndpoint().Build();
+
+        using (var server = mqttFactory.CreateMqttServer(mqttServerOptions))
+        {
+            // Make sure that the server will load the retained messages.
+            server.LoadingRetainedMessageAsync += async eventArgs =>
+            {
+                try
+                {
+                    eventArgs.LoadedRetainedMessages = await JsonSerializer.DeserializeAsync<List<MqttApplicationMessage>>(File.OpenRead(storePath));
+                    Console.WriteLine("Retained messages loaded.");
+                }
+                catch (FileNotFoundException)
+                {
+                    // Ignore because nothing is stored yet.
+                    Console.WriteLine("No retained messages stored yet.");
+                }
+                catch (Exception exception)
+                {
+                    Console.WriteLine(exception);
+                }
+            };
+
+            // Make sure to persist the changed retained messages.
+            server.RetainedMessageChangedAsync += async eventArgs =>
+            {
+                try
+                {
+                    // This sample uses the property _StoredRetainedMessages_ which will contain all(!) retained messages.
+                    // The event args also contain the affected retained message (property ChangedRetainedMessage). This can be
+                    // used to write all retained messages to dedicated files etc. Then all files must be loaded and a full list
+                    // of retained messages must be provided in the loaded event.
+
+                    var buffer = JsonSerializer.SerializeToUtf8Bytes(eventArgs.StoredRetainedMessages);
+                    await File.WriteAllBytesAsync(storePath, buffer);
+                    Console.WriteLine("Retained messages saved.");
+                }
+                catch (Exception exception)
+                {
+                    Console.WriteLine(exception);
+                }
+            };
+
+            // Make sure to clear the retained messages when they are all deleted via API.
+            server.RetainedMessagesClearedAsync += _ =>
+            {
+                File.Delete(storePath);
+                return Task.CompletedTask;
+            };
+
+            await server.StartAsync();
+
+            Console.WriteLine("Press Enter to exit.");
+            Console.ReadLine();
+        }
+    }
+}

Samples/Server/Server_Simple_Samples.cs

@@ -2,6 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+// ReSharper disable UnusedType.Global
+// ReSharper disable UnusedMember.Global
+// ReSharper disable InconsistentNaming
+
+using MQTTnet.Diagnostics;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 
@@ -9,27 +14,95 @@ namespace MQTTnet.Samples.Server;
 
 public static class Server_Simple_Samples
 {
+    public static async Task Force_Disconnecting_Client()
+    {
+        /*
+         * This sample will disconnect a client.
+         *
+         * See _Run_Minimal_Server_ for more information.
+         */
+
+        using (var mqttServer = await StartMqttServer())
+        {
+            // Let the client connect.
+            await Task.Delay(TimeSpan.FromSeconds(5));
+
+            // Now disconnect the client (if connected).
+            var affectedClient = (await mqttServer.GetClientsAsync()).FirstOrDefault(c => c.Id == "MyClient");
+            if (affectedClient != null)
+            {
+                await affectedClient.DisconnectAsync();
+            }
+        }
+    }
+
+    public static async Task Publish_Message_From_Broker()
+    {
+        /*
+         * This sample will publish a message directly at the broker.
+         *
+         * See _Run_Minimal_Server_ for more information.
+         */
+
+        using (var mqttServer = await StartMqttServer())
+        {
+            // Create a new message using the builder as usual.
+            var message = new MqttApplicationMessageBuilder().WithTopic("HelloWorld").WithPayload("Test").Build();
+
+            // Now inject the new message at the broker.
+            await mqttServer.InjectApplicationMessage(
+                new InjectedMqttApplicationMessage(message)
+                {
+                    SenderClientId = "SenderClientId"
+                });
+        }
+    }
+
     public static async Task Run_Minimal_Server()
     {
         /*
          * This sample starts a simple MQTT server which will accept any TCP connection.
          */
-        
+
         var mqttFactory = new MqttFactory();
 
         // The port for the default endpoint is 1883.
         // The default endpoint is NOT encrypted!
         // Use the builder classes where possible.
-        var mqttServerOptions = new MqttServerOptionsBuilder()
-            .WithDefaultEndpoint()
-            .Build();
-        
+        var mqttServerOptions = new MqttServerOptionsBuilder().WithDefaultEndpoint().Build();
+
         // The port can be changed using the following API (not used in this example).
-        new MqttServerOptionsBuilder()
-            .WithDefaultEndpoint()
-            .WithDefaultEndpointPort(1234)
-            .Build();
-        
+        // new MqttServerOptionsBuilder()
+        //     .WithDefaultEndpoint()
+        //     .WithDefaultEndpointPort(1234)
+        //     .Build();
+
+        using (var mqttServer = mqttFactory.CreateMqttServer(mqttServerOptions))
+        {
+            await mqttServer.StartAsync();
+
+            Console.WriteLine("Press Enter to exit.");
+            Console.ReadLine();
+
+            // Stop and dispose the MQTT server if it is no longer needed!
+            await mqttServer.StopAsync();
+        }
+    }
+
+    public static async Task Run_Server_With_Logging()
+    {
+        /*
+         * This sample starts a simple MQTT server and prints the logs to the output.
+         *
+         * IMPORTANT! Do not enable logging in live environment. It will decrease performance.
+         *
+         * See sample "Run_Minimal_Server" for more details.
+         */
+
+        var mqttFactory = new MqttFactory(new ConsoleLogger());
+
+        var mqttServerOptions = new MqttServerOptionsBuilder().WithDefaultEndpoint().Build();
+
         using (var mqttServer = mqttFactory.CreateMqttServer(mqttServerOptions))
         {
             await mqttServer.StartAsync();
@@ -52,9 +125,7 @@ public static class Server_Simple_Samples
 
         var mqttFactory = new MqttFactory();
 
-        var mqttServerOptions = new MqttServerOptionsBuilder()
-            .WithDefaultEndpoint()
-            .Build();
+        var mqttServerOptions = new MqttServerOptionsBuilder().WithDefaultEndpoint().Build();
 
         using (var mqttServer = mqttFactory.CreateMqttServer(mqttServerOptions))
         {
@@ -67,7 +138,7 @@ public static class Server_Simple_Samples
                     e.ReasonCode = MqttConnectReasonCode.ClientIdentifierNotValid;
                 }
 
-                if (e.Username != "ValidUser")
+                if (e.UserName != "ValidUser")
                 {
                     e.ReasonCode = MqttConnectReasonCode.BadUserNameOrPassword;
                 }
@@ -88,4 +159,61 @@ public static class Server_Simple_Samples
             await mqttServer.StopAsync();
         }
     }
-}
+
+    static async Task<MqttServer> StartMqttServer()
+    {
+        var mqttFactory = new MqttFactory();
+
+        // Due to security reasons the "default" endpoint (which is unencrypted) is not enabled by default!
+        var mqttServerOptions = mqttFactory.CreateServerOptionsBuilder().WithDefaultEndpoint().Build();
+        var server = mqttFactory.CreateMqttServer(mqttServerOptions);
+        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);
+                }
+            }
+        }
+    }
+}

Source/MQTTnet.AspNetCore.Tests/MQTTnet.AspNetCore.Tests.csproj

@@ -9,9 +9,9 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.0" />
-        <PackageReference Include="MSTest.TestAdapter" Version="2.1.2" />
-        <PackageReference Include="MSTest.TestFramework" Version="2.1.2" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.2.0" />
+        <PackageReference Include="MSTest.TestAdapter" Version="2.2.10" />
+        <PackageReference Include="MSTest.TestFramework" Version="2.2.10" />
         <PackageReference Include="Microsoft.CodeAnalysis.Compilers" Version="4.0.1" />
         <PackageReference Include="Microsoft.Net.Compilers" Version="4.0.1" />
     </ItemGroup>

Source/MQTTnet.AspnetCore/MQTTnet.AspNetCore.csproj

@@ -43,7 +43,7 @@
     </ItemGroup>
     
     <ItemGroup>
-        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
+        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
     </ItemGroup>
 
     <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">

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/ChannelAdapterBenchmark.cs

@@ -46,7 +46,7 @@ namespace MQTTnet.Benchmarks
 
             _stream.Position = 0;
 
-            var channel = new TestMqttChannel(_stream);
+            var channel = new MemoryMqttChannel(_stream);
             
             _channelAdapter = new MqttChannelAdapter(channel, serializer, null, new MqttNetEventLogger());
         }

Source/MQTTnet.Benchmarks/LoggerBenchmark.cs

@@ -8,7 +8,7 @@ using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [RPlotExporter]
     [MemoryDiagnoser]
     public class LoggerBenchmark

Source/MQTTnet.Benchmarks/MQTTnet.Benchmarks.csproj

@@ -12,14 +12,10 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="BenchmarkDotNet" Version="0.12.1" />
+        <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/MessageProcessingBenchmark.cs

@@ -6,11 +6,10 @@ using BenchmarkDotNet.Attributes;
 using BenchmarkDotNet.Jobs;
 using MQTTnet.Client;
 using MQTTnet.Server;
-using MqttClient = MQTTnet.Client.MqttClient;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [RPlotExporter, RankColumn]
     [MemoryDiagnoser]
     public class MessageProcessingBenchmark

Source/MQTTnet.Benchmarks/MessageProcessingMqttConnectionContextBenchmark.cs

@@ -13,7 +13,7 @@ using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [MemoryDiagnoser]
     public class MessageProcessingMqttConnectionContextBenchmark
     {

Source/MQTTnet.Benchmarks/MqttPacketReaderWriterBenchmark.cs

@@ -7,7 +7,7 @@ using MQTTnet.Tests.Mockups;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [MemoryDiagnoser]
     public class MqttPacketReaderWriterBenchmark
     {

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;
@@ -14,7 +15,7 @@ using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [MemoryDiagnoser]
     public sealed class MqttTcpChannelBenchmark
     {
@@ -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/RoundtripProcessingBenchmark.cs

@@ -5,7 +5,7 @@ using MQTTnet.Tests.Server;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [RPlotExporter, RankColumn]
     [MemoryDiagnoser]
     public class RoundtripProcessingBenchmark

Source/MQTTnet.Benchmarks/SerializerBenchmark.cs

@@ -17,7 +17,7 @@ using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [RPlotExporter]
     [MemoryDiagnoser]
     public class SerializerBenchmark : BaseBenchmark
@@ -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.Benchmarks/ServerProcessingBenchmark.cs

@@ -5,7 +5,7 @@ using MQTTnet.Tests.Server;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [RPlotExporter, RankColumn]
     [MemoryDiagnoser]
     public class ServerProcessingBenchmark

Source/MQTTnet.Benchmarks/TcpPipesBenchmark.cs

@@ -13,7 +13,7 @@ using MQTTnet.AspNetCore.Client.Tcp;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [MemoryDiagnoser]
     public class TcpPipesBenchmark
     {

Source/MQTTnet.Benchmarks/TopicFilterComparerBenchmark.cs

@@ -8,7 +8,7 @@ using BenchmarkDotNet.Jobs;
 
 namespace MQTTnet.Benchmarks
 {
-    [SimpleJob(RuntimeMoniker.NetCoreApp50)]
+    [SimpleJob(RuntimeMoniker.Net60)]
     [RPlotExporter]
     [MemoryDiagnoser]
     public class TopicFilterComparerBenchmark

Source/MQTTnet.Extensions.ManagedClient/MQTTnet.Extensions.ManagedClient.csproj

@@ -57,7 +57,7 @@
     </ItemGroup>
     
     <ItemGroup>
-        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
+        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
     </ItemGroup>
 
     <ItemGroup>

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs

@@ -51,9 +51,9 @@ namespace MQTTnet.Extensions.ManagedClient
         ///         cref="MaintainConnectionAsync" />
         ///     .
         /// </summary>
-        readonly Dictionary<string, MqttQualityOfServiceLevel> _reconnectSubscriptions = new Dictionary<string, MqttQualityOfServiceLevel>();
+        readonly Dictionary<string, MqttTopicFilter> _reconnectSubscriptions = new Dictionary<string, MqttTopicFilter>();
 
-        readonly Dictionary<string, MqttQualityOfServiceLevel> _subscriptions = new Dictionary<string, MqttQualityOfServiceLevel>();
+        readonly Dictionary<string, MqttTopicFilter> _subscriptions = new Dictionary<string, MqttTopicFilter>();
         readonly SemaphoreSlim _subscriptionsQueuedSignal = new SemaphoreSlim(0);
         readonly HashSet<string> _unsubscriptions = new HashSet<string>();
 
@@ -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)
                     {
@@ -201,7 +201,7 @@ namespace MQTTnet.Extensions.ManagedClient
             }
             finally
             {
-                if (_applicationMessageSkippedEvent.HasHandlers)
+                if (applicationMessageSkippedEventArgs != null && _applicationMessageSkippedEvent.HasHandlers)
                 {
                     await _applicationMessageSkippedEvent.InvokeAsync(applicationMessageSkippedEventArgs).ConfigureAwait(false);
                 }
@@ -289,14 +289,14 @@ namespace MQTTnet.Extensions.ManagedClient
             {
                 foreach (var topicFilter in topicFilters)
                 {
-                    _subscriptions[topicFilter.Topic] = topicFilter.QualityOfServiceLevel;
+                    _subscriptions[topicFilter.Topic] = topicFilter;
                     _unsubscriptions.Remove(topicFilter.Topic);
                 }
             }
 
             _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)
@@ -455,18 +455,11 @@ namespace MQTTnet.Extensions.ManagedClient
             {
                 if (_reconnectSubscriptions.Any())
                 {
-                    var subscriptions = _reconnectSubscriptions.Select(
-                        i => new MqttTopicFilter
-                        {
-                            Topic = i.Key,
-                            QualityOfServiceLevel = i.Value
-                        });
-
                     var topicFilters = new List<MqttTopicFilter>();
 
-                    foreach (var sub in subscriptions)
+                    foreach (var sub in _reconnectSubscriptions)
                     {
-                        topicFilters.Add(sub);
+                        topicFilters.Add(sub.Value);
 
                         if (topicFilters.Count == Options.MaxTopicFiltersInSubscribeUnsubscribePackets)
                         {
@@ -495,14 +488,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
                 lock (_subscriptions)
                 {
-                    subscriptions = _subscriptions.Select(
-                            i => new MqttTopicFilter
-                            {
-                                Topic = i.Key,
-                                QualityOfServiceLevel = i.Value
-                            })
-                        .ToList();
-
+                    subscriptions = _subscriptions.Values.ToList();
                     _subscriptions.Clear();
 
                     unsubscriptions = new HashSet<string>(_unsubscriptions);
@@ -523,7 +509,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
                 foreach (var subscription in subscriptions)
                 {
-                    _reconnectSubscriptions[subscription.Topic] = subscription.QualityOfServiceLevel;
+                    _reconnectSubscriptions[subscription.Topic] = subscription;
                 }
 
                 var addedTopicFilters = new List<MqttTopicFilter>();
@@ -707,7 +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
@@ -738,7 +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/MQTTnet.Extensions.Rpc.csproj

@@ -57,7 +57,7 @@
     </ItemGroup>
     
     <ItemGroup>
-        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
+        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
     </ItemGroup>
 
     <ItemGroup>

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/MQTTnet.Extensions.WebSocket4Net.csproj

@@ -57,15 +57,15 @@
     </ItemGroup>
     
     <ItemGroup>
-        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All"/>
+        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
     </ItemGroup>
 
     <ItemGroup>
-        <PackageReference Include="WebSocket4Net" Version="0.15.2"/>
+        <PackageReference Include="WebSocket4Net" Version="0.15.2" />
     </ItemGroup>
 
     <ItemGroup>
-        <ProjectReference Include="..\MQTTnet\MQTTnet.csproj"/>
+        <ProjectReference Include="..\MQTTnet\MQTTnet.csproj" />
     </ItemGroup>
 
 </Project>

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;
 
@@ -98,7 +99,6 @@ namespace MQTTnet.Extensions.WebSocket4Net
 #else
                     certificates.Add(certificate);
 #endif
-                    
                 }
             }
 
@@ -125,7 +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)
@@ -155,10 +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/MQTTnet.TestApp.csproj

@@ -12,7 +12,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />
+        <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
         <PackageReference Include="Microsoft.CodeAnalysis.Compilers" Version="4.0.1" />
         <PackageReference Include="Microsoft.Net.Compilers" Version="4.0.1" />
     </ItemGroup>
@@ -23,10 +23,4 @@
         <ProjectReference Include="..\..\Source\MQTTnet\MQTTnet.csproj" />
     </ItemGroup>
 
-    <ItemGroup>
-        <None Update="Start.bat">
-            <CopyToOutputDirectory>Always</CopyToOutputDirectory>
-        </None>
-    </ItemGroup>
-
 </Project>

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 =>
@@ -111,20 +109,20 @@ namespace MQTTnet.TestApp
                         e.CloseConnection = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
                 
                 mqttServer.ValidatingConnectionAsync += e =>
                 {
                     if (e.ClientId == "SpecialClient")
                     {
-                        if (e.Username != "USER" || e.Password != "PASS")
+                        if (e.UserName != "USER" || e.Password != "PASS")
                         {
                             e.ReasonCode = MqttConnectReasonCode.BadUserNameOrPassword;
                         }
                     }
 
-                    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.TestApp/Start.bat

@@ -1 +0,0 @@
-dotnet MQTTnet.TestApp.NetCore.dll

Source/MQTTnet.Tests/Client/ManagedMqttClient_Tests.cs

@@ -11,12 +11,10 @@ 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
 {
@@ -39,7 +37,7 @@ namespace MQTTnet.Tests.Client
                 managedClient.ConnectingFailedAsync += args =>
                 {
                     connectingFailedEventArgs = args;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await managedClient.StartAsync(
@@ -117,7 +115,7 @@ 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 +279,7 @@ 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 +288,29 @@ 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 +318,9 @@ 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 +429,7 @@ namespace MQTTnet.Tests.Client
             managedClient.ConnectedAsync += e =>
             {
                 connected.TrySetResult(true);
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             };
 
             return connected.Task;
@@ -457,7 +458,7 @@ namespace MQTTnet.Tests.Client
                     result.TrySetResult(receivedMessages);
                 }
 
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             };
 
             return result.Task;
@@ -486,7 +487,7 @@ 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

@@ -563,7 +563,7 @@ namespace MQTTnet.Tests
 
             using (var headerStream = new MemoryStream(buffer1.Join().ToArray()))
             {
-                using (var channel = new TestMqttChannel(headerStream))
+                using (var channel = new MemoryMqttChannel(headerStream))
                 {
                     using (var adapter = new MqttChannelAdapter(
                                channel,
@@ -583,7 +583,7 @@ namespace MQTTnet.Tests
 
         MqttProtocolVersion DeserializeAndDetectVersion(MqttPacketFormatterAdapter packetFormatterAdapter, byte[] buffer)
         {
-            var channel = new TestMqttChannel(buffer);
+            var channel = new MemoryMqttChannel(buffer);
             var adapter = new MqttChannelAdapter(channel, packetFormatterAdapter, null, new MqttNetEventLogger());
 
             adapter.ReceivePacketAsync(CancellationToken.None).GetAwaiter().GetResult();
@@ -603,7 +603,7 @@ namespace MQTTnet.Tests
             var serializer = MqttPacketFormatterAdapter.GetMqttPacketFormatter(protocolVersion, writer);
             var buffer = serializer.Encode(packet);
 
-            using (var channel = new TestMqttChannel(buffer.Join().ToArray()))
+            using (var channel = new MemoryMqttChannel(buffer.Join().ToArray()))
             {
                 var adapter = new MqttChannelAdapter(channel, new MqttPacketFormatterAdapter(protocolVersion, new MqttBufferWriter(4096, 65535)), null, new MqttNetEventLogger());
                 return (TPacket)adapter.ReceivePacketAsync(CancellationToken.None).GetAwaiter().GetResult();

Source/MQTTnet.Tests/Internal/AsyncEvent_Tests.cs

@@ -0,0 +1,140 @@
+// 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 Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Internal;
+
+namespace MQTTnet.Tests.Internal
+{
+    [TestClass]
+    public sealed class AsyncEvent_Tests
+    {
+        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()
+        {
+            var testClass = new TestClass();
+
+            testClass.TestEventAsync += OnTestEventAsync;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+
+            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()
+        {
+            var testClass = new TestClass();
+
+            testClass.TestEventAsync += OnTestEventAsync;
+            testClass.TestEventAsync -= OnTestEventAsync;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+
+            Assert.AreEqual(0, _testEventAsyncCount);
+        }
+
+        [TestMethod]
+        public void Has_Handlers()
+        {
+            var testClass = new TestClass();
+            testClass.TestEventAsync += OnTestEventAsync;
+
+            Assert.AreEqual(true, testClass.HasTestHandlers);
+        }
+
+        [TestMethod]
+        public void No_Handlers()
+        {
+            var testClass = new TestClass();
+
+            Assert.AreEqual(false, testClass.HasTestHandlers);
+        }
+
+        [TestMethod]
+        public void Remove_Handlers()
+        {
+            var testClass = new TestClass();
+            testClass.TestEventAsync += OnTestEventAsync;
+            testClass.TestEventAsync -= OnTestEventAsync;
+
+            Assert.AreEqual(false, testClass.HasTestHandlers);
+        }
+
+        void OnTestEvent(EventArgs arg)
+        {
+            Interlocked.Increment(ref _testEventAsyncCount);
+        }
+
+        Task OnTestEventAsync(EventArgs arg)
+        {
+            Interlocked.Increment(ref _testEventAsyncCount);
+            return CompletedTask.Instance;
+        }
+
+        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/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>
@@ -10,9 +10,9 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="MSTest.TestAdapter" Version="2.1.2" />
-        <PackageReference Include="MSTest.TestFramework" Version="2.1.2" />
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.0" />
+        <PackageReference Include="MSTest.TestAdapter" Version="2.2.10" />
+        <PackageReference Include="MSTest.TestFramework" Version="2.2.10" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.2.0" />
 
         <PackageReference Include="Microsoft.CodeAnalysis.Compilers" Version="4.0.1" />
         <PackageReference Include="Microsoft.Net.Compilers" Version="4.0.1" />

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()
@@ -212,7 +211,7 @@ namespace MQTTnet.Tests.MQTTv5
                 
                 var client1 = await testEnvironment.ConnectClient(o => o.WithProtocolVersion(MqttProtocolVersion.V500).WithClientId("client1"));
 
-                var testMessageHandler = new TestApplicationMessageReceivedHandler(client1);
+                var testMessageHandler = testEnvironment.CreateApplicationMessageHandler(client1);
 
                 await client1.SubscribeAsync("a");
 
@@ -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

@@ -2,24 +2,26 @@
 // 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
 {
-    public sealed class TestMqttChannel : IMqttChannel
+    public sealed class MemoryMqttChannel : IMqttChannel
     {
         readonly MemoryStream _stream;
 
-        public TestMqttChannel(MemoryStream stream)
+        public MemoryMqttChannel(MemoryStream stream)
         {
             _stream = stream;
         }
 
-        public TestMqttChannel(byte[] buffer)
+        public MemoryMqttChannel(byte[] buffer)
         {
             _stream = new MemoryStream(buffer);
         }
@@ -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/MqttPacketAsserts.cs

@@ -1,22 +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 Microsoft.VisualStudio.TestTools.UnitTesting;
-using MQTTnet.Packets;
-
-namespace MQTTnet.Tests.Mockups
-{
-    public sealed class MqttPacketAsserts
-    {
-        public void AssertIsConnectPacket(MqttPacket packet)
-        {
-            Assert.AreEqual(packet.GetType(), typeof(MqttConnectPacket));
-        }
-        
-        public void AssertIsConnAckPacket(MqttPacket packet)
-        {
-            Assert.AreEqual(packet.GetType(), typeof(MqttConnAckPacket));
-        }
-    }
-}

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/MqttPacketSerializationHelper.cs

@@ -27,7 +27,7 @@ namespace MQTTnet.Tests
 
         public MqttPacket Decode(MqttPacketBuffer buffer)
         {
-            using (var channel = new TestMqttChannel(buffer.ToArray()))
+            using (var channel = new MemoryMqttChannel(buffer.ToArray()))
             {
                 var formatterAdapter = new MqttPacketFormatterAdapter(_protocolVersion, new MqttBufferWriter(4096, 65535));
 

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");
@@ -798,38 +797,7 @@ namespace MQTTnet.Tests.Server
                 Assert.IsTrue(longBody.SequenceEqual(receivedBody ?? new byte[0]));
             }
         }
-
-        [TestMethod]
-        public async Task Session_Takeover()
-        {
-            using (var testEnvironment = CreateTestEnvironment())
-            {
-                await testEnvironment.StartServer();
-
-                var options = new MqttClientOptionsBuilder().WithCleanSession(false)
-                    .WithProtocolVersion(MqttProtocolVersion.V500) // Disconnect reason is only available in MQTT 5+
-                    .WithClientId("a");
-
-                var client1 = await testEnvironment.ConnectClient(options);
-                await Task.Delay(500);
-
-                var disconnectReason = MqttClientDisconnectReason.NormalDisconnection;
-                client1.DisconnectedAsync += c =>
-                {
-                    disconnectReason = c.Reason;
-                    return PlatformAbstractionLayer.CompletedTask;
-                };
-
-                var client2 = await testEnvironment.ConnectClient(options);
-                await Task.Delay(500);
-
-                Assert.IsFalse(client1.IsConnected);
-                Assert.IsTrue(client2.IsConnected);
-
-                Assert.AreEqual(MqttClientDisconnectReason.SessionTakenOver, disconnectReason);
-            }
-        }
-
+        
         [TestMethod]
         public async Task Set_Subscription_At_Server()
         {
@@ -853,7 +821,7 @@ namespace MQTTnet.Tests.Server
                         receivedMessages.Add(e.ApplicationMessage);
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await Task.Delay(500);
@@ -881,7 +849,7 @@ namespace MQTTnet.Tests.Server
                 c1.DisconnectedAsync += e =>
                 {
                     disconnectCalled++;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await Task.Delay(100);
@@ -944,26 +912,6 @@ namespace MQTTnet.Tests.Server
             }
         }
 
-        [TestMethod]
-        public async Task Use_Clean_Session()
-        {
-            using (var testEnvironment = CreateTestEnvironment())
-            {
-                await testEnvironment.StartServer();
-
-                var client = testEnvironment.CreateClient();
-                var connectResult = await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("localhost", testEnvironment.ServerPort).WithCleanSession().Build());
-
-                // Create the session including the subscription.
-                var client1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("a").WithCleanSession(false));
-                await client1.SubscribeAsync("x");
-                await client1.DisconnectAsync();
-                await Task.Delay(500);
-
-                Assert.IsFalse(connectResult.IsSessionPresent);
-            }
-        }
-
         [TestMethod]
         public async Task Use_Empty_Client_ID()
         {
@@ -988,93 +936,57 @@ namespace MQTTnet.Tests.Server
         }
 
         [TestMethod]
-        public async Task Will_Message_Do_Not_Send_On_Clean_Disconnect()
+        public async Task Disconnect_Client_with_Reason()
         {
             using (var testEnvironment = CreateTestEnvironment())
             {
-                var receivedMessagesCount = 0;
+                var disconnectPacketReceived = false;
+
+                string testClientId = null;
 
                 await testEnvironment.StartServer();
 
-                var clientOptions = new MqttClientOptionsBuilder().WithWillTopic("My/last/will");
-
-                var c1 = await testEnvironment.ConnectClient();
-
-                c1.ApplicationMessageReceivedAsync += e =>
+                testEnvironment.Server.ClientConnectedAsync += e =>
                 {
-                    Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    testClientId = e.ClientId;
+                    return CompletedTask.Instance;
                 };
 
-                await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
+                var client = testEnvironment.CreateClient();
 
-                var c2 = await testEnvironment.ConnectClient(clientOptions);
-                await c2.DisconnectAsync().ConfigureAwait(false);
-
-                await Task.Delay(1000);
-
-                Assert.AreEqual(0, receivedMessagesCount);
-            }
-        }
-
-        [TestMethod]
-        public async Task Will_Message_Do_Not_Send_On_Takeover()
-        {
-            using (var testEnvironment = CreateTestEnvironment())
-            {
-                var receivedMessagesCount = 0;
-
-                await testEnvironment.StartServer();
-
-                // C1 will receive the last will!
-                var c1 = await testEnvironment.ConnectClient();
-                c1.ApplicationMessageReceivedAsync += e =>
+                client.InspectPackage += e =>
                 {
-                    Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    if (e.Buffer.Length > 0)
+                    {
+                        if (e.Buffer[0] == (byte)MqttControlPacketType.Disconnect << 4)
+                        {
+                            disconnectPacketReceived = true;
+                        }
+                    }
+                    return CompletedTask.Instance;
                 };
 
-                await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
+                await client.ConnectAsync(
+                    new MqttClientOptionsBuilder().WithTcpServer("localhost", testEnvironment.ServerPort)
+                    .WithProtocolVersion(MQTTnet.Formatter.MqttProtocolVersion.V311)
+                    .Build()
+                    );
 
-                // C2 has the last will defined.
-                var clientOptions = new MqttClientOptionsBuilder().WithWillTopic("My/last/will").WithClientId("WillOwner");
+                await LongTestDelay();
 
-                var c2 = await testEnvironment.ConnectClient(clientOptions);
+                // Test client should be connected now
 
-                // C3 will do the connection takeover.
-                var c3 = await testEnvironment.ConnectClient(clientOptions);
+                Assert.IsTrue(testClientId != null);
 
-                await Task.Delay(1000);
+                // Have the server disconnect the client with SessionTakenOver reason; no DISCONNECT should be sent to MQTT 3.1.1 clients
 
-                Assert.AreEqual(0, receivedMessagesCount);
-            }
-        }
+                await testEnvironment.Server.DisconnectClientAsync(testClientId, Protocol.MqttDisconnectReasonCode.SessionTakenOver);
 
-        [TestMethod]
-        public async Task Will_Message_Send()
-        {
-            using (var testEnvironment = CreateTestEnvironment())
-            {
-                await testEnvironment.StartServer();
+                await LongTestDelay();
 
-                var c1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder());
+                // MQTT 3.1.1 client should not receive a disconnect packet
 
-                var receivedMessagesCount = 0;
-                c1.ApplicationMessageReceivedAsync += e =>
-                {
-                    Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
-                };
-
-                await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
-
-                var clientOptions = new MqttClientOptionsBuilder().WithWillTopic("My/last/will").WithWillQualityOfServiceLevel(MqttQualityOfServiceLevel.AtMostOnce);
-                var c2 = await testEnvironment.ConnectClient(clientOptions);
-                c2.Dispose(); // Dispose will not send a DISCONNECT pattern first so the will message must be sent.
-
-                await Task.Delay(1000);
-
-                Assert.AreEqual(1, receivedMessagesCount);
+                Assert.IsFalse(disconnectPacketReceived, "Disconnect packet received when none is expected");
             }
         }
 

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/+");
@@ -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/QoS_Tests.cs

@@ -0,0 +1,124 @@
+// 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.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Internal;
+using MQTTnet.Protocol;
+using MQTTnet.Server;
+
+namespace MQTTnet.Tests.Server
+{
+    [TestClass]
+    public sealed class QoS_Tests : BaseTestClass
+    {
+        [TestMethod]
+        public async Task Fire_Event_On_Client_Acknowledges_QoS_0()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                ClientAcknowledgedPublishPacketEventArgs eventArgs = null;
+                server.ClientAcknowledgedPublishPacketAsync += args =>
+                {
+                    eventArgs = args;
+                    return CompletedTask.Instance;
+                };
+
+                var client1 = await testEnvironment.ConnectClient();
+                await client1.SubscribeAsync("A");
+
+                var client2 = await testEnvironment.ConnectClient();
+                await client2.PublishStringAsync("A");
+
+                await LongTestDelay();
+
+                // Must be null because no event should be fired for QoS 0.
+                Assert.IsNull(eventArgs);
+            }
+        }
+
+        [TestMethod]
+        public async Task Fire_Event_On_Client_Acknowledges_QoS_1()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                ClientAcknowledgedPublishPacketEventArgs eventArgs = null;
+                server.ClientAcknowledgedPublishPacketAsync += args =>
+                {
+                    eventArgs = args;
+                    return CompletedTask.Instance;
+                };
+
+                var client1 = await testEnvironment.ConnectClient();
+                await client1.SubscribeAsync("A", MqttQualityOfServiceLevel.AtLeastOnce);
+
+                var client2 = await testEnvironment.ConnectClient();
+                await client2.PublishStringAsync("A", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+
+                await LongTestDelay();
+
+                Assert.IsNotNull(eventArgs);
+                Assert.IsNotNull(eventArgs.PublishPacket);
+                Assert.IsNotNull(eventArgs.AcknowledgePacket);
+                Assert.IsTrue(eventArgs.IsCompleted);
+
+                Assert.AreEqual("A", eventArgs.PublishPacket.Topic);
+            }
+        }
+
+        [TestMethod]
+        public async Task Fire_Event_On_Client_Acknowledges_QoS_2()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                var eventArgs = new List<ClientAcknowledgedPublishPacketEventArgs>();
+                server.ClientAcknowledgedPublishPacketAsync += args =>
+                {
+                    lock (eventArgs)
+                    {
+                        eventArgs.Add(args);
+                    }
+
+                    return CompletedTask.Instance;
+                };
+
+                var client1 = await testEnvironment.ConnectClient();
+                await client1.SubscribeAsync("A", MqttQualityOfServiceLevel.ExactlyOnce);
+
+                var client2 = await testEnvironment.ConnectClient();
+                await client2.PublishStringAsync("A", qualityOfServiceLevel: MqttQualityOfServiceLevel.ExactlyOnce);
+
+                await LongTestDelay();
+
+                Assert.AreEqual(2, eventArgs.Count);
+
+                var firstEvent = eventArgs[0];
+
+                Assert.IsNotNull(firstEvent);
+                Assert.IsNotNull(firstEvent.PublishPacket);
+                Assert.IsNotNull(firstEvent.AcknowledgePacket);
+                Assert.IsFalse(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/Security_Tests.cs

@@ -2,12 +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.Collections.Generic;
 using System.Threading.Tasks;
 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
@@ -15,6 +16,104 @@ namespace MQTTnet.Tests.Server
     [TestClass]
     public sealed class Security_Tests : BaseTestClass
     {
+        [TestMethod]
+        public async Task Do_Not_Affect_Authorized_Clients()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                testEnvironment.IgnoreClientLogErrors = true;
+                
+                await testEnvironment.StartServer();
+
+                var publishedApplicationMessages = new List<MqttApplicationMessage>();
+                testEnvironment.Server.InterceptingPublishAsync += eventArgs =>
+                {
+                    lock (publishedApplicationMessages)
+                    {
+                        publishedApplicationMessages.Add(eventArgs.ApplicationMessage);
+                    }
+
+                    return CompletedTask.Instance;
+                };
+
+                testEnvironment.Server.ValidatingConnectionAsync += eventArgs =>
+                {
+                    if (eventArgs.UserName == "SECRET")
+                    {
+                        eventArgs.ReasonCode = MqttConnectReasonCode.Success;
+                    }
+                    else
+                    {
+                        eventArgs.ReasonCode = MqttConnectReasonCode.NotAuthorized;
+                    }
+
+                    return CompletedTask.Instance;
+                };
+
+                using (var validClient = testEnvironment.CreateClient())
+                {
+                    await validClient.ConnectAsync(
+                        testEnvironment.Factory.CreateClientOptionsBuilder()
+                            .WithTcpServer("localhost", testEnvironment.ServerPort)
+                            .WithCredentials("SECRET")
+                            .WithClientId("CLIENT")
+                            .Build());
+
+                    await validClient.PublishStringAsync("HELLO 1");
+
+                    // The following code tries to connect a new client with the same client ID but invalid
+                    // credentials. This should block the second client but keep the first one up and running.
+                    try
+                    {
+                        using (var invalidClient = testEnvironment.CreateClient())
+                        {
+                            await invalidClient.ConnectAsync(
+                                testEnvironment.Factory.CreateClientOptionsBuilder()
+                                    .WithTcpServer("localhost", testEnvironment.ServerPort)
+                                    .WithCredentials("???")
+                                    .WithClientId("CLIENT")
+                                    .Build());
+                        }
+                    }
+                    catch
+                    {
+                        // Ignore errors from the second client.
+                    }
+
+                    await LongTestDelay();
+                    
+                    await validClient.PublishStringAsync("HELLO 2");
+                    
+                    await LongTestDelay();
+                    
+                    await validClient.PublishStringAsync("HELLO 3");
+                    
+                    await LongTestDelay();
+                    
+                    Assert.AreEqual(3, publishedApplicationMessages.Count);
+                    Assert.AreEqual(1, testEnvironment.Server.GetClientsAsync().GetAwaiter().GetResult().Count);
+                }
+            }
+        }
+
+        [TestMethod]
+        public Task Handle_Wrong_Password()
+        {
+            return TestCredentials("UserName", "x");
+        }
+
+        [TestMethod]
+        public Task Handle_Wrong_UserName()
+        {
+            return TestCredentials("x", "Password1");
+        }
+
+        [TestMethod]
+        public Task Handle_Wrong_UserName_And_Password()
+        {
+            return TestCredentials("x", "x");
+        }
+
         [TestMethod]
         public async Task Use_Username_Null_Password_Empty()
         {
@@ -29,10 +128,7 @@ namespace MQTTnet.Tests.Server
 
                 var client = testEnvironment.CreateClient();
 
-                var clientOptions = new MqttClientOptionsBuilder()
-                    .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
-                    .WithCredentials(username, password)
-                    .Build();
+                var clientOptions = new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort).WithCredentials(username, password).Build();
 
                 var ex = await Assert.ThrowsExceptionAsync<MqttConnectingFailedException>(async () => await client.ConnectAsync(clientOptions));
                 Assert.IsInstanceOfType(ex.InnerException, typeof(MqttProtocolViolationException));
@@ -41,25 +137,7 @@ namespace MQTTnet.Tests.Server
             }
         }
 
-        [TestMethod]
-        public Task Handle_Wrong_UserName()
-        {
-            return TestCredentials("x", "Password1");
-        }
-
-        [TestMethod]
-        public Task Handle_Wrong_Password()
-        {
-            return TestCredentials("UserName", "x");
-        }
-
-        [TestMethod]
-        public Task Handle_Wrong_UserName_And_Password()
-        {
-            return TestCredentials("x", "x");
-        }
-
-        private async Task TestCredentials(string userName, string password)
+        async Task TestCredentials(string userName, string password)
         {
             using (var testEnvironment = CreateTestEnvironment())
             {
@@ -69,7 +147,7 @@ namespace MQTTnet.Tests.Server
 
                 server.ValidatingConnectionAsync += e =>
                 {
-                    if (e.Username != "UserName1")
+                    if (e.UserName != "UserName1")
                     {
                         e.ReasonCode = MqttConnectReasonCode.BadUserNameOrPassword;
                     }
@@ -79,15 +157,12 @@ namespace MQTTnet.Tests.Server
                         e.ReasonCode = MqttConnectReasonCode.BadUserNameOrPassword;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = testEnvironment.CreateClient();
 
-                var clientOptions = new MqttClientOptionsBuilder()
-                    .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
-                    .WithCredentials(userName, password)
-                    .Build();
+                var clientOptions = new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort).WithCredentials(userName, password).Build();
 
                 var ex = await Assert.ThrowsExceptionAsync<MqttConnectingFailedException>(async () => await client.ConnectAsync(clientOptions));
                 Assert.AreEqual(MqttClientConnectResultCode.BadUserNameOrPassword, ex.Result.ResultCode);

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

@@ -10,11 +10,9 @@ 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;
-using MqttClient = MQTTnet.Client.MqttClient;
 
 namespace MQTTnet.Tests.Server
 {
@@ -30,7 +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
 
@@ -39,7 +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
@@ -58,7 +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();
 
 
@@ -84,7 +82,7 @@ namespace MQTTnet.Tests.Server
                 server.SessionDeletedAsync += e =>
                 {
                     deletedEventFired = true;
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -112,7 +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();
@@ -124,8 +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())
             {
@@ -133,7 +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;
 
@@ -147,11 +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);
@@ -162,26 +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)]
@@ -201,7 +188,7 @@ namespace MQTTnet.Tests.Server
                     c.AutoAcknowledge = false;
                     ++count;
                     Console.WriteLine("process");
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await subscriber.SubscribeAsync("#", qos);
@@ -220,6 +207,37 @@ namespace MQTTnet.Tests.Server
             }
         }
 
+        [TestMethod]
+        public async Task Session_Takeover()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                await testEnvironment.StartServer();
+
+                var options = new MqttClientOptionsBuilder().WithCleanSession(false)
+                    .WithProtocolVersion(MqttProtocolVersion.V500) // Disconnect reason is only available in MQTT 5+
+                    .WithClientId("a");
+
+                var client1 = await testEnvironment.ConnectClient(options);
+                await Task.Delay(500);
+
+                var disconnectReason = MqttClientDisconnectReason.NormalDisconnection;
+                client1.DisconnectedAsync += c =>
+                {
+                    disconnectReason = c.Reason;
+                    return CompletedTask.Instance;
+                };
+
+                var client2 = await testEnvironment.ConnectClient(options);
+                await Task.Delay(500);
+
+                Assert.IsFalse(client1.IsConnected);
+                Assert.IsTrue(client2.IsConnected);
+
+                Assert.AreEqual(MqttClientDisconnectReason.SessionTakenOver, disconnectReason);
+            }
+        }
+
         [TestMethod]
         public async Task Set_Session_Item()
         {
@@ -233,7 +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 =>
@@ -246,13 +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;
@@ -261,7 +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");
@@ -277,41 +295,82 @@ namespace MQTTnet.Tests.Server
             }
         }
 
-        async Task<IMqttClient> ConnectAndSubscribe(TestEnvironment testEnvironment, MqttClientOptionsBuilder options, Action onReceive)
+        [TestMethod]
+        public async Task Use_Clean_Session()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                await testEnvironment.StartServer();
+
+                var client = testEnvironment.CreateClient();
+                var connectResult = await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("localhost", testEnvironment.ServerPort).WithCleanSession().Build());
+
+                // Create the session including the subscription.
+                var client1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("a").WithCleanSession(false));
+                await client1.SubscribeAsync("x");
+                await client1.DisconnectAsync();
+                await Task.Delay(500);
+
+                Assert.IsFalse(connectResult.IsSessionPresent);
+            }
+        }
+
+        [TestMethod]
+        public async Task Will_Message_Do_Not_Send_On_Takeover()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var receivedMessagesCount = 0;
+
+                await testEnvironment.StartServer();
+
+                // C1 will receive the last will!
+                var c1 = await testEnvironment.ConnectClient();
+                c1.ApplicationMessageReceivedAsync += e =>
+                {
+                    Interlocked.Increment(ref receivedMessagesCount);
+                    return CompletedTask.Instance;
+                };
+
+                await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
+
+                // C2 has the last will defined.
+                var clientOptions = new MqttClientOptionsBuilder().WithWillTopic("My/last/will").WithClientId("WillOwner");
+
+                await testEnvironment.ConnectClient(clientOptions);
+
+                // C3 will do the connection takeover.
+                await testEnvironment.ConnectClient(clientOptions);
+
+                await Task.Delay(1000);
+
+                Assert.AreEqual(0, receivedMessagesCount);
+            }
+        }
+
+        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

@@ -9,9 +9,10 @@ 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;
 
 namespace MQTTnet.Tests.Server
 {
@@ -32,7 +33,7 @@ namespace MQTTnet.Tests.Server
                         e.Response.ReasonCode = MqttSubscribeReasonCode.TopicFilterInvalid;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var client = await testEnvironment.ConnectClient();
@@ -45,6 +46,30 @@ namespace MQTTnet.Tests.Server
             }
         }
 
+        [TestMethod]
+        public async Task Enqueue_Message_After_Subscription()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                server.ClientSubscribedTopicAsync += e =>
+                {
+                    server.InjectApplicationMessage(new InjectedMqttApplicationMessage(new MqttApplicationMessageBuilder().WithTopic("test_topic").Build()));
+                    return CompletedTask.Instance;
+                };
+
+                var client = await testEnvironment.ConnectClient();
+                var receivedMessages = testEnvironment.CreateApplicationMessageHandler(client);
+
+                await client.SubscribeAsync("test_topic");
+
+                await LongTestDelay();
+
+                Assert.AreEqual(1, receivedMessages.ReceivedEventArgs.Count);
+            }
+        }
+
         [TestMethod]
         public async Task Intercept_Subscription()
         {
@@ -56,7 +81,7 @@ 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;
@@ -74,7 +99,7 @@ namespace MQTTnet.Tests.Server
                         topicBReceived = true;
                     }
 
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 await client.SubscribeAsync("b");
@@ -101,7 +126,7 @@ namespace MQTTnet.Tests.Server
                     .WithTopicFilter("c", MqttQualityOfServiceLevel.ExactlyOnce)
                     .WithTopicFilter("d")
                     .Build();
-                
+
                 var response = await client.SubscribeAsync(subscribeOptions);
 
                 Assert.AreEqual(subscribeOptions.TopicFilters.Count, response.Items.Count);
@@ -121,7 +146,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 for (var i = 0; i < 500; i++)
@@ -164,7 +189,7 @@ namespace MQTTnet.Tests.Server
                 c1.ApplicationMessageReceivedAsync += e =>
                 {
                     Interlocked.Increment(ref receivedMessagesCount);
-                    return PlatformAbstractionLayer.CompletedTask;
+                    return CompletedTask.Instance;
                 };
 
                 var optionsBuilder = new MqttClientSubscribeOptionsBuilder();
@@ -204,7 +229,7 @@ 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());
@@ -242,7 +267,7 @@ 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());
@@ -276,7 +301,7 @@ 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"));
@@ -291,7 +316,7 @@ 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 });
@@ -306,7 +331,7 @@ 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/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/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

@@ -0,0 +1,71 @@
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Internal;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Tests.Server
+{
+    [TestClass]
+    public sealed class Will_Tests : BaseTestClass
+    {
+        [TestMethod]
+        public async Task Will_Message_Do_Not_Send_On_Clean_Disconnect()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var receivedMessagesCount = 0;
+
+                await testEnvironment.StartServer();
+
+                var clientOptions = new MqttClientOptionsBuilder().WithWillTopic("My/last/will");
+
+                var c1 = await testEnvironment.ConnectClient();
+
+                c1.ApplicationMessageReceivedAsync += e =>
+                {
+                    Interlocked.Increment(ref receivedMessagesCount);
+                    return CompletedTask.Instance;
+                };
+
+                await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
+
+                var c2 = await testEnvironment.ConnectClient(clientOptions);
+                await c2.DisconnectAsync().ConfigureAwait(false);
+
+                await Task.Delay(1000);
+
+                Assert.AreEqual(0, receivedMessagesCount);
+            }
+        }
+
+        [TestMethod]
+        public async Task Will_Message_Send()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                await testEnvironment.StartServer();
+
+                var c1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder());
+
+                var receivedMessagesCount = 0;
+                c1.ApplicationMessageReceivedAsync += e =>
+                {
+                    Interlocked.Increment(ref receivedMessagesCount);
+                    return CompletedTask.Instance;
+                };
+
+                await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
+
+                var clientOptions = new MqttClientOptionsBuilder().WithWillTopic("My/last/will").WithWillQualityOfServiceLevel(MqttQualityOfServiceLevel.AtMostOnce);
+                var c2 = await testEnvironment.ConnectClient(clientOptions);
+                c2.Dispose(); // Dispose will not send a DISCONNECT pattern first so the will message must be sent.
+
+                await Task.Delay(1000);
+
+                Assert.AreEqual(1, receivedMessagesCount);
+            }
+        }
+    }
+}

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -23,7 +23,7 @@ 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 +73,26 @@ 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 +195,7 @@ 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 +207,14 @@ 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);
@@ -341,7 +363,7 @@ 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/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/Options/MqttClientOptionsBuilder.cs

@@ -264,8 +264,7 @@ namespace MQTTnet.Client
 
             return this;
         }
-
-        // TODO: Consider creating _MqttClientTcpOptionsBuilder_ as overload.
+        
         public MqttClientOptionsBuilder WithTcpServer(Action<MqttClientTcpOptions> optionsBuilder)
         {
             if (optionsBuilder == null)

Source/MQTTnet/Client/Options/MqttClientTcpOptions.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.Net;
 using System.Net.Sockets;
 
 namespace MQTTnet.Client
@@ -10,8 +11,19 @@ namespace MQTTnet.Client
     {
         public AddressFamily AddressFamily { get; set; } = AddressFamily.Unspecified;
 
+        /// <summary>
+        /// Gets the local endpoint (network card) which is used by the client.
+        /// Set it to _null_ to let the OS select the network card.
+        /// </summary>
+        public EndPoint LocalEndpoint { get; set; }
+        
         public int BufferSize { get; set; } = 8192;
 
+        /// <summary>
+        /// Gets or sets whether the underlying socket should run in dual mode.
+        /// Leaving this _null_ will avoid setting this value at socket level.
+        /// Setting this a value other than _null_ will throw an exception when only IPv4 is supported on the machine.
+        /// </summary>
         public bool? DualMode { get; set; }
 
         public LingerOption LingerState { get; set; } = new LingerOption(true, 0);

Source/MQTTnet/Client/Options/MqttClientWebSocketOptions.cs

@@ -7,23 +7,23 @@ using System.Net;
 
 namespace MQTTnet.Client
 {
-    public class MqttClientWebSocketOptions : IMqttClientChannelOptions
+    public sealed class MqttClientWebSocketOptions : IMqttClientChannelOptions
     {
-        public string Uri { get; set; }
+        public CookieContainer CookieContainer { get; set; }
+        
+        public MqttClientWebSocketProxyOptions ProxyOptions { get; set; }
 
         public IDictionary<string, string> RequestHeaders { get; set; }
 
         public ICollection<string> SubProtocols { get; set; } = new List<string> { "mqtt" };
 
-        public CookieContainer CookieContainer { get; set; }
-
-        public MqttClientWebSocketProxyOptions ProxyOptions { get; set; }
-
         public MqttClientTlsOptions TlsOptions { get; set; } = new MqttClientTlsOptions();
+        
+        public string Uri { get; set; }
 
         public override string ToString()
         {
             return Uri;
         }
     }
-}
+}

Source/MQTTnet/Client/Options/MqttClientWebSocketProxyOptions.cs

@@ -4,7 +4,7 @@
 
 namespace MQTTnet.Client
 {
-    public class MqttClientWebSocketProxyOptions
+    public sealed class MqttClientWebSocketProxyOptions
     {
         public string Address { get; set; }
 

Source/MQTTnet/Client/Receiving/MqttApplicationMessageReceivedEventArgs.cs

@@ -13,74 +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;
+            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 ir 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 ushort PacketIdentifier => PublishPacket.PacketIdentifier;
+
+        /// <summary>
+        ///     Indicates if the processing of this PUBLISH packet has failed.
+        ///     If the processing has failed the client will not send an ACK packet etc.
+        /// </summary>
+        public bool ProcessingFailed { get; set; }
+
+        /// <summary>
+        ///     Gets or sets the reason code which will be sent to the server.
+        /// </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; }
-        
-        public Task AcknowledgeAsync(CancellationToken cancellationToken) 
+
+        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/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/MqttConnAckPacketFactory.cs

@@ -4,6 +4,7 @@
 
 using System;
 using MQTTnet.Packets;
+using MQTTnet.Protocol;
 using MQTTnet.Server;
 
 namespace MQTTnet.Formatter
@@ -25,6 +26,7 @@ namespace MQTTnet.Formatter
                 SubscriptionIdentifiersAvailable = true,
                 SharedSubscriptionAvailable = false,
                 TopicAliasMaximum = ushort.MaxValue,
+                MaximumQoS = MqttQualityOfServiceLevel.ExactlyOnce,
                 WildcardSubscriptionAvailable = true,
 
                 AuthenticationMethod = validatingConnectionEventArgs.AuthenticationMethod,
@@ -32,7 +34,11 @@ namespace MQTTnet.Formatter
                 AssignedClientIdentifier = validatingConnectionEventArgs.AssignedClientIdentifier,
                 ReasonString = validatingConnectionEventArgs.ReasonString,
                 ServerReference = validatingConnectionEventArgs.ServerReference,
-                UserProperties = validatingConnectionEventArgs.ResponseUserProperties
+                UserProperties = validatingConnectionEventArgs.ResponseUserProperties,
+
+                ResponseInformation = null,
+                MaximumPacketSize = 0, // Unlimited,
+                ReceiveMaximum = 0 // Unlimited
             };
 
             return connAckPacket;

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/V5/MqttV5PropertiesWriter.cs

@@ -72,11 +72,11 @@ namespace MQTTnet.Formatter.V5
 
             if (value == MqttQualityOfServiceLevel.AtLeastOnce)
             {
-                Write(MqttPropertyId.MaximumQoS, true);
+                Write(MqttPropertyId.MaximumQoS, 0x1);
             }
             else
             {
-                Write(MqttPropertyId.MaximumQoS, false);
+                Write(MqttPropertyId.MaximumQoS, 0x0);
             }
         }
 

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

@@ -3,6 +3,9 @@
 // See the LICENSE file in the project root for more information.
 
 #if !WINDOWS_UWP
+using MQTTnet.Channel;
+using MQTTnet.Client;
+using MQTTnet.Exceptions;
 using System;
 using System.IO;
 using System.Net.Security;
@@ -11,9 +14,7 @@ using System.Runtime.ExceptionServices;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
-using MQTTnet.Channel;
-using MQTTnet.Client;
-using MQTTnet.Exceptions;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Implementations
 {
@@ -68,6 +69,11 @@ namespace MQTTnet.Implementations
                     socket = new CrossPlatformSocket(_tcpOptions.AddressFamily);
                 }
 
+                if (_tcpOptions.LocalEndpoint != null)
+                {
+                    socket.Bind(_tcpOptions.LocalEndpoint);
+                }
+                
                 socket.ReceiveBufferSize = _tcpOptions.BufferSize;
                 socket.SendBufferSize = _tcpOptions.BufferSize;
                 socket.SendTimeout = (int)_clientOptions.Timeout.TotalMilliseconds;
@@ -77,7 +83,7 @@ namespace MQTTnet.Implementations
                 {
                     socket.LingerState = _tcpOptions.LingerState;
                 }
-                
+
                 if (_tcpOptions.DualMode.HasValue)
                 {
                     // It is important to avoid setting the flag if no specific value is set by the user
@@ -103,15 +109,20 @@ namespace MQTTnet.Implementations
                             ApplicationProtocols = _tcpOptions.TlsOptions.ApplicationProtocols,
                             ClientCertificates = LoadCertificates(),
                             EnabledSslProtocols = _tcpOptions.TlsOptions.SslProtocol,
-                            CertificateRevocationCheckMode = _tcpOptions.TlsOptions.IgnoreCertificateRevocationErrors ? X509RevocationMode.NoCheck : _tcpOptions.TlsOptions.RevocationMode,
+                            CertificateRevocationCheckMode =
+ _tcpOptions.TlsOptions.IgnoreCertificateRevocationErrors ? X509RevocationMode.NoCheck : _tcpOptions.TlsOptions.RevocationMode,
                             TargetHost = _tcpOptions.Server,
                             CipherSuitesPolicy = _tcpOptions.TlsOptions.CipherSuitesPolicy
                         };
 
                         await sslStream.AuthenticateAsClientAsync(sslOptions, cancellationToken).ConfigureAwait(false);
 #else
-                        await sslStream.AuthenticateAsClientAsync(_tcpOptions.Server, LoadCertificates(), _tcpOptions.TlsOptions.SslProtocol,
-                            !_tcpOptions.TlsOptions.IgnoreCertificateRevocationErrors).ConfigureAwait(false);
+                        await sslStream.AuthenticateAsClientAsync(
+                                _tcpOptions.Server,
+                                LoadCertificates(),
+                                _tcpOptions.TlsOptions.SslProtocol,
+                                !_tcpOptions.TlsOptions.IgnoreCertificateRevocationErrors)
+                            .ConfigureAwait(false);
 #endif
                     }
                     catch
@@ -144,7 +155,7 @@ namespace MQTTnet.Implementations
         public Task DisconnectAsync(CancellationToken cancellationToken)
         {
             Dispose();
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public void Dispose()
@@ -215,7 +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();
 
@@ -229,12 +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
             }
@@ -269,6 +280,11 @@ namespace MQTTnet.Implementations
                 return certificateValidationHandler(eventArgs);
             }
 
+            if (_tcpOptions?.TlsOptions?.IgnoreCertificateChainErrors ?? false)
+            {
+                sslPolicyErrors &= ~SslPolicyErrors.RemoteCertificateChainErrors;
+            }
+
             return sslPolicyErrors == SslPolicyErrors.None;
         }
 

Source/MQTTnet/Implementations/MqttTcpServerAdapter.cs

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

Source/MQTTnet/Implementations/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.
@@ -105,89 +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()
@@ -231,5 +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
+            }
+        }
     }
 }