Add README to nugets

* Move "Client is not connected" error to it's own exception

* Update MqttClientNotConnectedException.cs

* Update release notes

---------

Co-authored-by: Christian <6939810+chkr1011@users.noreply.github.com>

.github/workflows/ReleaseNotes.md

@@ -1,2 +1,4 @@
-* [Client] Fixed wrong TLS options handling (#1830).
-* [Client] Fixed NullReferenceExeption when performing a Ping when the client is not connected (#1831).
+* [Core] Optimized packet serialization of PUBACK and PUBREC packets for protocol version 5.0.0 (#1939, thanks to @Y-Sindo).
+* [Core] The package inspector is now fully async (#1941).
+* [Client] Added a dedicated exception when the client is not connected (#1954, thanks to @marcpiulachs).
+* [ManagedClient] Added a new event (SubscriptionsChangedAsync) which is fired when a subscription or unsubscription was made (#1894, thanks to @pdufrene).

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ name: CI
 on: [push, pull_request]
 
 env:
-  VERSION: "4.3.1.${{github.run_number}}"
+  VERSION: "4.3.4.${{github.run_number}}"
 
 jobs:
   build:
@@ -54,7 +54,7 @@ jobs:
         uses: darenm/Setup-VSTest@v1
 
       - name: Core Tests
-        run: vstest.console.exe Source\MQTTnet.Tests\bin\Release\net6.0\MQTTnet.Tests.dll
+        run: vstest.console.exe Source\MQTTnet.Tests\bin\Release\net7.0\MQTTnet.Tests.dll
 
       - name: ASP.NET Tests
         run: vstest.console.exe Source\MQTTnet.AspNetCore.Tests\bin\Release\netcoreapp3.1\MQTTnet.AspNetCore.Tests.dll

MQTTnet.sln.DotSettings

@@ -230,6 +230,7 @@
 	<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_002EFeature_002EServices_002ECodeCleanup_002EFileHeader_002EFileHeaderSettingsMigrate/@EntryIndexedValue">True</s:Boolean>
 	<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>
@@ -240,6 +241,7 @@ See the LICENSE file in the project root for more information.</s:String>
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ESettingsUpgrade_002ECSharpPlaceAttributeOnSameLineMigration/@EntryIndexedValue">True</s:Boolean>
 	<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/=catched/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=CONNACK/@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>

Samples/Client/Client_Connection_Samples.cs

@@ -7,6 +7,7 @@
 // ReSharper disable InconsistentNaming
 
 using System.Security.Authentication;
+using System.Security.Cryptography.X509Certificates;
 using MQTTnet.Client;
 using MQTTnet.Extensions.WebSocket4Net;
 using MQTTnet.Formatter;
@@ -435,4 +436,54 @@ public static class Client_Connection_Samples
             Console.ReadLine();
         }
     }
+
+    public static async Task ConnectTls_WithCaFile()
+    {
+        var mqttFactory = new MqttFactory();
+
+        X509Certificate2Collection caChain = new X509Certificate2Collection();
+        caChain.ImportFromPem(mosquitto_org); // from https://test.mosquitto.org/ssl/mosquitto.org.crt
+
+        using (var mqttClient = mqttFactory.CreateMqttClient())
+        {
+            var mqttClientOptions = new MqttClientOptionsBuilder()
+                .WithTcpServer("test.mosquitto.org", 8883)
+                .WithTlsOptions(new MqttClientTlsOptionsBuilder()
+                    .WithTrustChain(caChain) 
+                    .Build())
+                .Build();
+
+            var connAck = await mqttClient.ConnectAsync(mqttClientOptions);
+            Console.WriteLine("Connected to test.moquitto.org:8883 with CaFile mosquitto.org.crt: " + connAck.ResultCode);
+        }
+
+
+    }
+    const string mosquitto_org = @"
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+";
+
 }

Samples/Client/Client_Subscribe_Samples.cs

@@ -184,7 +184,7 @@ public static class Client_Subscribe_Samples
          * This sample shows how to achieve concurrent processing and not have message AutoAcknowledged
          * This to have a proper QoS1 (at-least-once) experience for what at least MQTT specification can provide
          */
-        mqttClient.ApplicationMessageReceivedAsync += async ea =>
+        mqttClient.ApplicationMessageReceivedAsync += ea =>
         {
             ea.AutoAcknowledge = false;
 
@@ -201,6 +201,8 @@ public static class Client_Subscribe_Samples
             }
 
             _ = Task.Run(ProcessAsync, shutdownToken);
+
+            return Task.CompletedTask;
         };
     }
 

Samples/MQTTnet.Samples.csproj

@@ -2,7 +2,7 @@
 
     <PropertyGroup>
         <OutputType>Exe</OutputType>
-        <TargetFramework>net6.0</TargetFramework>
+        <TargetFramework>net7.0</TargetFramework>
         <ImplicitUsings>enable</ImplicitUsings>
         <Nullable>enable</Nullable>
         <IsPackable>false</IsPackable>

Samples/ManagedClient/Managed_Client_Subscribe_Samples.cs

@@ -0,0 +1,87 @@
+// 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;
+
+namespace MQTTnet.Samples.ManagedClient;
+
+public sealed class Managed_Client_Subscribe_Samples
+{
+    public static async Task Connect_Client()
+    {
+        /*
+         * This sample creates a simple managed MQTT client and connects to a public broker, subscribe to a topic and verifies subscription result.
+         *
+         * The managed client extends the existing _MqttClient_. It adds the following features.
+         * - Reconnecting when connection is lost.
+         * - Storing pending messages in an internal queue so that an enqueue is possible while the client remains not connected.
+         */
+        
+        var mqttFactory = new MqttFactory();
+        var subscribed = false;
+
+        using (var managedMqttClient = mqttFactory.CreateManagedMqttClient())
+        {
+            var mqttClientOptions = new MqttClientOptionsBuilder()
+                .WithTcpServer("broker.hivemq.com")
+                .Build();
+
+            var managedMqttClientOptions = new ManagedMqttClientOptionsBuilder()
+                .WithClientOptions(mqttClientOptions)
+                .Build();
+
+            await managedMqttClient.StartAsync(managedMqttClientOptions);
+
+            // The application message is not sent. It is stored in an internal queue and
+            // will be sent when the client is connected.
+            await managedMqttClient.EnqueueAsync("Topic", "Payload");
+
+            Console.WriteLine("The managed MQTT client is connected.");
+            
+            // Wait until the queue is fully processed.
+            SpinWait.SpinUntil(() => managedMqttClient.PendingApplicationMessagesCount == 0, 10000);
+            
+            Console.WriteLine($"Pending messages = {managedMqttClient.PendingApplicationMessagesCount}");
+
+            managedMqttClient.SubscriptionsChangedAsync += args => SubscriptionsResultAsync(args, subscribed);
+            await managedMqttClient.SubscribeAsync("Topic").ConfigureAwait(false);
+
+            SpinWait.SpinUntil(() => subscribed, 1000);
+            Console.WriteLine("Subscription properly done");
+        }
+    }
+
+    private static Task SubscriptionsResultAsync(SubscriptionsChangedEventArgs arg, bool subscribed)
+    {
+        foreach (var mqttClientSubscribeResult in arg.SubscribeResult)
+        {
+            Console.WriteLine($"Subscription reason {mqttClientSubscribeResult.ReasonString}");
+            foreach (var item in mqttClientSubscribeResult.Items)
+            {
+                Console.WriteLine($"For topic filter {item.TopicFilter}, result code: {item.ResultCode}");
+
+                if (item.TopicFilter.Topic == "Topic" && item.ResultCode == MqttClientSubscribeResultCode.GrantedQoS0 && !subscribed)
+                {
+                    subscribed = true;
+                }
+            }
+        }
+
+        foreach (var mqttClientUnsubscribeResult in arg.UnsubscribeResult)
+        {
+            Console.WriteLine($"Unsubscription reason {mqttClientUnsubscribeResult.ReasonString}");
+            foreach (var item in mqttClientUnsubscribeResult.Items)
+            {
+                Console.WriteLine($"For topic filter {item.TopicFilter}, result code: {item.ResultCode}");
+            }
+        }
+
+        return Task.CompletedTask;
+    }
+}

Source/MQTTnet.AspNetCore.Tests/MqttConnectionContextTest.cs

@@ -48,7 +48,6 @@ namespace MQTTnet.AspNetCore.Tests
         {
             var mockup = new ConnectionHandlerMockup();
 
-
             using (var host = new WebHostBuilder().UseKestrel(kestrel => kestrel.ListenLocalhost(1883, listener => listener.Use((ctx, next) => mockup.OnConnectedAsync(ctx))))
                        .UseStartup<Startup>()
                        .ConfigureServices(

Source/MQTTnet.AspnetCore/Client/MqttClientConnectionContextFactory.cs

@@ -6,7 +6,6 @@ using MQTTnet.Adapter;
 using MQTTnet.AspNetCore.Client.Tcp;
 using MQTTnet.Formatter;
 using System;
-using System.Net;
 using MQTTnet.Client;
 using MQTTnet.Diagnostics;
 
@@ -22,8 +21,7 @@ namespace MQTTnet.AspNetCore.Client
             {
                 case MqttClientTcpOptions tcpOptions:
                     {
-                        var endpoint = new DnsEndPoint(tcpOptions.Server, tcpOptions.GetPort());
-                        var tcpConnection = new TcpConnection(endpoint);
+                        var tcpConnection = new SocketConnection(tcpOptions.RemoteEndpoint);
                         
                         var formatter = new MqttPacketFormatterAdapter(options.ProtocolVersion, new MqttBufferWriter(4096, 65535));
                         return new MqttConnectionContext(formatter, tcpConnection);

Source/MQTTnet.AspnetCore/Client/Tcp/SocketConnection.cs

@@ -15,15 +15,15 @@ using System.Threading.Tasks;
 
 namespace MQTTnet.AspNetCore.Client.Tcp
 {
-    public class TcpConnection : ConnectionContext
+    public sealed class SocketConnection : ConnectionContext
     {
-        private volatile bool _aborted;
-        private readonly EndPoint _endPoint;
-        private SocketSender _sender;
-        private SocketReceiver _receiver;
+        volatile bool _aborted;
+        readonly EndPoint _endPoint;
+        SocketSender _sender;
+        SocketReceiver _receiver;
 
-        private Socket _socket;
-        private IDuplexPipe _application;
+        Socket _socket;
+        IDuplexPipe _application;
 
         public bool IsConnected { get; private set; }
         public override string ConnectionId { get; set; }
@@ -31,12 +31,12 @@ namespace MQTTnet.AspNetCore.Client.Tcp
         public override IDictionary<object, object> Items { get; set; }
         public override IDuplexPipe Transport { get; set; }
 
-        public TcpConnection(EndPoint endPoint)
+        public SocketConnection(EndPoint endPoint)
         {
             _endPoint = endPoint;
         }
 
-        public TcpConnection(Socket socket)
+        public SocketConnection(Socket socket)
         {
             _socket = socket;
             _endPoint = socket.RemoteEndPoint;
@@ -87,7 +87,7 @@ namespace MQTTnet.AspNetCore.Client.Tcp
             IsConnected = true;
         }
 
-        private async Task ExecuteAsync()
+        async Task ExecuteAsync()
         {
             Exception sendError = null;
             try
@@ -114,7 +114,7 @@ namespace MQTTnet.AspNetCore.Client.Tcp
             }
             catch (Exception ex)
             {
-                Console.WriteLine($"Unexpected exception in {nameof(TcpConnection)}.{nameof(StartAsync)}: " + ex);
+                Console.WriteLine($"Unexpected exception in {nameof(SocketConnection)}.{nameof(StartAsync)}: " + ex);
             }
             finally
             {
@@ -122,7 +122,8 @@ namespace MQTTnet.AspNetCore.Client.Tcp
                 _application.Input.Complete(sendError);
             }
         }
-        private async Task DoReceive()
+
+        async Task DoReceive()
         {
             Exception error = null;
 
@@ -171,7 +172,7 @@ namespace MQTTnet.AspNetCore.Client.Tcp
             }
         }
 
-        private async Task ProcessReceives()
+        async Task ProcessReceives()
         {
             while (true)
             {
@@ -204,12 +205,12 @@ namespace MQTTnet.AspNetCore.Client.Tcp
             }
         }
 
-        private Exception ConnectionAborted()
+        Exception ConnectionAborted()
         {
             return new MqttCommunicationException("Connection Aborted");
         }
 
-        private async Task<Exception> DoSend()
+        async Task<Exception> DoSend()
         {
             Exception error = null;
 
@@ -240,7 +241,7 @@ namespace MQTTnet.AspNetCore.Client.Tcp
             return error;
         }
 
-        private async Task ProcessSends()
+        async Task ProcessSends()
         {
             while (true)
             {

Source/MQTTnet.AspnetCore/MQTTnet.AspNetCore.csproj

@@ -11,6 +11,7 @@
         <Description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker) and supports v3.1.0, v3.1.1 and v5.0.0 of the MQTT protocol.</Description>
         <Authors>The contributors of MQTTnet</Authors>
         <PackageId>MQTTnet.AspNetCore</PackageId>
+        <PackageReadmeFile>README.md</PackageReadmeFile>
         <SignAssembly>false</SignAssembly>
         <DelaySign>false</DelaySign>
         <PublishRepositoryUrl>true</PublishRepositoryUrl>
@@ -37,12 +38,10 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <None Include="..\..\Images\nuget.png">
-            <Pack>True</Pack>
-            <PackagePath>\</PackagePath>
-        </None>
+        <None Include="..\..\Images\nuget.png" Pack="True" PackagePath="\" />
+        <None Include="..\..\README.md" Pack="true" PackagePath="\" />
     </ItemGroup>
-    
+
     <ItemGroup>
         <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
     </ItemGroup>
@@ -51,11 +50,11 @@
         <DefineConstants>RELEASE;NETSTANDARD2_0</DefineConstants>
     </PropertyGroup>
 
-    <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp3.1' or '$(TargetFramework)' == 'net5.0' or '$(TargetFramework)' == 'net6.0' or '$(TargetFramework)' == 'net7.0' ">
+    <ItemGroup Condition="'$(TargetFramework)' != 'netstandard2.0' and '$(TargetFramework)' != 'netcoreapp2.1' ">
         <FrameworkReference Include="Microsoft.AspNetCore.App" />
     </ItemGroup>
 
-    <ItemGroup Condition="'$(TargetFramework)' != 'netcoreapp3.1' and '$(TargetFramework)' != 'net5.0' and '$(TargetFramework)' != 'net6.0' and '$(TargetFramework)' != 'net7.0' ">
+    <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0' or '$(TargetFramework)' == 'netcoreapp2.1' ">
         <PackageReference Include="Microsoft.AspNetCore.Http.Connections" Version="1.1.0" />
     </ItemGroup>
 

Source/MQTTnet.AspnetCore/MqttConnectionContext.cs

@@ -33,7 +33,7 @@ namespace MQTTnet.AspNetCore
             PacketFormatterAdapter = packetFormatterAdapter ?? throw new ArgumentNullException(nameof(packetFormatterAdapter));
             _connection = connection ?? throw new ArgumentNullException(nameof(connection));
 
-            if (!(_connection is TcpConnection tcp) || tcp.IsConnected)
+            if (!(_connection is SocketConnection tcp) || tcp.IsConnected)
             {
                 _input = connection.Transport.Input;
                 _output = connection.Transport.Output;
@@ -83,8 +83,6 @@ namespace MQTTnet.AspNetCore
             }
         }
 
-        public bool IsReadingPacket { get; private set; }
-
         public bool IsSecureConnection
         {
             get
@@ -111,7 +109,7 @@ namespace MQTTnet.AspNetCore
 
         public async Task ConnectAsync(CancellationToken cancellationToken)
         {
-            if (_connection is TcpConnection tcp && !tcp.IsConnected)
+            if (_connection is SocketConnection tcp && !tcp.IsConnected)
             {
                 await tcp.StartAsync().ConfigureAwait(false);
             }
@@ -164,11 +162,6 @@ namespace MQTTnet.AspNetCore
                                 BytesReceived += received;
                                 return packet;
                             }
-                            else
-                            {
-                                // we did receive something but the message is not yet complete
-                                IsReadingPacket = true;
-                            }
                         }
                         else if (readResult.IsCompleted)
                         {
@@ -189,12 +182,9 @@ namespace MQTTnet.AspNetCore
                 // completing the channel makes sure that there is no more data read after a protocol error
                 _input?.Complete(exception);
                 _output?.Complete(exception);
+
                 throw;
             }
-            finally
-            {
-                IsReadingPacket = false;
-            }
 
             cancellationToken.ThrowIfCancellationRequested();
             return null;

Source/MQTTnet.AspnetCore/MqttHostedServer.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 System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
@@ -14,20 +15,27 @@ namespace MQTTnet.AspNetCore
 {
     public sealed class MqttHostedServer : MqttServer, IHostedService
     {
-        public MqttHostedServer(MqttServerOptions options, IEnumerable<IMqttServerAdapter> adapters, IMqttNetLogger logger) 
-            : base(options, adapters, logger)
+        readonly MqttFactory _mqttFactory;
+
+        public MqttHostedServer(MqttFactory mqttFactory, MqttServerOptions options, IEnumerable<IMqttServerAdapter> adapters, IMqttNetLogger logger) : base(
+            options,
+            adapters,
+            logger)
         {
+            _mqttFactory = mqttFactory ?? throw new ArgumentNullException(nameof(mqttFactory));
         }
 
-        public Task StartAsync(CancellationToken cancellationToken)
+        public async Task StartAsync(CancellationToken cancellationToken)
         {
-            _ =  StartAsync();
-            return Task.CompletedTask;
+            // The yield makes sure that the hosted service is considered up and running.
+            await Task.Yield();
+
+            _ = StartAsync();
         }
 
         public Task StopAsync(CancellationToken cancellationToken)
         {
-            return StopAsync();
+            return StopAsync(_mqttFactory.CreateMqttServerStopOptionsBuilder().Build());
         }
     }
 }

Source/MQTTnet.AspnetCore/ServiceCollectionExtensions.cs

@@ -4,6 +4,7 @@
 
 using System;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
 using Microsoft.Extensions.Hosting;
 using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
@@ -14,86 +15,74 @@ namespace MQTTnet.AspNetCore
 {
     public static class ServiceCollectionExtensions
     {
-        public static IServiceCollection AddMqttServer(this IServiceCollection serviceCollection, Action<MqttServerOptionsBuilder> configure = null)
-        {
-            if (serviceCollection is null)
-            {
-                throw new ArgumentNullException(nameof(serviceCollection));
-            }
-
-            serviceCollection.AddMqttConnectionHandler();
-            serviceCollection.AddHostedMqttServer(configure);
-            
-            return serviceCollection;
-        }
-
         public static IServiceCollection AddHostedMqttServer(this IServiceCollection services, MqttServerOptions options)
         {
-            if (options == null) throw new ArgumentNullException(nameof(options));
+            if (services == null)
+            {
+                throw new ArgumentNullException(nameof(services));
+            }
+
+            if (options == null)
+            {
+                throw new ArgumentNullException(nameof(options));
+            }
 
             services.AddSingleton(options);
-
             services.AddHostedMqttServer();
 
             return services;
         }
 
-        public static IServiceCollection AddHostedMqttServer(this IServiceCollection services, Action<MqttServerOptionsBuilder> configure = null)
+        public static IServiceCollection AddHostedMqttServer(this IServiceCollection services, Action<MqttServerOptionsBuilder> configure)
         {
-            services.AddSingleton(s =>
+            if (services == null)
             {
-                var serverOptionsBuilder = new MqttServerOptionsBuilder();
-                configure?.Invoke(serverOptionsBuilder);
-                return serverOptionsBuilder.Build();
-            });
+                throw new ArgumentNullException(nameof(services));
+            }
 
-            services.AddHostedMqttServer();
+            if (configure == null)
+            {
+                throw new ArgumentNullException(nameof(configure));
+            }
 
-            return services;
+            var serverOptionsBuilder = new MqttServerOptionsBuilder();
+            configure.Invoke(serverOptionsBuilder);
+            var options = serverOptionsBuilder.Build();
+
+            return AddHostedMqttServer(services, options);
+        }
+
+        public static void AddHostedMqttServer(this IServiceCollection services)
+        {
+            // The user may have these services already registered.
+            services.TryAddSingleton<IMqttNetLogger>(MqttNetNullLogger.Instance);
+            services.TryAddSingleton(new MqttFactory());
+
+            services.AddSingleton<MqttHostedServer>();
+            services.AddSingleton<IHostedService>(s => s.GetService<MqttHostedServer>());
+            services.AddSingleton<MqttServer>(s => s.GetService<MqttHostedServer>());
         }
 
         public static IServiceCollection AddHostedMqttServerWithServices(this IServiceCollection services, Action<AspNetMqttServerOptionsBuilder> configure)
         {
-            services.AddSingleton(s =>
+            if (services == null)
             {
-                var builder = new AspNetMqttServerOptionsBuilder(s);
-                configure(builder);
-                return builder.Build();
-            });
+                throw new ArgumentNullException(nameof(services));
+            }
+
+            services.AddSingleton(
+                s =>
+                {
+                    var builder = new AspNetMqttServerOptionsBuilder(s);
+                    configure(builder);
+                    return builder.Build();
+                });
 
             services.AddHostedMqttServer();
 
             return services;
         }
 
-        static IServiceCollection AddHostedMqttServer(this IServiceCollection services)
-        {
-            var logger = new MqttNetEventLogger();
-
-            services.AddSingleton<IMqttNetLogger>(logger);
-            services.AddSingleton<MqttHostedServer>();
-            services.AddSingleton<IHostedService>(s => s.GetService<MqttHostedServer>());
-            services.AddSingleton<MqttServer>(s => s.GetService<MqttHostedServer>());
-
-            return services;
-        }
-
-        public static IServiceCollection AddMqttWebSocketServerAdapter(this IServiceCollection services)
-        {
-            services.AddSingleton<MqttWebSocketServerAdapter>();
-            services.AddSingleton<IMqttServerAdapter>(s => s.GetService<MqttWebSocketServerAdapter>());
-
-            return services;
-        }
-
-        public static IServiceCollection AddMqttTcpServerAdapter(this IServiceCollection services)
-        {
-            services.AddSingleton<MqttTcpServerAdapter>();
-            services.AddSingleton<IMqttServerAdapter>(s => s.GetService<MqttTcpServerAdapter>());
-
-            return services;
-        }
-
         public static IServiceCollection AddMqttConnectionHandler(this IServiceCollection services)
         {
             services.AddSingleton<MqttConnectionHandler>();
@@ -101,5 +90,44 @@ namespace MQTTnet.AspNetCore
 
             return services;
         }
+
+        public static void AddMqttLogger(this IServiceCollection services, IMqttNetLogger logger)
+        {
+            if (services == null)
+            {
+                throw new ArgumentNullException(nameof(services));
+            }
+
+            services.AddSingleton(logger);
+        }
+
+        public static IServiceCollection AddMqttServer(this IServiceCollection serviceCollection, Action<MqttServerOptionsBuilder> configure = null)
+        {
+            if (serviceCollection is null)
+            {
+                throw new ArgumentNullException(nameof(serviceCollection));
+            }
+
+            serviceCollection.AddMqttConnectionHandler();
+            serviceCollection.AddHostedMqttServer(configure);
+
+            return serviceCollection;
+        }
+
+        public static IServiceCollection AddMqttTcpServerAdapter(this IServiceCollection services)
+        {
+            services.AddSingleton<MqttTcpServerAdapter>();
+            services.AddSingleton<IMqttServerAdapter>(s => s.GetService<MqttTcpServerAdapter>());
+
+            return services;
+        }
+
+        public static IServiceCollection AddMqttWebSocketServerAdapter(this IServiceCollection services)
+        {
+            services.AddSingleton<MqttWebSocketServerAdapter>();
+            services.AddSingleton<IMqttServerAdapter>(s => s.GetService<MqttWebSocketServerAdapter>());
+
+            return services;
+        }
     }
-}
+}

Source/MQTTnet.Benchmarks/TcpPipesBenchmark.cs

@@ -29,12 +29,12 @@ namespace MQTTnet.Benchmarks
 
             var task = Task.Run(() => server.AcceptSocket());
 
-            var clientConnection = new TcpConnection(new IPEndPoint(IPAddress.Loopback, 1883));
+            var clientConnection = new SocketConnection(new IPEndPoint(IPAddress.Loopback, 1883));
 
             clientConnection.StartAsync().GetAwaiter().GetResult();
             _client = clientConnection.Transport;
 
-            var serverConnection = new TcpConnection(task.GetAwaiter().GetResult());
+            var serverConnection = new SocketConnection(task.GetAwaiter().GetResult());
             serverConnection.StartAsync().GetAwaiter().GetResult();
             _server = serverConnection.Transport;
         }

Source/MQTTnet.Extensions.ManagedClient/IManagedMqttClient.cs

@@ -25,6 +25,8 @@ namespace MQTTnet.Extensions.ManagedClient
         
         event Func<ManagedProcessFailedEventArgs, Task> SynchronizingSubscriptionsFailedAsync;
         
+        event Func<SubscriptionsChangedEventArgs, Task> SubscriptionsChangedAsync;
+        
         IMqttClient InternalClient { get; }
         
         bool IsConnected { get; }

Source/MQTTnet.Extensions.ManagedClient/InterceptingPublishMessageEventArgs.cs

@@ -0,0 +1,20 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace MQTTnet.Extensions.ManagedClient
+{
+    public sealed class InterceptingPublishMessageEventArgs : EventArgs
+    {
+        public InterceptingPublishMessageEventArgs(ManagedMqttApplicationMessage applicationMessage)
+        {
+            ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
+        }
+
+        public ManagedMqttApplicationMessage ApplicationMessage { get; }
+
+        public bool AcceptPublish { get; set; } = true;
+    }
+}

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

@@ -13,6 +13,7 @@
         <Description>This is an extension library which provides a managed MQTT client with additional features using MQTTnet.</Description>
         <Authors>The contributors of MQTTnet</Authors>
         <PackageId>MQTTnet.Extensions.ManagedClient</PackageId>
+        <PackageReadmeFile>README.md</PackageReadmeFile>
         <SignAssembly>false</SignAssembly>
         <DelaySign>false</DelaySign>
         <PublishRepositoryUrl>true</PublishRepositoryUrl>
@@ -50,10 +51,8 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <None Include="..\..\Images\nuget.png">
-            <Pack>True</Pack>
-            <PackagePath>\</PackagePath>
-        </None>
+        <None Include="..\..\Images\nuget.png" Pack="True" PackagePath="\" />
+        <None Include="..\..\README.md" Pack="true" PackagePath="\" />
     </ItemGroup>
     
     <ItemGroup>

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs

@@ -19,12 +19,14 @@ namespace MQTTnet.Extensions.ManagedClient
 {
     public sealed class ManagedMqttClient : Disposable, IManagedMqttClient
     {
+        readonly AsyncEvent<InterceptingPublishMessageEventArgs> _interceptingPublishMessageEvent = new AsyncEvent<InterceptingPublishMessageEventArgs>();
         readonly AsyncEvent<ApplicationMessageProcessedEventArgs> _applicationMessageProcessedEvent = new AsyncEvent<ApplicationMessageProcessedEventArgs>();
         readonly AsyncEvent<ApplicationMessageSkippedEventArgs> _applicationMessageSkippedEvent = new AsyncEvent<ApplicationMessageSkippedEventArgs>();
         readonly AsyncEvent<ConnectingFailedEventArgs> _connectingFailedEvent = new AsyncEvent<ConnectingFailedEventArgs>();
         readonly AsyncEvent<EventArgs> _connectionStateChangedEvent = new AsyncEvent<EventArgs>();
         readonly AsyncEvent<ManagedProcessFailedEventArgs> _synchronizingSubscriptionsFailedEvent = new AsyncEvent<ManagedProcessFailedEventArgs>();
-        
+        readonly AsyncEvent<SubscriptionsChangedEventArgs> _subscriptionsChangedEvent = new AsyncEvent<SubscriptionsChangedEventArgs>();
+
         readonly MqttNetSourceLogger _logger;
         readonly BlockingQueue<ManagedMqttApplicationMessage> _messageQueue = new BlockingQueue<ManagedMqttApplicationMessage>();
 
@@ -88,6 +90,14 @@ namespace MQTTnet.Extensions.ManagedClient
             remove => _applicationMessageProcessedEvent.RemoveHandler(value);
         }
 
+
+        public event Func<InterceptingPublishMessageEventArgs, Task> InterceptPublishMessageAsync
+        {
+            add => _interceptingPublishMessageEvent.AddHandler(value);
+            remove => _interceptingPublishMessageEvent.RemoveHandler(value);
+        }
+
+
         public event Func<MqttApplicationMessageReceivedEventArgs, Task> ApplicationMessageReceivedAsync
         {
             add => InternalClient.ApplicationMessageReceivedAsync += value;
@@ -124,6 +134,12 @@ namespace MQTTnet.Extensions.ManagedClient
             remove => _synchronizingSubscriptionsFailedEvent.RemoveHandler(value);
         }
 
+        public event Func<SubscriptionsChangedEventArgs, Task> SubscriptionsChangedAsync
+        {
+            add => _subscriptionsChangedEvent.AddHandler(value);
+            remove => _subscriptionsChangedEvent.RemoveHandler(value);
+        }
+
         public IMqttClient InternalClient { get; }
 
         public bool IsConnected => InternalClient.IsConnected;
@@ -363,6 +379,14 @@ namespace MQTTnet.Extensions.ManagedClient
             }
         }
 
+        async Task HandleSubscriptionsResultAsync(SendSubscribeUnsubscribeResult subscribeUnsubscribeResult)
+        {
+            if (_subscriptionsChangedEvent.HasHandlers)
+            {
+                await _subscriptionsChangedEvent.InvokeAsync(new SubscriptionsChangedEventArgs(subscribeUnsubscribeResult.SubscribeResults, subscribeUnsubscribeResult.UnsubscribeResults)).ConfigureAwait(false);
+            }
+        }
+
         async Task MaintainConnectionAsync(CancellationToken cancellationToken)
         {
             try
@@ -464,19 +488,22 @@ namespace MQTTnet.Extensions.ManagedClient
                 if (_reconnectSubscriptions.Any())
                 {
                     topicFilters = new List<MqttTopicFilter>();
-
+                    SendSubscribeUnsubscribeResult subscribeUnsubscribeResult;
+                    
                     foreach (var sub in _reconnectSubscriptions)
                     {
                         topicFilters.Add(sub.Value);
 
                         if (topicFilters.Count == Options.MaxTopicFiltersInSubscribeUnsubscribePackets)
                         {
-                            await SendSubscribeUnsubscribe(topicFilters, null).ConfigureAwait(false);
+                            subscribeUnsubscribeResult = await SendSubscribeUnsubscribe(topicFilters, null).ConfigureAwait(false);
                             topicFilters.Clear();
+                            await HandleSubscriptionsResultAsync(subscribeUnsubscribeResult).ConfigureAwait(false);
                         }
                     }
 
-                    await SendSubscribeUnsubscribe(topicFilters, null).ConfigureAwait(false);
+                    subscribeUnsubscribeResult = await SendSubscribeUnsubscribe(topicFilters, null).ConfigureAwait(false);
+                    await HandleSubscriptionsResultAsync(subscribeUnsubscribeResult).ConfigureAwait(false);
                 }
             }
             catch (Exception exception)
@@ -492,6 +519,7 @@ namespace MQTTnet.Extensions.ManagedClient
             while (await _subscriptionsQueuedSignal.WaitAsync(GetRemainingTime(endTime), cancellationToken).ConfigureAwait(false))
             {
                 List<MqttTopicFilter> subscriptions;
+                SendSubscribeUnsubscribeResult subscribeUnsubscribeResult;
                 HashSet<string> unsubscriptions;
 
                 lock (_subscriptions)
@@ -527,12 +555,14 @@ namespace MQTTnet.Extensions.ManagedClient
 
                     if (addedTopicFilters.Count == Options.MaxTopicFiltersInSubscribeUnsubscribePackets)
                     {
-                        await SendSubscribeUnsubscribe(addedTopicFilters, null).ConfigureAwait(false);
+                        subscribeUnsubscribeResult = await SendSubscribeUnsubscribe(addedTopicFilters, null).ConfigureAwait(false);
                         addedTopicFilters.Clear();
+                        await HandleSubscriptionsResultAsync(subscribeUnsubscribeResult).ConfigureAwait(false);
                     }
                 }
-
-                await SendSubscribeUnsubscribe(addedTopicFilters, null).ConfigureAwait(false);
+                
+                subscribeUnsubscribeResult = await SendSubscribeUnsubscribe(addedTopicFilters, null).ConfigureAwait(false);
+                await HandleSubscriptionsResultAsync(subscribeUnsubscribeResult).ConfigureAwait(false);
 
                 var removedTopicFilters = new List<string>();
                 foreach (var unSub in unsubscriptions)
@@ -541,12 +571,14 @@ namespace MQTTnet.Extensions.ManagedClient
 
                     if (removedTopicFilters.Count == Options.MaxTopicFiltersInSubscribeUnsubscribePackets)
                     {
-                        await SendSubscribeUnsubscribe(null, removedTopicFilters).ConfigureAwait(false);
+                        subscribeUnsubscribeResult = await SendSubscribeUnsubscribe(null, removedTopicFilters).ConfigureAwait(false);
                         removedTopicFilters.Clear();
+                        await HandleSubscriptionsResultAsync(subscribeUnsubscribeResult).ConfigureAwait(false);
                     }
                 }
 
-                await SendSubscribeUnsubscribe(null, removedTopicFilters).ConfigureAwait(false);
+                subscribeUnsubscribeResult = await SendSubscribeUnsubscribe(null, removedTopicFilters).ConfigureAwait(false);
+                await HandleSubscriptionsResultAsync(subscribeUnsubscribeResult).ConfigureAwait(false);
             }
         }
 
@@ -579,8 +611,10 @@ namespace MQTTnet.Extensions.ManagedClient
             }
         }
 
-        async Task SendSubscribeUnsubscribe(List<MqttTopicFilter> addedSubscriptions, List<string> removedSubscriptions)
+        async Task<SendSubscribeUnsubscribeResult> SendSubscribeUnsubscribe(List<MqttTopicFilter> addedSubscriptions, List<string> removedSubscriptions)
         {
+            var subscribeResults = new List<MqttClientSubscribeResult>();
+            var unsubscribeResults = new List<MqttClientUnsubscribeResult>();
             try
             {
                 if (removedSubscriptions != null && removedSubscriptions.Any())
@@ -592,7 +626,8 @@ namespace MQTTnet.Extensions.ManagedClient
                         unsubscribeOptionsBuilder.WithTopicFilter(removedSubscription);
                     }
 
-                    await InternalClient.UnsubscribeAsync(unsubscribeOptionsBuilder.Build()).ConfigureAwait(false);
+                    var unsubscribeResult = await InternalClient.UnsubscribeAsync(unsubscribeOptionsBuilder.Build()).ConfigureAwait(false);
+                    unsubscribeResults.Add(unsubscribeResult);
 
                     //clear because these worked, maybe the subscribe below will fail, only report those
                     removedSubscriptions.Clear();
@@ -607,13 +642,16 @@ namespace MQTTnet.Extensions.ManagedClient
                         subscribeOptionsBuilder.WithTopicFilter(addedSubscription);
                     }
 
-                    await InternalClient.SubscribeAsync(subscribeOptionsBuilder.Build()).ConfigureAwait(false);
+                    var subscribeResult = await InternalClient.SubscribeAsync(subscribeOptionsBuilder.Build()).ConfigureAwait(false);
+                    subscribeResults.Add(subscribeResult);
                 }
             }
             catch (Exception exception)
             {
                 await HandleSubscriptionExceptionAsync(exception, addedSubscriptions, removedSubscriptions).ConfigureAwait(false);
             }
+
+            return new SendSubscribeUnsubscribeResult(subscribeResults, unsubscribeResults);
         }
 
         void StartPublishing()
@@ -700,9 +738,20 @@ namespace MQTTnet.Extensions.ManagedClient
         async Task TryPublishQueuedMessageAsync(ManagedMqttApplicationMessage message)
         {
             Exception transmitException = null;
+            bool acceptPublish = true;
             try
             {
-                await InternalClient.PublishAsync(message.ApplicationMessage).ConfigureAwait(false);
+                if (_interceptingPublishMessageEvent.HasHandlers)
+                {
+                    var interceptEventArgs = new InterceptingPublishMessageEventArgs(message);
+                    await _interceptingPublishMessageEvent.InvokeAsync(interceptEventArgs).ConfigureAwait(false);
+                    acceptPublish = interceptEventArgs.AcceptPublish;
+                }
+
+                if (acceptPublish)
+                {
+                    await InternalClient.PublishAsync(message.ApplicationMessage).ConfigureAwait(false);
+                }
 
                 using (await _messageQueueLock.EnterAsync().ConfigureAwait(false)) //lock to avoid conflict with this.PublishAsync
                 {

Source/MQTTnet.Extensions.ManagedClient/SendSubscribeUnsubscribeResult.cs

@@ -0,0 +1,23 @@
+// 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.Collections.Generic;
+using MQTTnet.Client;
+
+namespace MQTTnet.Extensions.ManagedClient
+{
+    public sealed class SendSubscribeUnsubscribeResult
+    {
+        public SendSubscribeUnsubscribeResult(List<MqttClientSubscribeResult> subscribeResults, List<MqttClientUnsubscribeResult> unsubscribeResults)
+        {
+            SubscribeResults = subscribeResults ?? throw new ArgumentNullException(nameof(subscribeResults));
+            UnsubscribeResults = unsubscribeResults ?? throw new ArgumentNullException(nameof(unsubscribeResults));
+        }
+        
+        public List<MqttClientSubscribeResult> SubscribeResults { get; private set; }
+        
+        public List<MqttClientUnsubscribeResult> UnsubscribeResults { get; private set; }
+    }
+}

Source/MQTTnet.Extensions.ManagedClient/SubscriptionsChangedEventArgs.cs

@@ -0,0 +1,23 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using MQTTnet.Client;
+using System;
+using System.Collections.Generic;
+
+namespace MQTTnet.Extensions.ManagedClient
+{
+    public sealed class SubscriptionsChangedEventArgs : EventArgs
+    {
+        public SubscriptionsChangedEventArgs(List<MqttClientSubscribeResult> subscribeResult, List<MqttClientUnsubscribeResult> unsubscribeResult)
+        {
+            SubscribeResult = subscribeResult ?? throw new ArgumentNullException(nameof(subscribeResult));
+            UnsubscribeResult = unsubscribeResult ?? throw new ArgumentNullException(nameof(unsubscribeResult));
+        }
+
+        public List<MqttClientSubscribeResult> SubscribeResult { get; }
+
+        public List<MqttClientUnsubscribeResult> UnsubscribeResult { get; }
+    }
+}

Source/MQTTnet.Extensions.Rpc/MQTTnet.Extensions.Rpc.csproj

@@ -13,6 +13,7 @@
         <Description>This is an extension library which allows executing synchronous device calls including a response using MQTTnet.</Description>
         <Authors>The contributors of MQTTnet</Authors>
         <PackageId>MQTTnet.Extensions.Rpc</PackageId>
+        <PackageReadmeFile>README.md</PackageReadmeFile>
         <SignAssembly>false</SignAssembly>
         <DelaySign>false</DelaySign>
         <PublishRepositoryUrl>true</PublishRepositoryUrl>
@@ -51,10 +52,8 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <None Include="..\..\Images\nuget.png">
-            <Pack>True</Pack>
-            <PackagePath>\</PackagePath>
-        </None>
+        <None Include="..\..\Images\nuget.png" Pack="True" PackagePath="\" />
+        <None Include="..\..\README.md" Pack="true" PackagePath="\" />
     </ItemGroup>
     
     <ItemGroup>

Source/MQTTnet.Extensions.WebSocket4Net/MQTTnet.Extensions.WebSocket4Net.csproj

@@ -13,6 +13,7 @@
         <Description>This is an extension library which allows using _WebSocket4Net_ as transport for MQTTnet clients.</Description>
         <Authors>The contributors of MQTTnet</Authors>
         <PackageId>MQTTnet.Extensions.WebSocket4Net</PackageId>
+        <PackageReadmeFile>README.md</PackageReadmeFile>
         <SignAssembly>false</SignAssembly>
         <DelaySign>false</DelaySign>
         <PublishRepositoryUrl>true</PublishRepositoryUrl>
@@ -51,10 +52,8 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <None Include="..\..\Images\nuget.png">
-            <Pack>True</Pack>
-            <PackagePath>\</PackagePath>
-        </None>
+        <None Include="..\..\Images\nuget.png" Pack="True" PackagePath="\" />
+        <None Include="..\..\README.md" Pack="true" PackagePath="\" />
     </ItemGroup>
     
     <ItemGroup>

Source/MQTTnet.Extensions.WebSocket4Net/WebSocket4NetMqttChannel.cs

@@ -76,7 +76,7 @@ namespace MQTTnet.Extensions.WebSocket4Net
             List<KeyValuePair<string, string>> customHeaders = null;
             if (_webSocketOptions.RequestHeaders != null)
             {
-                customHeaders = _webSocketOptions.RequestHeaders.Select(i => new KeyValuePair<string, string>(i.Key, i.Value)).ToList();
+                customHeaders = _webSocketOptions.RequestHeaders.ToList();
             }
 
             EndPoint proxy = null;

Source/MQTTnet.TestApp/ClientTest.cs

@@ -4,6 +4,7 @@
 
 using MQTTnet.Client;
 using System;
+using System.Net;
 using System.Text;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
@@ -27,7 +28,7 @@ namespace MQTTnet.TestApp
                 {
                     ChannelOptions = new MqttClientTcpOptions
                     {
-                        Server = "127.0.0.1"
+                        RemoteEndpoint = new DnsEndPoint("127.0.0.1", 0)
                     }
                 };
 

Source/MQTTnet.TestApp/MQTTnet.TestApp.csproj

@@ -10,6 +10,7 @@
         <EnforceCodeStyleInBuild>false</EnforceCodeStyleInBuild>
         <DisableImplicitAspNetCoreAnalyzers>true</DisableImplicitAspNetCoreAnalyzers>
         <NoWarn>1591;NETSDK1138</NoWarn>
+        <LangVersion>7.3</LangVersion>
     </PropertyGroup>
 
     <ItemGroup>

Source/MQTTnet.TestApp/ManagedClientTest.cs

@@ -7,6 +7,7 @@ using System.Threading.Tasks;
 using System.IO;
 using Newtonsoft.Json;
 using System.Collections.Generic;
+using System.Net;
 using MQTTnet.Client;
 using MQTTnet.Extensions.ManagedClient;
 using MQTTnet.Internal;
@@ -28,7 +29,7 @@ namespace MQTTnet.TestApp
                     Credentials = new RandomPassword(),
                     ChannelOptions = new MqttClientTcpOptions
                     {
-                        Server = "broker.hivemq.com"
+                        RemoteEndpoint = new DnsEndPoint("broker.hivemq.com", 0)
                     }
                 },
 

Source/MQTTnet.TestApp/PerformanceTest.cs

@@ -5,13 +5,13 @@
 using System;
 using System.Diagnostics;
 using System.Linq;
+using System.Net;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Client;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
-using MqttClient = MQTTnet.Client.MqttClient;
 
 namespace MQTTnet.TestApp
 {
@@ -25,7 +25,7 @@ namespace MQTTnet.TestApp
                 {
                     ChannelOptions = new MqttClientTcpOptions
                     {
-                        Server = "127.0.0.1"
+                        RemoteEndpoint = new DnsEndPoint("127.0.0.1", 0)
                     },
                     CleanSession = true
                 };
@@ -71,7 +71,7 @@ namespace MQTTnet.TestApp
                 {
                     ChannelOptions = new MqttClientTcpOptions
                     {
-                        Server = "127.0.0.1"
+                        RemoteEndpoint = new DnsEndPoint("127.0.0.1", 0)
                     }
                 };
 
@@ -114,7 +114,10 @@ namespace MQTTnet.TestApp
             {
                 var options = new MqttClientOptions
                 {
-                    ChannelOptions = new MqttClientTcpOptions { Server = "localhost" },
+                    ChannelOptions = new MqttClientTcpOptions
+                    {
+                        RemoteEndpoint = new DnsEndPoint("localhost", 0)
+                    },
                     ClientId = "Client1",
                     CleanSession = true
                 };
@@ -218,7 +221,7 @@ namespace MQTTnet.TestApp
                 {
                     ChannelOptions = new MqttClientTcpOptions
                     {
-                        Server = "127.0.0.1"
+                        RemoteEndpoint = new DnsEndPoint("127.0.0.1", 0)
                     },
                     CleanSession = true
                 };
@@ -269,7 +272,7 @@ namespace MQTTnet.TestApp
                 {
                     ChannelOptions = new MqttClientTcpOptions
                     {
-                        Server = "127.0.0.1"
+                        RemoteEndpoint = new DnsEndPoint("127.0.0.1", 0)
                     },
                     CleanSession = true
                 };
@@ -320,7 +323,7 @@ namespace MQTTnet.TestApp
                 {
                     ChannelOptions = new MqttClientTcpOptions
                     {
-                        Server = "127.0.0.1"
+                        RemoteEndpoint = new DnsEndPoint("127.0.0.1", 0)
                     },
                     CleanSession = true
                 };

Source/MQTTnet.TestApp/PublicBrokerTest.cs

@@ -42,45 +42,45 @@ namespace MQTTnet.TestApp
             };
 
             // mqtt.eclipseprojects.io
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "mqtt.eclipseprojects.io TCP",
                 new MqttClientOptionsBuilder().WithTcpServer("mqtt.eclipseprojects.io", 1883).WithProtocolVersion(MqttProtocolVersion.V311).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "mqtt.eclipseprojects.io WS",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("mqtt.eclipseprojects.io:80/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).Build());
 
 #if NET5_0_OR_GREATER
-            await ExecuteTestAsync("mqtt.eclipseprojects.io WS TLS13",
+            await ExecuteTestsAsync("mqtt.eclipseprojects.io WS TLS13",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("mqtt.eclipseprojects.io:443/mqtt"))
                     .WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls13).Build());
             
-            await ExecuteTestAsync("mqtt.eclipseprojects.io WS TLS13 (WebSocket4Net)",
+            await ExecuteTestsAsync("mqtt.eclipseprojects.io WS TLS13 (WebSocket4Net)",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("mqtt.eclipseprojects.io:443/mqtt"))
                     .WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls13).Build(),
                     true);
 #endif
 
             // test.mosquitto.org
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "test.mosquitto.org TCP",
                 new MqttClientOptionsBuilder().WithTcpServer("test.mosquitto.org", 1883).WithProtocolVersion(MqttProtocolVersion.V311).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "test.mosquitto.org TCP - Authenticated",
                 new MqttClientOptionsBuilder().WithTcpServer("test.mosquitto.org", 1884).WithCredentials("rw", "readwrite").WithProtocolVersion(MqttProtocolVersion.V311).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "test.mosquitto.org TCP TLS12",
                 new MqttClientOptionsBuilder().WithTcpServer("test.mosquitto.org", 8883).WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls12).Build());
 
 #if NET5_0_OR_GREATER
-            await ExecuteTestAsync("test.mosquitto.org TCP TLS13",
+            await ExecuteTestsAsync("test.mosquitto.org TCP TLS13",
                 new MqttClientOptionsBuilder().WithTcpServer("test.mosquitto.org", 8883)
                     .WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls13).Build());
 #endif
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "test.mosquitto.org TCP TLS12 - Authenticated",
                 new MqttClientOptionsBuilder().WithTcpServer("test.mosquitto.org", 8885)
                     .WithCredentials("rw", "readwrite")
@@ -88,67 +88,67 @@ namespace MQTTnet.TestApp
                     .WithTlsOptions(unsafeTls12)
                     .Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "test.mosquitto.org WS",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("test.mosquitto.org:8080/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "test.mosquitto.org WS (WebSocket4Net)",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("test.mosquitto.org:8080/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).Build(),
                 true);
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "test.mosquitto.org WS TLS12",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("test.mosquitto.org:8081/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls12).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "test.mosquitto.org WS TLS12 (WebSocket4Net)",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("test.mosquitto.org:8081/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls12).Build(),
                 true);
 
             // broker.emqx.io
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "broker.emqx.io TCP",
                 new MqttClientOptionsBuilder().WithTcpServer("broker.emqx.io", 1883).WithProtocolVersion(MqttProtocolVersion.V311).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "broker.emqx.io TCP TLS12",
                 new MqttClientOptionsBuilder().WithTcpServer("broker.emqx.io", 8883).WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls12).Build());
 
 #if NET5_0_OR_GREATER
-            await ExecuteTestAsync("broker.emqx.io TCP TLS13",
+            await ExecuteTestsAsync("broker.emqx.io TCP TLS13",
                 new MqttClientOptionsBuilder().WithTcpServer("broker.emqx.io", 8883)
                     .WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls13).Build());
 #endif
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "broker.emqx.io WS",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("broker.emqx.io:8083/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "broker.emqx.io WS (WebSocket4Net)",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("broker.emqx.io:8084/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).Build(),
                 true);
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "broker.emqx.io WS TLS12",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("broker.emqx.io:8084/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls12).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "broker.emqx.io WS TLS12 (WebSocket4Net)",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("broker.emqx.io:8084/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).WithTlsOptions(unsafeTls12).Build(),
                 true);
 
             // broker.hivemq.com
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "broker.hivemq.com TCP",
                 new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com", 1883).WithProtocolVersion(MqttProtocolVersion.V311).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "broker.hivemq.com WS",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("broker.hivemq.com:8000/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).Build());
 
-            await ExecuteTestAsync(
+            await ExecuteTestsAsync(
                 "broker.hivemq.com WS (WebSocket4Net)",
                 new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("broker.hivemq.com:8000/mqtt")).WithProtocolVersion(MqttProtocolVersion.V311).Build(),
                 true);
@@ -160,6 +160,15 @@ namespace MQTTnet.TestApp
             Console.ReadLine();
         }
 
+        static async Task ExecuteTestsAsync(string name, MqttClientOptions options, bool useWebSocket4Net = false)
+        {
+            options.ProtocolVersion = MqttProtocolVersion.V311;
+            await ExecuteTestAsync(name + " V3.1.1", options, useWebSocket4Net);
+
+            options.ProtocolVersion = MqttProtocolVersion.V500;
+            await ExecuteTestAsync(name + " V5.0.0", options, useWebSocket4Net);
+        }
+        
         static async Task ExecuteTestAsync(string name, MqttClientOptions options, bool useWebSocket4Net = false)
         {
             try

Source/MQTTnet.Tests/Clients/LowLevelMqttClient/LowLevelMqttClient_Tests.cs

@@ -13,6 +13,7 @@ using MQTTnet.Exceptions;
 using MQTTnet.LowLevelClient;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using MQTTnet.Server;
 
 namespace MQTTnet.Tests.Clients.LowLevelMqttClient
 {

Source/MQTTnet.Tests/Clients/ManagedMqttClient/ManagedMqttClient_Tests.cs

@@ -22,99 +22,6 @@ namespace MQTTnet.Tests.Clients.ManagedMqttClient
     [TestClass]
     public sealed class ManagedMqttClient_Tests : BaseTestClass
     {
-        [TestMethod]
-        public async Task Expose_Custom_Connection_Error()
-        {
-            using (var testEnvironment = CreateTestEnvironment())
-            {
-                var server = await testEnvironment.StartServer();
-                
-                server.ValidatingConnectionAsync += args =>
-                {
-                    args.ReasonCode = MqttConnectReasonCode.BadUserNameOrPassword;
-                    return CompletedTask.Instance;
-                };
-
-                var managedClient = testEnvironment.Factory.CreateManagedMqttClient();
-
-                MqttClientDisconnectedEventArgs disconnectedArgs = null;
-                managedClient.DisconnectedAsync += args =>
-                {
-                    disconnectedArgs = args;
-                    return CompletedTask.Instance;
-                };
-                
-                var clientOptions = testEnvironment.Factory.CreateManagedMqttClientOptionsBuilder().WithClientOptions(testEnvironment.CreateDefaultClientOptions()).Build();
-                await managedClient.StartAsync(clientOptions);
-
-                await LongTestDelay();
-
-                Assert.IsNotNull(disconnectedArgs);
-                Assert.AreEqual(MqttClientConnectResultCode.BadUserNameOrPassword, disconnectedArgs.ConnectResult.ResultCode);
-            }
-        }
-        
-        [TestMethod]
-        public async Task Receive_While_Not_Cleanly_Disconnected()
-        {
-            using (var testEnvironment = CreateTestEnvironment(MqttProtocolVersion.V500))
-            {
-                await testEnvironment.StartServer(o => o.WithPersistentSessions());
-
-                var senderClient = await testEnvironment.ConnectClient();
-
-                // Prepare managed client.
-                var managedClient = testEnvironment.Factory.CreateManagedMqttClient();
-                await managedClient.SubscribeAsync("#");
-                var receivedMessages = testEnvironment.CreateApplicationMessageHandler(managedClient);
-                
-                var managedClientOptions = new ManagedMqttClientOptions
-                {
-                    ClientOptions = testEnvironment.Factory.CreateClientOptionsBuilder()
-                        .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
-                        .WithClientId(nameof(Receive_While_Not_Cleanly_Disconnected) + "_managed")
-                        .WithCleanSession(false)
-                        .Build()
-                };
-
-                await managedClient.StartAsync(managedClientOptions);
-                await LongTestDelay();
-                await LongTestDelay();
-                
-                // Send test data.
-                await senderClient.PublishStringAsync("topic1");
-                await LongTestDelay();
-                await LongTestDelay();
-                
-                receivedMessages.AssertReceivedCountEquals(1);
-
-                // Stop the managed client but keep session at server (not clean disconnect required).
-                await managedClient.StopAsync(false);
-                await LongTestDelay();
-                
-                // Send new messages in the meantime.
-                await senderClient.PublishStringAsync("topic2", qualityOfServiceLevel: MqttQualityOfServiceLevel.ExactlyOnce);
-                await LongTestDelay();
-                
-                // Start the managed client, it should receive the new message.
-                await managedClient.StartAsync(managedClientOptions);
-                await LongTestDelay();
-                
-                receivedMessages.AssertReceivedCountEquals(2);
-                
-                // Stop and start again, no new message should be received.
-                for (var i = 0; i < 3; i++)
-                {
-                    await managedClient.StopAsync(false);
-                    await LongTestDelay();
-                    await managedClient.StartAsync(managedClientOptions);
-                    await LongTestDelay();
-                }
-
-                receivedMessages.AssertReceivedCountEquals(2);
-            }
-        }
-
         [TestMethod]
         public async Task Connect_To_Invalid_Server()
         {
@@ -181,6 +88,38 @@ namespace MQTTnet.Tests.Clients.ManagedMqttClient
             }
         }
 
+        [TestMethod]
+        public async Task Expose_Custom_Connection_Error()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                server.ValidatingConnectionAsync += args =>
+                {
+                    args.ReasonCode = MqttConnectReasonCode.BadUserNameOrPassword;
+                    return CompletedTask.Instance;
+                };
+
+                var managedClient = testEnvironment.Factory.CreateManagedMqttClient();
+
+                MqttClientDisconnectedEventArgs disconnectedArgs = null;
+                managedClient.DisconnectedAsync += args =>
+                {
+                    disconnectedArgs = args;
+                    return CompletedTask.Instance;
+                };
+
+                var clientOptions = testEnvironment.Factory.CreateManagedMqttClientOptionsBuilder().WithClientOptions(testEnvironment.CreateDefaultClientOptions()).Build();
+                await managedClient.StartAsync(clientOptions);
+
+                await LongTestDelay();
+
+                Assert.IsNotNull(disconnectedArgs);
+                Assert.AreEqual(MqttClientConnectResultCode.BadUserNameOrPassword, disconnectedArgs.ConnectResult.ResultCode);
+            }
+        }
+
         [TestMethod]
         public async Task ManagedClients_Will_Message_Send()
         {
@@ -224,6 +163,67 @@ namespace MQTTnet.Tests.Clients.ManagedMqttClient
             }
         }
 
+        [TestMethod]
+        public async Task Receive_While_Not_Cleanly_Disconnected()
+        {
+            using (var testEnvironment = CreateTestEnvironment(MqttProtocolVersion.V500))
+            {
+                await testEnvironment.StartServer(o => o.WithPersistentSessions());
+
+                var senderClient = await testEnvironment.ConnectClient();
+
+                // Prepare managed client.
+                var managedClient = testEnvironment.Factory.CreateManagedMqttClient();
+                await managedClient.SubscribeAsync("#");
+                var receivedMessages = testEnvironment.CreateApplicationMessageHandler(managedClient);
+
+                var managedClientOptions = new ManagedMqttClientOptions
+                {
+                    ClientOptions = testEnvironment.Factory.CreateClientOptionsBuilder()
+                        .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                        .WithClientId(nameof(Receive_While_Not_Cleanly_Disconnected) + "_managed")
+                        .WithCleanSession(false)
+                        .Build()
+                };
+
+                await managedClient.StartAsync(managedClientOptions);
+                await LongTestDelay();
+                await LongTestDelay();
+
+                // Send test data.
+                await senderClient.PublishStringAsync("topic1");
+                await LongTestDelay();
+                await LongTestDelay();
+
+                receivedMessages.AssertReceivedCountEquals(1);
+
+                // Stop the managed client but keep session at server (not clean disconnect required).
+                await managedClient.StopAsync(false);
+                await LongTestDelay();
+
+                // Send new messages in the meantime.
+                await senderClient.PublishStringAsync("topic2", qualityOfServiceLevel: MqttQualityOfServiceLevel.ExactlyOnce);
+                await LongTestDelay();
+
+                // Start the managed client, it should receive the new message.
+                await managedClient.StartAsync(managedClientOptions);
+                await LongTestDelay();
+
+                receivedMessages.AssertReceivedCountEquals(2);
+
+                // Stop and start again, no new message should be received.
+                for (var i = 0; i < 3; i++)
+                {
+                    await managedClient.StopAsync(false);
+                    await LongTestDelay();
+                    await managedClient.StartAsync(managedClientOptions);
+                    await LongTestDelay();
+                }
+
+                receivedMessages.AssertReceivedCountEquals(2);
+            }
+        }
+
         [TestMethod]
         public async Task Start_Stop()
         {
@@ -375,7 +375,7 @@ namespace MQTTnet.Tests.Clients.ManagedMqttClient
                 var clientOptions = new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", testEnvironment.ServerPort);
 
                 var receivedManagedMessages = new List<MqttApplicationMessage>();
-                
+
                 var managedClient = testEnvironment.Factory.CreateManagedMqttClient(testEnvironment.CreateClient());
                 managedClient.ApplicationMessageReceivedAsync += e =>
                 {
@@ -403,7 +403,7 @@ namespace MQTTnet.Tests.Clients.ManagedMqttClient
                 // Make sure that it gets received after subscribing again.
                 await managedClient.SubscribeAsync("topic");
                 await LongTestDelay();
-                
+
                 Assert.AreEqual(2, receivedManagedMessages.Count);
             }
         }
@@ -421,7 +421,7 @@ namespace MQTTnet.Tests.Clients.ManagedMqttClient
                 var receivingClient = await CreateManagedClientAsync(testEnvironment, null, connectionCheckInterval);
                 var sendingClient = await testEnvironment.ConnectClient();
 
-                await sendingClient.PublishAsync(new MqttApplicationMessage { Topic = "topic", PayloadSegment = new ArraySegment<byte>( new byte[] { 1 }), Retain = true });
+                await sendingClient.PublishAsync(new MqttApplicationMessage { Topic = "topic", PayloadSegment = new ArraySegment<byte>(new byte[] { 1 }), Retain = true });
 
                 var subscribeTime = DateTime.UtcNow;
 
@@ -437,6 +437,109 @@ namespace MQTTnet.Tests.Clients.ManagedMqttClient
             }
         }
 
+
+        [TestMethod]
+        public async Task ManagedClients_CanInterceptPublishedMessage_PreventingPublish()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                await testEnvironment.StartServer();
+
+                var receivedMessagesCount = 0;
+                var interceptedMessagesCount = 0;
+
+                var clientOptions = new MqttClientOptionsBuilder()
+                    .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                    .Build();
+
+                var sendingClient = testEnvironment.CreateClient();
+                var sendingManagedClient = new MQTTnet.Extensions.ManagedClient.ManagedMqttClient(sendingClient, testEnvironment.ClientLogger);
+
+                await sendingManagedClient.StartAsync(new ManagedMqttClientOptionsBuilder().WithClientOptions(clientOptions).Build());
+
+                // Wait until the managed client is fully set up and running.
+                await Task.Delay(1000);
+
+                var receivingClient = await testEnvironment.ConnectClient();
+
+                receivingClient.ApplicationMessageReceivedAsync += e =>
+                {
+                    Interlocked.Increment(ref receivedMessagesCount);
+                    return CompletedTask.Instance;
+                };
+                sendingManagedClient.InterceptPublishMessageAsync += e =>
+                {
+                    Interlocked.Increment(ref interceptedMessagesCount);
+                    e.AcceptPublish = false;
+                    return CompletedTask.Instance;
+                };
+
+                await receivingClient.SubscribeAsync("My/last/will");
+
+                // Wait for arrival of the will message at the receiver.
+                await Task.Delay(5000);
+
+                await sendingManagedClient.EnqueueAsync("My/last/will", "hello world");
+
+                // Wait for arrival of the will message at the receiver.
+                await Task.Delay(5000);
+
+                Assert.AreEqual(0, receivedMessagesCount);
+                Assert.AreEqual(1, interceptedMessagesCount);
+            }
+        }
+
+        [TestMethod]
+        public async Task ManagedClients_CanInterceptPublishedMessage_AllowingPublish()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                await testEnvironment.StartServer();
+
+                var receivedMessagesCount = 0;
+                var interceptedMessagesCount = 0;
+
+                var clientOptions = new MqttClientOptionsBuilder()
+                    .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                    .Build();
+
+                var sendingClient = testEnvironment.CreateClient();
+                var sendingManagedClient = new MQTTnet.Extensions.ManagedClient.ManagedMqttClient(sendingClient, testEnvironment.ClientLogger);
+
+                await sendingManagedClient.StartAsync(new ManagedMqttClientOptionsBuilder().WithClientOptions(clientOptions).Build());
+
+                // Wait until the managed client is fully set up and running.
+                await Task.Delay(1000);
+
+                var receivingClient = await testEnvironment.ConnectClient();
+
+                receivingClient.ApplicationMessageReceivedAsync += e =>
+                {
+                    Interlocked.Increment(ref receivedMessagesCount);
+                    return CompletedTask.Instance;
+                };
+                sendingManagedClient.InterceptPublishMessageAsync += e =>
+                {
+                    Interlocked.Increment(ref interceptedMessagesCount);
+                    e.AcceptPublish = true;
+                    return CompletedTask.Instance;
+                };
+
+                await receivingClient.SubscribeAsync("My/last/will");
+
+                // Wait for arrival of the will message at the receiver.
+                await Task.Delay(5000);
+
+                await sendingManagedClient.EnqueueAsync("My/last/will", "hello world");
+
+                // Wait for arrival of the will message at the receiver.
+                await Task.Delay(5000);
+
+                Assert.AreEqual(1, receivedMessagesCount);
+                Assert.AreEqual(1, interceptedMessagesCount);
+            }
+        }
+
         // This case also serves as a regression test for the previous behavior which re-published
         // each and every existing subscriptions with every new subscription that was made
         // (causing performance problems and having the visible symptom of retained messages being received again)

Source/MQTTnet.Tests/Clients/MqttClient/MqttClient_Connection_Tests.cs

@@ -65,7 +65,7 @@ namespace MQTTnet.Tests.Clients.MqttClient
                         return CompletedTask.Instance;
                     };
 
-                    await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("1.2.3.4").Build());
+                    await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1").Build());
 
                     Assert.Fail("Must fail!");
                 }
@@ -167,6 +167,38 @@ namespace MQTTnet.Tests.Clients.MqttClient
             }
         }
 
+        [TestMethod]
+        public async Task No_Unobserved_Exception()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                testEnvironment.IgnoreClientLogErrors = true;
+
+                var client = testEnvironment.CreateClient();
+                var options = new MqttClientOptionsBuilder().WithTcpServer("1.2.3.4").WithTimeout(TimeSpan.FromSeconds(2)).Build();
+
+                try
+                {
+                    using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(0.5)))
+                    {
+                        await client.ConnectAsync(options, timeout.Token);
+                    }
+                }
+                catch (OperationCanceledException)
+                {
+                }
+
+                client.Dispose();
+
+                // These delays and GC calls are required in order to make calling the finalizer reproducible.
+                GC.Collect();
+                GC.WaitForPendingFinalizers();
+                await LongTestDelay();
+                await LongTestDelay();
+                await LongTestDelay();
+            }
+        }
+
         [TestMethod]
         public async Task Return_Non_Success()
         {
@@ -208,9 +240,9 @@ namespace MQTTnet.Tests.Clients.MqttClient
                     await mqttClient.SubscribeAsync("test", MqttQualityOfServiceLevel.AtLeastOnce);
                 }
             }
-            catch (MqttCommunicationException exception)
+            catch (MqttClientNotConnectedException exception)
             {
-                if (exception.Message == "The client is not connected.")
+                if (exception.Message == "The MQTT client is not connected.")
                 {
                     return;
                 }

Source/MQTTnet.Tests/Clients/MqttClient/MqttClient_Tests.cs

@@ -17,6 +17,7 @@ using MQTTnet.Formatter;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using MQTTnet.Server;
 using MQTTnet.Tests.Mockups;
 
 // ReSharper disable InconsistentNaming

Source/MQTTnet.Tests/Extensions/Rpc_Tests.cs

@@ -52,7 +52,7 @@ namespace MQTTnet.Tests.Extensions
             var paramValue = "123";
             var parameters = new Dictionary<string, object>
             {
-                { TestParametersTopicGenerationStrategy.ExpectedParamName, "123" },
+                { TestParametersTopicGenerationStrategy.ExpectedParamName, "123" }
             };
 
             using (var testEnvironment = CreateTestEnvironment())
@@ -164,7 +164,7 @@ namespace MQTTnet.Tests.Extensions
 
                 using (var rpcClient = new MqttRpcClient(requestSender, new MqttRpcClientOptionsBuilder().Build()))
                 {
-                    var response = await rpcClient.ExecuteAsync(TimeSpan.FromSeconds(2), "ping", "", MqttQualityOfServiceLevel.AtMostOnce);
+                    await rpcClient.ExecuteAsync(TimeSpan.FromSeconds(2), "ping", "", MqttQualityOfServiceLevel.AtMostOnce);
                 }
             }
         }

Source/MQTTnet.Tests/MQTTnet.Tests.csproj

@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFrameworks>net452;net461;net48;netcoreapp3.1;net6.0;net7.0</TargetFrameworks>
+        <TargetFrameworks>netcoreapp3.1;net7.0</TargetFrameworks>
+        <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">$(TargetFrameworks);net452;net48</TargetFrameworks>
         <IsPackable>false</IsPackable>
         <LangVersion>7.3</LangVersion>
         <EnableNETAnalyzers>false</EnableNETAnalyzers>

Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs

@@ -273,7 +273,7 @@ namespace MQTTnet.Tests.MQTTv5
                 Assert.AreEqual(applicationMessage.ResponseTopic, receivedMessage.ResponseTopic);
                 Assert.AreEqual(applicationMessage.MessageExpiryInterval, receivedMessage.MessageExpiryInterval);
                 CollectionAssert.AreEqual(applicationMessage.CorrelationData, receivedMessage.CorrelationData);
-                CollectionAssert.AreEqual(applicationMessage.Payload, receivedMessage.Payload);
+                CollectionAssert.AreEqual(applicationMessage.PayloadSegment.ToArray(), receivedMessage.PayloadSegment.ToArray());
                 CollectionAssert.AreEqual(applicationMessage.UserProperties, receivedMessage.UserProperties);
             }
         }

Source/MQTTnet.Tests/MQTTv5/Server_Tests.cs

@@ -10,6 +10,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Internal;
 using MQTTnet.Protocol;
+using MQTTnet.Server;
 
 namespace MQTTnet.Tests.MQTTv5
 {

Source/MQTTnet.Tests/Mockups/TestEnvironment.cs

@@ -3,7 +3,6 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -25,9 +24,9 @@ namespace MQTTnet.Tests.Mockups
     public sealed class TestEnvironment : IDisposable
     {
         readonly List<string> _clientErrors = new List<string>();
-        readonly ConcurrentBag<IMqttClient> _clients = new ConcurrentBag<IMqttClient>();
+        readonly List<IMqttClient> _clients = new List<IMqttClient>();
         readonly List<Exception> _exceptions = new List<Exception>();
-        readonly ConcurrentBag<ILowLevelMqttClient> _lowLevelClients = new ConcurrentBag<ILowLevelMqttClient>();
+        readonly List<ILowLevelMqttClient> _lowLevelClients = new List<ILowLevelMqttClient>();
         readonly MqttProtocolVersion _protocolVersion;
         readonly List<string> _serverErrors = new List<string>();
 
@@ -40,6 +39,8 @@ namespace MQTTnet.Tests.Mockups
             _protocolVersion = protocolVersion;
             TestContext = testContext;
 
+            TaskScheduler.UnobservedTaskException += TrackUnobservedTaskException;
+
             ServerLogger.LogMessagePublished += (s, e) =>
             {
                 if (Debugger.IsAttached)
@@ -216,8 +217,9 @@ namespace MQTTnet.Tests.Mockups
         public IMqttClient CreateClient()
         {
             var logger = EnableLogger ? (IMqttNetLogger)ClientLogger : MqttNetNullLogger.Instance;
-            
+
             var client = Factory.CreateMqttClient(logger);
+
             client.ConnectingAsync += e =>
             {
                 if (TestContext != null)
@@ -232,9 +234,12 @@ namespace MQTTnet.Tests.Mockups
 
                 return CompletedTask.Instance;
             };
-            
-            _clients.Add(client);
-            
+
+            lock (_clients)
+            {
+                _clients.Add(client);
+            }
+
             return client;
         }
 
@@ -245,13 +250,20 @@ namespace MQTTnet.Tests.Mockups
 
         public MqttClientOptionsBuilder CreateDefaultClientOptionsBuilder()
         {
-            return Factory.CreateClientOptionsBuilder().WithProtocolVersion(_protocolVersion).WithTcpServer("127.0.0.1", ServerPort).WithClientId(TestContext.TestName + "_" + Guid.NewGuid());
+            return Factory.CreateClientOptionsBuilder()
+                .WithProtocolVersion(_protocolVersion)
+                .WithTcpServer("127.0.0.1", ServerPort)
+                .WithClientId(TestContext.TestName + "_" + Guid.NewGuid());
         }
 
         public ILowLevelMqttClient CreateLowLevelClient()
         {
             var client = Factory.CreateLowLevelMqttClient(ClientLogger);
-            _lowLevelClients.Add(client);
+
+            lock (_lowLevelClients)
+            {
+                _lowLevelClients.Add(client);
+            }
 
             return client;
         }
@@ -287,45 +299,68 @@ namespace MQTTnet.Tests.Mockups
 
         public void Dispose()
         {
-            foreach (var mqttClient in _clients)
+            try
             {
+                lock (_clients)
+                {
+                    foreach (var mqttClient in _clients)
+                    {
+                        try
+                        {
+                            //mqttClient.DisconnectAsync().GetAwaiter().GetResult();
+                        }
+                        catch
+                        {
+                            // This can happen when the test already disconnected the client.
+                        }
+                        finally
+                        {
+                            mqttClient?.Dispose();
+                        }
+                    }
+
+                    _clients.Clear();
+                }
+
+                lock (_lowLevelClients)
+                {
+                    foreach (var lowLevelMqttClient in _lowLevelClients)
+                    {
+                        lowLevelMqttClient.Dispose();
+                    }
+
+                    _lowLevelClients.Clear();
+                }
+
                 try
                 {
-                    //mqttClient.DisconnectAsync().GetAwaiter().GetResult();
+                    Server?.StopAsync().GetAwaiter().GetResult();
                 }
                 catch
                 {
-                    // This can happen when the test already disconnected the client.
+                    // This can happen when the test already stopped the server.
                 }
                 finally
                 {
-                    mqttClient?.Dispose();
+                    Server?.Dispose();
                 }
-            }
 
-            foreach (var lowLevelMqttClient in _lowLevelClients)
-            {
-                lowLevelMqttClient.Dispose();
-            }
+                Server = null;
 
-            try
-            {
-                Server?.StopAsync().GetAwaiter().GetResult();
-            }
-            catch
-            {
-                // This can happen when the test already stopped the server.
+                ThrowIfLogErrors();
+
+                GC.Collect();
+                GC.WaitForFullGCComplete();
+                GC.WaitForPendingFinalizers();
+                
+                if (_exceptions.Any())
+                {
+                    throw new Exception($"{_exceptions.Count} exceptions tracked.\r\n" + string.Join(Environment.NewLine, _exceptions));
+                }
             }
             finally
             {
-                Server?.Dispose();
-            }
-
-            ThrowIfLogErrors();
-
-            if (_exceptions.Any())
-            {
-                throw new Exception($"{_exceptions.Count} exceptions tracked.\r\n" + string.Join(Environment.NewLine, _exceptions));
+                TaskScheduler.UnobservedTaskException -= TrackUnobservedTaskException;
             }
         }
 
@@ -342,7 +377,7 @@ namespace MQTTnet.Tests.Mockups
 
             var options = optionsBuilder.Build();
             var server = CreateServer(options);
-            await server.StartAsync();
+            await server.StartAsync().ConfigureAwait(false);
 
             // The OS has chosen the port to we have to properly expose it to the tests.
             ServerPort = options.DefaultEndpointOptions.Port;
@@ -370,13 +405,16 @@ namespace MQTTnet.Tests.Mockups
 
         public void ThrowIfLogErrors()
         {
-            lock (_serverErrors)
+            if (!IgnoreServerLogErrors)
             {
-                if (!IgnoreServerLogErrors && _serverErrors.Count > 0)
+                lock (_serverErrors)
                 {
-                    var message = $"Server had {_serverErrors.Count} errors (${string.Join(Environment.NewLine, _serverErrors)}).";
-                    Console.WriteLine(message);
-                    throw new Exception(message);
+                    if (_serverErrors.Count > 0)
+                    {
+                        var message = $"Server had {_serverErrors.Count} errors (${string.Join(Environment.NewLine, _serverErrors)}).";
+                        Console.WriteLine(message);
+                        throw new Exception(message);
+                    }
                 }
             }
 
@@ -396,10 +434,20 @@ namespace MQTTnet.Tests.Mockups
 
         public void TrackException(Exception exception)
         {
+            if (exception == null)
+            {
+                return;
+            }
+
             lock (_exceptions)
             {
                 _exceptions.Add(exception);
             }
         }
+
+        void TrackUnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)
+        {
+            TrackException(e.Exception);
+        }
     }
 }

Source/MQTTnet.Tests/MqttApplicationMessageTest.cs

@@ -1,3 +1,5 @@
+#pragma warning disable CS0618 // Type or member is obsolete
+
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using System;
 using System.Collections.Generic;
@@ -8,7 +10,7 @@ using System.Threading.Tasks;
 namespace MQTTnet.Tests
 {
     [TestClass]
-    public class MqttApplicationMessageTest
+    public sealed class MqttApplicationMessageTest
     {
         [TestMethod]
         public void PayloadSegment()
@@ -17,32 +19,27 @@ namespace MQTTnet.Tests
             Assert.AreEqual(0, message.Payload.Length);
             Assert.AreEqual(0, message.PayloadSegment.Count);
 
-            Assert.IsTrue(object.ReferenceEquals(message.Payload, message.Payload));
-            Assert.IsTrue(object.ReferenceEquals(message.Payload, message.PayloadSegment.Array));
+            Assert.IsTrue(ReferenceEquals(message.Payload, message.PayloadSegment.Array));
 
             message.Payload = new byte[] { 1, 2 };
             Assert.AreEqual(2, message.Payload.Length);
             Assert.AreEqual(2, message.PayloadSegment.Count);
-            Assert.IsTrue(object.ReferenceEquals(message.Payload, message.Payload));
-            Assert.IsTrue(object.ReferenceEquals(message.Payload, message.PayloadSegment.Array));
+            Assert.IsTrue(ReferenceEquals(message.Payload, message.PayloadSegment.Array));
 
             message.Payload = new byte[] { 1, 2, 3 };
             Assert.AreEqual(3, message.Payload.Length);
             Assert.AreEqual(3, message.PayloadSegment.Count);
-            Assert.IsTrue(object.ReferenceEquals(message.Payload, message.Payload));
-            Assert.IsTrue(object.ReferenceEquals(message.Payload, message.PayloadSegment.Array));
+            Assert.IsTrue(ReferenceEquals(message.Payload, message.PayloadSegment.Array));
 
             message.PayloadSegment = new ArraySegment<byte>(new byte[] { 1, 2, 3 });
             Assert.AreEqual(3, message.Payload.Length);
             Assert.AreEqual(3, message.PayloadSegment.Count);
-            Assert.IsTrue(object.ReferenceEquals(message.Payload, message.Payload));
-            Assert.IsTrue(object.ReferenceEquals(message.Payload, message.PayloadSegment.Array));
+            Assert.IsTrue(ReferenceEquals(message.Payload, message.PayloadSegment.Array));
 
             message.PayloadSegment = new ArraySegment<byte>(new byte[] { 1, 2, 3 }, 1, 1);
             Assert.AreEqual(1, message.Payload.Length);
             Assert.AreEqual(1, message.PayloadSegment.Count);
-            Assert.IsTrue(object.ReferenceEquals(message.Payload, message.Payload));
-            Assert.IsFalse(object.ReferenceEquals(message.Payload, message.PayloadSegment.Array));
+            Assert.IsFalse(ReferenceEquals(message.Payload, message.PayloadSegment.Array));
         }
     }
 }

Source/MQTTnet.Tests/Server/General.cs

@@ -303,7 +303,7 @@ namespace MQTTnet.Tests.Server
                 var server = await testEnvironment.StartServer();
                 server.InterceptingPublishAsync += e =>
                 {
-                    e.ApplicationMessage.Payload = Encoding.ASCII.GetBytes("extended");
+                    e.ApplicationMessage.PayloadSegment = new ArraySegment<byte>(Encoding.ASCII.GetBytes("extended"));
                     return CompletedTask.Instance;
                 };
 
@@ -314,7 +314,7 @@ namespace MQTTnet.Tests.Server
                 var isIntercepted = false;
                 c2.ApplicationMessageReceivedAsync += e =>
                 {
-                    isIntercepted = string.Compare("extended", Encoding.UTF8.GetString(e.ApplicationMessage.Payload), StringComparison.Ordinal) == 0;
+                    isIntercepted = string.Compare("extended", Encoding.UTF8.GetString(e.ApplicationMessage.PayloadSegment.ToArray()), StringComparison.Ordinal) == 0;
                     return CompletedTask.Instance;
                 };
 
@@ -425,7 +425,7 @@ namespace MQTTnet.Tests.Server
                             new MqttApplicationMessage
                             {
                                 Topic = "/test/1",
-                                Payload = Encoding.UTF8.GetBytes("true"),
+                                PayloadSegment = new ArraySegment<byte>(Encoding.UTF8.GetBytes("true")),
                                 QualityOfServiceLevel = MqttQualityOfServiceLevel.ExactlyOnce
                             })
                         {
@@ -780,7 +780,7 @@ namespace MQTTnet.Tests.Server
                 var client1 = await testEnvironment.ConnectClient();
                 client1.ApplicationMessageReceivedAsync += e =>
                 {
-                    receivedBody = e.ApplicationMessage.Payload;
+                    receivedBody = e.ApplicationMessage.PayloadSegment.ToArray();
                     return CompletedTask.Instance;
                 };
 

Source/MQTTnet.Tests/Server/HotSwapCerts_Tests.cs

@@ -1,4 +1,4 @@
-#if !(NET452 || NET461)
+#if !(NET452 || NET461 || NET48)
 using System;
 using System.Collections.Concurrent;
 using System.Diagnostics;
@@ -149,18 +149,14 @@ namespace MQTTnet.Tests.Server
             }
         }
 
-        class ClientTestHarness : IDisposable
+        sealed class ClientTestHarness : IDisposable
         {
             readonly HotSwappableClientCertProvider _hotSwapClient = new HotSwappableClientCertProvider();
+
             IMqttClient _client;
 
             public string ClientId => _client.Options.ClientId;
 
-            public void ClearServerCerts()
-            {
-                _hotSwapClient.ClearServerCerts();
-            }
-
             public Task Connect()
             {
                 return Run_Client_Connection();
@@ -169,6 +165,7 @@ namespace MQTTnet.Tests.Server
             public void Dispose()
             {
                 _client.Dispose();
+                _hotSwapClient.Dispose();
             }
 
             public X509Certificate2 GetCurrentClientCert()
@@ -189,6 +186,8 @@ namespace MQTTnet.Tests.Server
 
             public void WaitForConnectOrFail(TimeSpan timeout)
             {
+                Thread.Sleep(100);
+
                 if (!_client.IsConnected)
                 {
                     _client.ReconnectAsync().Wait(timeout);
@@ -210,12 +209,12 @@ namespace MQTTnet.Tests.Server
                 Assert.IsFalse(_client.IsConnected, "Client connection success but test wanted fail");
             }
 
-            public void WaitForDisconnect(TimeSpan timeout)
+            void WaitForDisconnect(TimeSpan timeout)
             {
                 var timer = Stopwatch.StartNew();
                 while ((_client == null || _client.IsConnected) && timer.Elapsed < timeout)
                 {
-                    Thread.Sleep(5);
+                    Thread.Sleep(100);
                 }
             }
 
@@ -252,22 +251,17 @@ namespace MQTTnet.Tests.Server
                 var timer = Stopwatch.StartNew();
                 while ((_client == null || !_client.IsConnected) && timer.Elapsed < timeout)
                 {
-                    Thread.Sleep(5);
+                    Thread.Sleep(100);
                 }
             }
         }
 
-        class ServerTestHarness : IDisposable
+        sealed class ServerTestHarness : IDisposable
         {
             readonly HotSwappableServerCertProvider _hotSwapServer = new HotSwappableServerCertProvider();
 
             MqttServer _server;
 
-            public void ClearClientCerts()
-            {
-                _hotSwapServer.ClearClientCerts();
-            }
-
             public void Dispose()
             {
                 if (_server != null)
@@ -276,15 +270,12 @@ namespace MQTTnet.Tests.Server
                     _server.Dispose();
                 }
 
-                if (_hotSwapServer != null)
-                {
-                    _hotSwapServer.Dispose();
-                }
+                _hotSwapServer?.Dispose();
             }
 
-            public async Task ForceDisconnectAsync(ClientTestHarness client)
+            public Task ForceDisconnectAsync(ClientTestHarness client)
             {
-                await _server.DisconnectClientAsync(client.ClientId, MqttDisconnectReasonCode.UnspecifiedError);
+                return _server.DisconnectClientAsync(client.ClientId, MqttDisconnectReasonCode.UnspecifiedError);
             }
 
             public X509Certificate2 GetCurrentServerCert()
@@ -302,7 +293,7 @@ namespace MQTTnet.Tests.Server
                 _hotSwapServer.InstallNewClientCert(serverCert);
             }
 
-            public async Task StartServer()
+            public Task StartServer()
             {
                 var mqttFactory = new MqttFactory();
 
@@ -312,24 +303,33 @@ namespace MQTTnet.Tests.Server
                     .Build();
 
                 mqttServerOptions.TlsEndpointOptions.ClientCertificateRequired = true;
+
                 _server = mqttFactory.CreateMqttServer(mqttServerOptions);
-                await _server.StartAsync();
+                return _server.StartAsync();
             }
         }
 
-        class HotSwappableClientCertProvider : IMqttClientCertificatesProvider
+        class HotSwappableClientCertProvider : IMqttClientCertificatesProvider, IDisposable
         {
             X509Certificate2Collection _certificates;
-            ConcurrentBag<X509Certificate2> ServerCerts = new ConcurrentBag<X509Certificate2>();
+            ConcurrentBag<X509Certificate2> _serverCerts = new ConcurrentBag<X509Certificate2>();
 
             public HotSwappableClientCertProvider()
             {
                 _certificates = new X509Certificate2Collection(CreateSelfSignedCertificate("1.3.6.1.5.5.7.3.2"));
             }
-
-            public void ClearServerCerts()
+            
+            public void Dispose()
             {
-                ServerCerts = new ConcurrentBag<X509Certificate2>();
+                if (_certificates != null)
+                {
+                    foreach (var certs in _certificates)
+                    {
+#if !NET452
+                        certs.Dispose();
+#endif
+                    }
+                }
             }
 
             public X509CertificateCollection GetCertificates()
@@ -339,18 +339,17 @@ namespace MQTTnet.Tests.Server
 
             public void HotSwapCert()
             {
-                var newCert = new X509Certificate2Collection(CreateSelfSignedCertificate("1.3.6.1.5.5.7.3.2"));
-                var oldCerts = Interlocked.Exchange(ref _certificates, newCert);
+                _certificates = new X509Certificate2Collection(CreateSelfSignedCertificate("1.3.6.1.5.5.7.3.2"));
             }
 
             public void InstallNewServerCert(X509Certificate2 serverCert)
             {
-                ServerCerts.Add(serverCert);
+                _serverCerts.Add(serverCert);
             }
 
             public bool OnCertificateValidation(MqttClientCertificateValidationEventArgs certContext)
             {
-                var serverCerts = ServerCerts.ToArray();
+                var serverCerts = _serverCerts.ToArray();
 
                 var providedCert = certContext.Certificate.GetRawCertData();
                 for (int i = 0, n = serverCerts.Length; i < n; i++)
@@ -365,36 +364,18 @@ namespace MQTTnet.Tests.Server
 
                 return false;
             }
-
-            void Dispose()
-            {
-                if (_certificates != null)
-                {
-                    foreach (var certs in _certificates)
-                    {
-#if !NET452
-                        certs.Dispose();
-#endif
-                    }
-                }
-            }
         }
 
-        class HotSwappableServerCertProvider : ICertificateProvider, IDisposable
+        sealed class HotSwappableServerCertProvider : ICertificateProvider, IDisposable
         {
+            readonly ConcurrentBag<X509Certificate2> _clientCerts = new ConcurrentBag<X509Certificate2>();
             X509Certificate2 _certificate;
-            ConcurrentBag<X509Certificate2> ClientCerts = new ConcurrentBag<X509Certificate2>();
 
             public HotSwappableServerCertProvider()
             {
                 _certificate = CreateSelfSignedCertificate("1.3.6.1.5.5.7.3.1");
             }
 
-            public void ClearClientCerts()
-            {
-                ClientCerts = new ConcurrentBag<X509Certificate2>();
-            }
-
             public void Dispose()
             {
 #if !NET452
@@ -418,12 +399,12 @@ namespace MQTTnet.Tests.Server
 
             public void InstallNewClientCert(X509Certificate2 certificate)
             {
-                ClientCerts.Add(certificate);
+                _clientCerts.Add(certificate);
             }
 
             public bool RemoteCertificateValidationCallback(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
             {
-                var serverCerts = ClientCerts.ToArray();
+                var serverCerts = _clientCerts.ToArray();
 
                 var providedCert = certificate.GetRawCertData();
                 for (int i = 0, n = serverCerts.Length; i < n; i++)

Source/MQTTnet.Tests/Server/Publishing_Tests.cs

@@ -8,6 +8,7 @@ using MQTTnet.Client;
 using MQTTnet.Formatter;
 using MQTTnet.Internal;
 using MQTTnet.Protocol;
+using MQTTnet.Server;
 
 namespace MQTTnet.Tests.Server
 {

Source/MQTTnet.Tests/Server/Session_Tests.cs

@@ -283,7 +283,7 @@ namespace MQTTnet.Tests.Server
 
                 server.InterceptingPublishAsync += e =>
                 {
-                    e.ApplicationMessage.Payload = Encoding.UTF8.GetBytes(e.SessionItems["default_payload"] as string ?? string.Empty);
+                    e.ApplicationMessage.PayloadSegment = new ArraySegment<byte>(Encoding.UTF8.GetBytes(e.SessionItems["default_payload"] as string ?? string.Empty));
                     return CompletedTask.Instance;
                 };
 

Source/MQTTnet.Tests/Server/Tls_Tests.cs

@@ -101,7 +101,7 @@ namespace MQTTnet.Tests.Server
                 new MqttApplicationMessage
                 {
                     Topic = "TestTopic1",
-                    Payload = new byte[] { 1, 2, 3, 4 }
+                    PayloadSegment = new ArraySegment<byte>(new byte[] { 1, 2, 3, 4 })
                 });
 
             await testEnvironment.Server.InjectApplicationMessage(
@@ -109,7 +109,7 @@ namespace MQTTnet.Tests.Server
                     new MqttApplicationMessage
                     {
                         Topic = "TestTopic1",
-                        Payload = new byte[] { 1, 2, 3, 4 }
+                        PayloadSegment = new ArraySegment<byte>(new byte[] { 1, 2, 3, 4 })
                     }));
 
             certificateProvider.CurrentCertificate = CreateCertificate(secondOid);
@@ -137,7 +137,7 @@ namespace MQTTnet.Tests.Server
                 new MqttApplicationMessage
                 {
                     Topic = "TestTopic2",
-                    Payload = new byte[] { 1, 2, 3, 4 }
+                    PayloadSegment = new ArraySegment<byte>(new byte[] { 1, 2, 3, 4 })
                 });
 
             await testEnvironment.Server.InjectApplicationMessage(
@@ -145,7 +145,7 @@ namespace MQTTnet.Tests.Server
                     new MqttApplicationMessage
                     {
                         Topic = "TestTopic2",
-                        Payload = new byte[] { 1, 2, 3, 4 }
+                        PayloadSegment = new ArraySegment<byte>(new byte[] { 1, 2, 3, 4 })
                     }));
 
             // Ensure first client still works
@@ -153,7 +153,7 @@ namespace MQTTnet.Tests.Server
                 new MqttApplicationMessage
                 {
                     Topic = "TestTopic1",
-                    Payload = new byte[] { 1, 2, 3, 4 }
+                    PayloadSegment = new ArraySegment<byte>(new byte[] { 1, 2, 3, 4 })
                 });
 
             await testEnvironment.Server.InjectApplicationMessage(
@@ -161,7 +161,7 @@ namespace MQTTnet.Tests.Server
                     new MqttApplicationMessage
                     {
                         Topic = "TestTopic1",
-                        Payload = new byte[] { 1, 2, 3, 4 }
+                        PayloadSegment = new ArraySegment<byte>(new byte[] { 1, 2, 3, 4 })
                     }));
 
             await Task.Delay(1000);
@@ -178,12 +178,10 @@ namespace MQTTnet.Tests.Server
             var clientOptionsBuilder = testEnvironment.Factory.CreateClientOptionsBuilder();
             clientOptionsBuilder.WithClientId(Guid.NewGuid().ToString())
                 .WithTcpServer("localhost", 8883)
-                .WithTls(
-                    tls =>
+                .WithTlsOptions(
+                    o =>
                     {
-                        tls.UseTls = true;
-                        tls.SslProtocol = SslProtocols.Tls12;
-                        tls.CertificateValidationHandler = certValidator;
+                        o.WithSslProtocols(SslProtocols.Tls12).WithCertificateValidationHandler(certValidator);
                     });
 
             var clientOptions = clientOptionsBuilder.Build();

Source/MQTTnet/Adapter/IMqttChannelAdapter.cs

@@ -25,8 +25,6 @@ namespace MQTTnet.Adapter
 
         long BytesReceived { get; }
 
-        bool IsReadingPacket { get; }
-
         Task ConnectAsync(CancellationToken cancellationToken);
 
         Task DisconnectAsync(CancellationToken cancellationToken);

Source/MQTTnet/Adapter/IMqttServerAdapter.cs

@@ -14,6 +14,7 @@ namespace MQTTnet.Adapter
         Func<IMqttChannelAdapter, Task> ClientHandler { get; set; }
 
         Task StartAsync(MqttServerOptions options, IMqttNetLogger logger);
+
         Task StopAsync();
     }
 }

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -26,9 +26,7 @@ namespace MQTTnet.Adapter
         readonly IMqttChannel _channel;
         readonly byte[] _fixedHeaderBuffer = new byte[2];
         readonly MqttNetSourceLogger _logger;
-
         readonly byte[] _singleByteBuffer = new byte[1];
-
         readonly AsyncLock _syncRoot = new AsyncLock();
 
         Statistics _statistics; // mutable struct, don't make readonly!
@@ -47,6 +45,8 @@ namespace MQTTnet.Adapter
             _logger = logger.WithSource(nameof(MqttChannelAdapter));
         }
 
+        public bool AllowPacketFragmentation { get; set; } = true;
+
         public long BytesReceived => Volatile.Read(ref _statistics._bytesReceived);
 
         public long BytesSent => Volatile.Read(ref _statistics._bytesSent);
@@ -55,16 +55,12 @@ namespace MQTTnet.Adapter
 
         public string Endpoint => _channel.Endpoint;
 
-        public bool IsReadingPacket { get; private set; }
-
         public bool IsSecureConnection => _channel.IsSecureConnection;
 
         public MqttPacketFormatterAdapter PacketFormatterAdapter { get; }
 
         public MqttPacketInspector PacketInspector { get; set; }
 
-        public bool AllowPacketFragmentation { get; set; } = true;
-
         public async Task ConnectAsync(CancellationToken cancellationToken)
         {
             cancellationToken.ThrowIfCancellationRequested();
@@ -78,12 +74,30 @@ namespace MQTTnet.Adapter
                  * 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)))
                 {
+                    var connectTask = Task.Run(
+                        async () =>
+                        {
+                            try
+                            {
+                                await _channel.ConnectAsync(cancellationToken).ConfigureAwait(false);
+                            }
+                            catch
+                            {
+                                // If the timeout is already reached the exception is no longer of interest and
+                                // must be catched. Otherwise it will arrive at the TaskScheduler.UnobservedTaskException.
+                                if (!timeout.Task.IsCompleted)
+                                {
+                                    throw;
+                                }
+                            }
+                        },
+                        CancellationToken.None);
+
                     await Task.WhenAny(connectTask, timeout.Task).ConfigureAwait(false);
+
                     if (timeout.Task.IsCompleted && !connectTask.IsCompleted)
                     {
                         throw new OperationCanceledException("MQTT connect canceled.", cancellationToken);
@@ -127,7 +141,8 @@ namespace MQTTnet.Adapter
 
             try
             {
-                PacketInspector?.BeginReceivePacket();
+                var localPacketInspector = PacketInspector;
+                localPacketInspector?.BeginReceivePacket();
 
                 ReceivedMqttPacket receivedPacket;
                 var receivedPacketTask = ReceiveAsync(cancellationToken);
@@ -145,7 +160,10 @@ namespace MQTTnet.Adapter
                     return null;
                 }
 
-                PacketInspector?.EndReceivePacket();
+                if (localPacketInspector != null)
+                {
+                    await localPacketInspector.EndReceivePacket().ConfigureAwait(false);
+                }
 
                 Interlocked.Add(ref _statistics._bytesSent, receivedPacket.TotalLength);
 
@@ -201,10 +219,15 @@ namespace MQTTnet.Adapter
                 try
                 {
                     var packetBuffer = PacketFormatterAdapter.Encode(packet);
-                    PacketInspector?.BeginSendPacket(packetBuffer);
 
-                    _logger.Verbose("TX ({0} bytes) >>> {1}", packetBuffer.Length, packet);
+                    var localPacketInspector = PacketInspector;
+                    if (localPacketInspector != null)
+                    {
+                        await localPacketInspector.BeginSendPacket(packetBuffer).ConfigureAwait(false);    
+                    }
                     
+                    _logger.Verbose("TX ({0} bytes) >>> {1}", packetBuffer.Length, packet);
+
                     if (packetBuffer.Payload.Count == 0 || !AllowPacketFragmentation)
                     {
                         await _channel.WriteAsync(packetBuffer.Join(), true, cancellationToken).ConfigureAwait(false);
@@ -373,53 +396,45 @@ namespace MQTTnet.Adapter
                 return ReceivedMqttPacket.Empty;
             }
 
-            IsReadingPacket = true;
-            try
+            var fixedHeader = readFixedHeaderResult.FixedHeader;
+            if (fixedHeader.RemainingLength == 0)
             {
-                var fixedHeader = readFixedHeaderResult.FixedHeader;
-                if (fixedHeader.RemainingLength == 0)
+                return new ReceivedMqttPacket(fixedHeader.Flags, EmptyBuffer.ArraySegment, 2);
+            }
+
+            var bodyLength = fixedHeader.RemainingLength;
+            var body = new byte[bodyLength];
+
+            var bodyOffset = 0;
+            var chunkSize = Math.Min(ReadBufferSize, bodyLength);
+
+            do
+            {
+                var bytesLeft = body.Length - bodyOffset;
+                if (chunkSize > bytesLeft)
                 {
-                    return new ReceivedMqttPacket(fixedHeader.Flags, EmptyBuffer.ArraySegment, 2);
+                    chunkSize = bytesLeft;
                 }
 
-                var bodyLength = fixedHeader.RemainingLength;
-                var body = new byte[bodyLength];
+                var readBytes = await _channel.ReadAsync(body, bodyOffset, chunkSize, cancellationToken).ConfigureAwait(false);
 
-                var bodyOffset = 0;
-                var chunkSize = Math.Min(ReadBufferSize, bodyLength);
-
-                do
+                if (cancellationToken.IsCancellationRequested)
                 {
-                    var bytesLeft = body.Length - bodyOffset;
-                    if (chunkSize > bytesLeft)
-                    {
-                        chunkSize = bytesLeft;
-                    }
+                    return ReceivedMqttPacket.Empty;
+                }
 
-                    var readBytes = await _channel.ReadAsync(body, bodyOffset, chunkSize, cancellationToken).ConfigureAwait(false);
+                if (readBytes == 0)
+                {
+                    return ReceivedMqttPacket.Empty;
+                }
 
-                    if (cancellationToken.IsCancellationRequested)
-                    {
-                        return ReceivedMqttPacket.Empty;
-                    }
+                bodyOffset += readBytes;
+            } while (bodyOffset < bodyLength);
 
-                    if (readBytes == 0)
-                    {
-                        return ReceivedMqttPacket.Empty;
-                    }
+            PacketInspector?.FillReceiveBuffer(body);
 
-                    bodyOffset += readBytes;
-                } while (bodyOffset < bodyLength);
-
-                PacketInspector?.FillReceiveBuffer(body);
-
-                var bodySegment = new ArraySegment<byte>(body, 0, bodyLength);
-                return new ReceivedMqttPacket(fixedHeader.Flags, bodySegment, fixedHeader.TotalLength);
-            }
-            finally
-            {
-                IsReadingPacket = false;
-            }
+            var bodySegment = new ArraySegment<byte>(body, 0, bodyLength);
+            return new ReceivedMqttPacket(fixedHeader.Flags, bodySegment, fixedHeader.TotalLength);
         }
 
         static bool WrapAndThrowException(Exception exception)

Source/MQTTnet/Adapter/MqttPacketInspector.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.IO;
+using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
 using MQTTnet.Formatter;
 using MQTTnet.Internal;
@@ -12,16 +13,20 @@ namespace MQTTnet.Adapter
 {
     public sealed class MqttPacketInspector
     {
-        readonly MqttNetSourceLogger _logger;
         readonly AsyncEvent<InspectMqttPacketEventArgs> _asyncEvent;
-        
+        readonly MqttNetSourceLogger _logger;
+
         MemoryStream _receivedPacketBuffer;
-        
+
         public MqttPacketInspector(AsyncEvent<InspectMqttPacketEventArgs> asyncEvent, IMqttNetLogger logger)
         {
             _asyncEvent = asyncEvent ?? throw new ArgumentNullException(nameof(asyncEvent));
-            
-            if (logger == null) throw new ArgumentNullException(nameof(logger));
+
+            if (logger == null)
+            {
+                throw new ArgumentNullException(nameof(logger));
+            }
+
             _logger = logger.WithSource(nameof(MqttPacketInspector));
         }
 
@@ -36,28 +41,15 @@ namespace MQTTnet.Adapter
             {
                 _receivedPacketBuffer = new MemoryStream();
             }
-            
+
             _receivedPacketBuffer?.SetLength(0);
         }
 
-        public void EndReceivePacket()
+        public Task BeginSendPacket(MqttPacketBuffer buffer)
         {
             if (!_asyncEvent.HasHandlers)
             {
-                return;
-            }
-
-            var buffer = _receivedPacketBuffer.ToArray();
-            _receivedPacketBuffer.SetLength(0);
-
-            InspectPacket(buffer, MqttPacketFlowDirection.Inbound);
-        }
-
-        public void BeginSendPacket(MqttPacketBuffer buffer)
-        {
-            if (!_asyncEvent.HasHandlers)
-            {
-                return;
+                return CompletedTask.Instance;
             }
 
             // Create a copy of the actual packet so that the inspector gets no access
@@ -65,7 +57,20 @@ namespace MQTTnet.Adapter
             // intended for debugging etc. so that this is OK.
             var bufferCopy = buffer.ToArray();
 
-            InspectPacket(bufferCopy, MqttPacketFlowDirection.Outbound);
+            return InspectPacket(bufferCopy, MqttPacketFlowDirection.Outbound);
+        }
+
+        public Task EndReceivePacket()
+        {
+            if (!_asyncEvent.HasHandlers)
+            {
+                return CompletedTask.Instance;
+            }
+
+            var buffer = _receivedPacketBuffer.ToArray();
+            _receivedPacketBuffer.SetLength(0);
+
+            return InspectPacket(buffer, MqttPacketFlowDirection.Inbound);
         }
 
         public void FillReceiveBuffer(byte[] buffer)
@@ -74,16 +79,16 @@ namespace MQTTnet.Adapter
             {
                 return;
             }
-            
+
             _receivedPacketBuffer?.Write(buffer, 0, buffer.Length);
         }
 
-        void InspectPacket(byte[] buffer, MqttPacketFlowDirection direction)
+        async Task InspectPacket(byte[] buffer, MqttPacketFlowDirection direction)
         {
             try
             {
                 var eventArgs = new InspectMqttPacketEventArgs(direction, buffer);
-                _asyncEvent.InvokeAsync(eventArgs).GetAwaiter().GetResult();
+                await _asyncEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
             }
             catch (Exception exception)
             {

Source/MQTTnet/Client/MqttClient.cs

@@ -309,6 +309,7 @@ namespace MQTTnet.Client
                 AuthenticationMethod = Options.AuthenticationMethod,
                 AuthenticationData = data.AuthenticationData,
                 ReasonString = data.ReasonString,
+                ReasonCode = data.ReasonCode,
                 UserProperties = data.UserProperties
             };
 
@@ -708,12 +709,15 @@ namespace MQTTnet.Client
                         await eventArgs.AcknowledgeAsync(cancellationToken).ConfigureAwait(false);
                     }
                 }
+                catch (ObjectDisposedException)
+                {
+                }
                 catch (OperationCanceledException)
                 {
                 }
                 catch (Exception exception)
                 {
-                    _logger.Error(exception, "Error while handling application message.");
+                    _logger.Error(exception, "Error while handling application message");
                 }
             }
         }
@@ -787,7 +791,7 @@ namespace MQTTnet.Client
         {
             try
             {
-                _logger.Verbose("Start receiving packets.");
+                _logger.Verbose("Start receiving packets");
 
                 while (!cancellationToken.IsCancellationRequested)
                 {
@@ -825,20 +829,21 @@ namespace MQTTnet.Client
                 }
                 else if (exception is MqttCommunicationException)
                 {
-                    _logger.Warning(exception, "Communication error while receiving packets.");
+                    _logger.Warning(exception, "Communication error while receiving packets");
                 }
                 else
                 {
-                    _logger.Error(exception, "Error while receiving packets.");
+                    _logger.Error(exception, "Error while receiving packets");
                 }
 
-                _packetDispatcher.FailAll(exception);
+                // The packet dispatcher is set to null when the client is being disposed so it may already being gone!
+                _packetDispatcher?.FailAll(exception);
 
                 await DisconnectInternal(_packetReceiverTask, exception, null).ConfigureAwait(false);
             }
             finally
             {
-                _logger.Verbose("Stopped receiving packets.");
+                _logger.Verbose("Stopped receiving packets");
             }
         }
 
@@ -860,7 +865,7 @@ namespace MQTTnet.Client
                 }
                 catch (Exception exception)
                 {
-                    _logger.Warning(exception, "Error when sending request packet ({0}).", requestPacket.GetType().Name);
+                    _logger.Warning(exception, "Error when sending {0} request packet", requestPacket.GetType().Name);
                     packetAwaitable.Fail(exception);
                 }
 
@@ -872,7 +877,7 @@ namespace MQTTnet.Client
                 {
                     if (exception is MqttCommunicationTimedOutException)
                     {
-                        _logger.Warning("Timeout while waiting for response packet ({0}).", typeof(TResponsePacket).Name);
+                        _logger.Warning("Timeout while waiting for {0} response packet", typeof(TResponsePacket).Name);
                     }
 
                     throw;
@@ -925,7 +930,7 @@ namespace MQTTnet.Client
 
         static void ThrowNotConnected()
         {
-            throw new MqttCommunicationException("The client is not connected.");
+            throw new MqttClientNotConnectedException();
         }
 
         void TryInitiateDisconnect()
@@ -938,7 +943,7 @@ namespace MQTTnet.Client
                 }
                 catch (Exception exception)
                 {
-                    _logger.Warning(exception, "Error while initiating disconnect.");
+                    _logger.Warning(exception, "Error while initiating disconnect");
                 }
             }
         }
@@ -973,7 +978,7 @@ namespace MQTTnet.Client
                 }
                 else if (packet is MqttPingReqPacket)
                 {
-                    throw new MqttProtocolViolationException("The PINGREQ Packet is sent from a Client to the Server only.");
+                    throw new MqttProtocolViolationException("The PINGREQ Packet is sent from a client to the server only.");
                 }
                 else
                 {
@@ -995,14 +1000,15 @@ namespace MQTTnet.Client
                 }
                 else if (exception is MqttCommunicationException)
                 {
-                    _logger.Warning(exception, "Communication error while receiving packets.");
+                    _logger.Warning(exception, "Communication error while receiving packets");
                 }
                 else
                 {
-                    _logger.Error(exception, $"Error while processing received packet ({packet.GetType().Name}).");
+                    _logger.Error(exception, "Error while processing received {0} packet", packet.GetType().Name);
                 }
 
-                _packetDispatcher.FailAll(exception);
+                // The packet dispatcher may already be gone due to disconnect etc!
+                _packetDispatcher?.FailAll(exception);
 
                 await DisconnectInternal(_packetReceiverTask, exception, null).ConfigureAwait(false);
             }
@@ -1012,7 +1018,7 @@ namespace MQTTnet.Client
         {
             try
             {
-                _logger.Verbose("Start sending keep alive packets.");
+                _logger.Verbose("Start sending keep alive packets");
 
                 var keepAlivePeriod = Options.KeepAlivePeriod;
 
@@ -1050,18 +1056,18 @@ namespace MQTTnet.Client
                 }
                 else if (exception is MqttCommunicationException)
                 {
-                    _logger.Warning(exception, "Communication error while sending/receiving keep alive packets.");
+                    _logger.Warning(exception, "Communication error while sending/receiving keep alive packets");
                 }
                 else
                 {
-                    _logger.Error(exception, "Error exception while sending/receiving keep alive packets.");
+                    _logger.Error(exception, "Error exception while sending/receiving keep alive packets");
                 }
 
                 await DisconnectInternal(_keepAlivePacketsSenderTask, exception, null).ConfigureAwait(false);
             }
             finally
             {
-                _logger.Verbose("Stopped sending keep alive packets.");
+                _logger.Verbose("Stopped sending keep alive packets");
             }
         }
     }

Source/MQTTnet/Client/Options/MqttClientOptionsBuilder.cs

@@ -2,9 +2,12 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#pragma warning disable CS0612 // Type or member is obsolete
+
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Net;
 using System.Text;
 using MQTTnet.Formatter;
 using MQTTnet.Packets;
@@ -20,7 +23,9 @@ namespace MQTTnet.Client
 
         MqttClientTcpOptions _tcpOptions;
         MqttClientTlsOptions _tlsOptions;
-
+        EndPoint _remoteEndPoint;
+        int? _port;
+        
         [Obsolete] MqttClientOptionsBuilderTlsParameters _tlsParameters;
 
         MqttClientWebSocketOptions _webSocketOptions;
@@ -37,7 +42,7 @@ namespace MQTTnet.Client
             // on the order of called methods from the builder).
             // The builder prefers the explicitly set TLS options!
             var tlsOptions = _tlsOptions ?? _tcpOptions?.TlsOptions;
-            
+
             if (_tlsParameters != null)
             {
                 if (_tlsParameters?.UseTls == true)
@@ -61,6 +66,31 @@ namespace MQTTnet.Client
             if (_tcpOptions != null)
             {
                 _tcpOptions.TlsOptions = tlsOptions;
+
+                if (_remoteEndPoint == null)
+                {
+                    throw new ArgumentException("No endpoint is set.");
+                }
+
+                if (_remoteEndPoint is DnsEndPoint dns)
+                {
+                    if (dns.Port == 0)
+                    {
+                        if (_port.HasValue)
+                        {
+                            _remoteEndPoint = new DnsEndPoint(dns.Host, _port.Value);
+                        }
+                        else
+                        {
+                            _remoteEndPoint = new DnsEndPoint(dns.Host, tlsOptions?.UseTls == false ? MqttPorts.Default : MqttPorts.Secure);
+                        }
+                    }
+                }
+
+                if (_tcpOptions.RemoteEndpoint == null)
+                {
+                    _tcpOptions.RemoteEndpoint = _remoteEndPoint;    
+                }
             }
             else if (_webSocketOptions != null)
             {
@@ -305,11 +335,20 @@ namespace MQTTnet.Client
 
         public MqttClientOptionsBuilder WithTcpServer(string server, int? port = null)
         {
-            _tcpOptions = new MqttClientTcpOptions
-            {
-                Server = server,
-                Port = port
-            };
+            _tcpOptions = new MqttClientTcpOptions();
+
+            // The value 0 will be updated when building the options.
+            // This a backward compatibility feature.
+            _remoteEndPoint = new DnsEndPoint(server, port ?? 0);
+            _port = port;
+
+            return this;
+        }
+        
+        public MqttClientOptionsBuilder WithEndPoint(EndPoint endPoint)
+        {
+            _remoteEndPoint = endPoint ?? throw new ArgumentNullException(nameof(endPoint));
+            _tcpOptions = new MqttClientTcpOptions();
 
             return this;
         }

Source/MQTTnet/Client/Options/MqttClientTcpOptions.cs

@@ -11,34 +11,32 @@ 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.
+        ///     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);
 
+        /// <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 bool NoDelay { get; set; } = true;
 
-        public int? Port { get; set; }
-        
-        public string Server { get; set; }
+        public EndPoint RemoteEndpoint { get; set; }
 
         public MqttClientTlsOptions TlsOptions { get; set; } = new MqttClientTlsOptions();
 
         public override string ToString()
         {
-            return Server + ":" + this.GetPort();
+            return RemoteEndpoint?.ToString() ?? string.Empty;
         }
     }
 }

Source/MQTTnet/Client/Options/MqttClientTcpOptionsExtensions.cs

@@ -1,23 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-// See the LICENSE file in the project root for more information.
-
-using System;
-
-namespace MQTTnet.Client
-{
-    public static class MqttClientTcpOptionsExtensions
-    {
-        public static int GetPort(this MqttClientTcpOptions options)
-        {
-            if (options == null) throw new ArgumentNullException(nameof(options));
-
-            if (options.Port.HasValue)
-            {
-                return options.Port.Value;
-            }
-
-            return !(options.TlsOptions?.UseTls ?? false) ? 1883 : 8883;
-        }
-    }
-}

Source/MQTTnet/Client/Options/MqttClientTlsOptions.cs

@@ -50,8 +50,13 @@ namespace MQTTnet.Client
 
 #if NET48 || NETCOREAPP3_1_OR_GREATER
         public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12 | SslProtocols.Tls13;
+
 #else
         public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12 | (SslProtocols)0x00003000 /*Tls13*/;
 #endif
+
+#if NET7_0_OR_GREATER
+        public X509Certificate2Collection TrustChain { get; set; }
+#endif
     }
 }

Source/MQTTnet/Client/Options/MqttClientTlsOptionsBuilder.cs

@@ -131,6 +131,15 @@ namespace MQTTnet.Client
             _tlsOptions.EncryptionPolicy = encryptionPolicy;
             return this;
         }
+#endif
+
+#if NET7_0_OR_GREATER
+        public MqttClientTlsOptionsBuilder WithTrustChain(X509Certificate2Collection chain)
+        {
+            _tlsOptions.TrustChain = chain;
+            return this;
+        }
+
 #endif
     }
 }

Source/MQTTnet/Exceptions/MqttClientNotConnectedException.cs

@@ -0,0 +1,19 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace MQTTnet.Exceptions
+{
+    public class MqttClientNotConnectedException : MqttCommunicationException
+    {
+        public MqttClientNotConnectedException() : base("The MQTT client is not connected.")
+        {
+        }
+
+        public MqttClientNotConnectedException(Exception innerException) : base("The MQTT client is not connected.", innerException)
+        {
+        }
+    }
+}

Source/MQTTnet/Formatter/MqttDisconnectPacketFactory.cs

@@ -5,45 +5,107 @@
 using MQTTnet.Client;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using MQTTnet.Server;
+using MQTTnet.Server.Disconnecting;
 
 namespace MQTTnet.Formatter
 {
     public sealed class MqttDisconnectPacketFactory
     {
-        readonly MqttDisconnectPacket _normalDisconnection = new MqttDisconnectPacket
+        static readonly MqttDisconnectPacket DefaultNormalDisconnection = new MqttDisconnectPacket
         {
-            ReasonCode = MqttDisconnectReasonCode.NormalDisconnection
+            ReasonCode = MqttDisconnectReasonCode.NormalDisconnection,
+            UserProperties = null,
+            ReasonString = null,
+            ServerReference = null,
+            SessionExpiryInterval = 0
+        };
+
+        static readonly MqttDisconnectPacket DefaultServerShuttingDown = new MqttDisconnectPacket
+        {
+            ReasonCode = MqttDisconnectReasonCode.ServerShuttingDown,
+            UserProperties = null,
+            ReasonString = null,
+            ServerReference = null,
+            SessionExpiryInterval = 0
+        };
+
+        static readonly MqttDisconnectPacket DefaultUnspecifiedError = new MqttDisconnectPacket
+        {
+            ReasonCode = MqttDisconnectReasonCode.UnspecifiedError,
+            UserProperties = null,
+            ReasonString = null,
+            ServerReference = null,
+            SessionExpiryInterval = 0
         };
 
         public MqttDisconnectPacket Create(MqttDisconnectReasonCode reasonCode)
         {
             if (reasonCode == MqttDisconnectReasonCode.NormalDisconnection)
             {
-                return _normalDisconnection;
+                return DefaultNormalDisconnection;
+            }
+
+            if (reasonCode == MqttDisconnectReasonCode.ServerShuttingDown)
+            {
+                return DefaultServerShuttingDown;
+            }
+
+            if (reasonCode == MqttDisconnectReasonCode.UnspecifiedError)
+            {
+                return DefaultUnspecifiedError;
             }
 
             return new MqttDisconnectPacket
             {
-                ReasonCode = reasonCode
+                ReasonCode = reasonCode,
+                UserProperties = null,
+                ReasonString = null,
+                ServerReference = null,
+                SessionExpiryInterval = 0
+            };
+        }
+
+        public MqttDisconnectPacket Create(MqttServerStopOptions serverStopOptions)
+        {
+            if (serverStopOptions == null)
+            {
+                return DefaultServerShuttingDown;
+            }
+
+            return Create(serverStopOptions.DefaultClientDisconnectOptions);
+        }
+
+        public MqttDisconnectPacket Create(MqttServerClientDisconnectOptions clientDisconnectOptions)
+        {
+            if (clientDisconnectOptions == null)
+            {
+                return DefaultNormalDisconnection;
+            }
+
+            return new MqttDisconnectPacket
+            {
+                ReasonCode = clientDisconnectOptions.ReasonCode,
+                UserProperties = clientDisconnectOptions.UserProperties,
+                ReasonString = clientDisconnectOptions.ReasonString,
+                ServerReference = clientDisconnectOptions.ServerReference,
+                SessionExpiryInterval = 0 // TODO: Not yet supported!
             };
         }
 
         public MqttDisconnectPacket Create(MqttClientDisconnectOptions clientDisconnectOptions)
         {
-            var packet = new MqttDisconnectPacket();
-
             if (clientDisconnectOptions == null)
             {
-                packet.ReasonCode = MqttDisconnectReasonCode.NormalDisconnection;
-            }
-            else
-            {
-                packet.ReasonCode = (MqttDisconnectReasonCode)clientDisconnectOptions.Reason;
-                packet.UserProperties = clientDisconnectOptions.UserProperties;
-                packet.SessionExpiryInterval = clientDisconnectOptions.SessionExpiryInterval;
+                return DefaultNormalDisconnection;
             }
 
-            return packet;
+            return new MqttDisconnectPacket
+            {
+                ReasonCode = (MqttDisconnectReasonCode)clientDisconnectOptions.Reason,
+                UserProperties = clientDisconnectOptions.UserProperties,
+                SessionExpiryInterval = clientDisconnectOptions.SessionExpiryInterval
+            };
         }
     }
 }

Source/MQTTnet/Formatter/V5/MqttV5PacketEncoder.cs

@@ -279,7 +279,7 @@ namespace MQTTnet.Formatter.V5
             _propertiesWriter.WriteReasonString(packet.ReasonString);
             _propertiesWriter.WriteUserProperties(packet.UserProperties);
 
-            if (_bufferWriter.Length > 0 || packet.ReasonCode != MqttPubAckReasonCode.Success)
+            if (_propertiesWriter.Length > 0 || packet.ReasonCode != MqttPubAckReasonCode.Success)
             {
                 _bufferWriter.WriteByte((byte)packet.ReasonCode);
                 _propertiesWriter.WriteTo(_bufferWriter);
@@ -375,7 +375,7 @@ namespace MQTTnet.Formatter.V5
 
             _bufferWriter.WriteTwoByteInteger(packet.PacketIdentifier);
 
-            if (_bufferWriter.Length > 0 || packet.ReasonCode != MqttPubRecReasonCode.Success)
+            if (_propertiesWriter.Length > 0 || packet.ReasonCode != MqttPubRecReasonCode.Success)
             {
                 _bufferWriter.WriteByte((byte)packet.ReasonCode);
                 _propertiesWriter.WriteTo(_bufferWriter);
@@ -534,4 +534,4 @@ namespace MQTTnet.Formatter.V5
             }
         }
     }
-}
+}

Source/MQTTnet/Implementations/CrossPlatformSocket.cs

@@ -170,11 +170,16 @@ namespace MQTTnet.Implementations
             _socket.Bind(localEndPoint);
         }
 
-        public async Task ConnectAsync(string host, int port, CancellationToken cancellationToken)
+        public Task ConnectAsync(string host, int port, CancellationToken cancellationToken)
         {
-            if (host is null)
+            return ConnectAsync(new DnsEndPoint(host, port), cancellationToken);
+        }
+
+        public async Task ConnectAsync(EndPoint endPoint, CancellationToken cancellationToken)
+        {
+            if (endPoint is null)
             {
-                throw new ArgumentNullException(nameof(host));
+                throw new ArgumentNullException(nameof(endPoint));
             }
 
             cancellationToken.ThrowIfCancellationRequested();
@@ -191,15 +196,15 @@ namespace MQTTnet.Implementations
 #endif
 
 #if NET5_0_OR_GREATER
-                await _socket.ConnectAsync(host, port, cancellationToken).ConfigureAwait(false);
+                await _socket.ConnectAsync(endPoint, cancellationToken).ConfigureAwait(false);
 #else
                 // Workaround for: https://github.com/dotnet/corefx/issues/24430
                 using (cancellationToken.Register(_socketDisposeAction))
                 {
 #if NET452 || NET461
-                    await Task.Factory.FromAsync(_socket.BeginConnect, _socket.EndConnect, host, port, null).ConfigureAwait(false);
+                    await Task.Factory.FromAsync(_socket.BeginConnect, _socket.EndConnect, endPoint, null).ConfigureAwait(false);
 #else
-                    await _socket.ConnectAsync(host, port).ConfigureAwait(false);
+                    await _socket.ConnectAsync(endPoint).ConfigureAwait(false);
 #endif
                 }
 #endif
@@ -217,7 +222,7 @@ namespace MQTTnet.Implementations
                     throw new MqttCommunicationTimedOutException();
                 }
 
-                throw new MqttCommunicationException($"Error while connecting with host '{host}:{port}'.", socketException);
+                throw new MqttCommunicationException($"Error while connecting host '{endPoint}'.", socketException);
             }
             catch (ObjectDisposedException)
             {

Source/MQTTnet/Implementations/MqttTcpChannel.Uwp.cs

@@ -17,6 +17,7 @@ using MQTTnet.Channel;
 using MQTTnet.Client;
 using MQTTnet.Server;
 using System.Runtime.InteropServices.WindowsRuntime;
+using System.Net;
 
 namespace MQTTnet.Implementations
 {
@@ -65,9 +66,22 @@ namespace MQTTnet.Implementations
                 _socket.Control.KeepAlive = true;
             }
 
+            string hostName;
+            string serviceName;
+
+            if (_options.RemoteEndpoint is DnsEndPoint dns)
+            {
+                hostName = dns.Host;
+                serviceName = dns.Port.ToString();
+            }
+            else
+            {
+                throw new NotSupportedException("UWP only supports a DNS endpoint.");
+            }
+
             if (_options.TlsOptions?.UseTls != true)
             {
-                await _socket.ConnectAsync(new HostName(_options.Server), _options.GetPort().ToString()).AsTask().ConfigureAwait(false);
+                await _socket.ConnectAsync(new HostName(hostName), serviceName).AsTask().ConfigureAwait(false);
             }
             else
             {
@@ -88,7 +102,7 @@ namespace MQTTnet.Implementations
                     socketProtectionLevel = SocketProtectionLevel.Tls10;
                 }
 
-                await _socket.ConnectAsync(new HostName(_options.Server), _options.GetPort().ToString(), socketProtectionLevel).AsTask().ConfigureAwait(false);
+                await _socket.ConnectAsync(new HostName(hostName), serviceName, socketProtectionLevel).AsTask().ConfigureAwait(false);
             }
 
             Endpoint = _socket.Information.RemoteAddress + ":" + _socket.Information.RemotePort;

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -5,6 +5,7 @@
 #if !WINDOWS_UWP
 using System;
 using System.IO;
+using System.Net;
 using System.Net.Security;
 using System.Net.Sockets;
 using System.Runtime.ExceptionServices;
@@ -92,7 +93,7 @@ namespace MQTTnet.Implementations
                     socket.DualMode = _tcpOptions.DualMode.Value;
                 }
 
-                await socket.ConnectAsync(_tcpOptions.Server, _tcpOptions.GetPort(), cancellationToken).ConfigureAwait(false);
+                await socket.ConnectAsync(_tcpOptions.RemoteEndpoint, cancellationToken).ConfigureAwait(false);
 
                 cancellationToken.ThrowIfCancellationRequested();
 
@@ -103,9 +104,11 @@ namespace MQTTnet.Implementations
                     var targetHost = _tcpOptions.TlsOptions.TargetHost;
                     if (string.IsNullOrEmpty(targetHost))
                     {
-                        targetHost = _tcpOptions.Server;
+                        if (_tcpOptions.RemoteEndpoint is DnsEndPoint dns)
+                        {
+                            targetHost = dns.Host;    
+                        }
                     }
-
                     var sslStream = new SslStream(networkStream, false, InternalUserCertificateValidationCallback);
                     try
                     {
@@ -115,14 +118,31 @@ 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 = targetHost,
                             CipherSuitesPolicy = _tcpOptions.TlsOptions.CipherSuitesPolicy,
                             EncryptionPolicy = _tcpOptions.TlsOptions.EncryptionPolicy,
                             AllowRenegotiation = _tcpOptions.TlsOptions.AllowRenegotiation
                         };
 
+#if NET7_0_OR_GREATER
+                        if (_tcpOptions.TlsOptions.TrustChain?.Count > 0)
+                        {
+                            sslOptions.CertificateChainPolicy = new X509ChainPolicy
+                            {
+                                TrustMode = X509ChainTrustMode.CustomRootTrust,
+                                VerificationFlags = X509VerificationFlags.IgnoreEndRevocationUnknown,
+                                RevocationMode = _tcpOptions.TlsOptions.IgnoreCertificateRevocationErrors
+                                    ? X509RevocationMode.NoCheck
+                                    : _tcpOptions.TlsOptions.RevocationMode
+                            };
+
+                            sslOptions.CertificateChainPolicy.CustomTrustStore.AddRange(_tcpOptions.TlsOptions.TrustChain);
+                        }
+#endif
+
                         await sslStream.AuthenticateAsClientAsync(sslOptions, cancellationToken).ConfigureAwait(false);
 #else
                         await sslStream.AuthenticateAsClientAsync(

Source/MQTTnet/Internal/MqttPacketBus.cs

@@ -104,17 +104,22 @@ namespace MQTTnet.Internal
             _signal.Dispose();
         }
 
-        public void DropFirstItem(MqttPacketBusPartition partition)
+        public MqttPacketBusItem DropFirstItem(MqttPacketBusPartition partition)
         {
             lock (_syncRoot)
             {
                 var partitionInstance = _partitions[(int)partition];
 
-                if (partitionInstance.Any())
+                if (partitionInstance.Count > 0)
                 {
+                    var firstItem = partitionInstance.First.Value;
                     partitionInstance.RemoveFirst();
+
+                    return firstItem;
                 }
             }
+
+            return null;
         }
 
         public void EnqueueItem(MqttPacketBusItem item, MqttPacketBusPartition partition)

Source/MQTTnet/LowLevelClient/LowLevelMqttClient.cs

@@ -19,7 +19,6 @@ namespace MQTTnet.LowLevelClient
         readonly IMqttClientAdapterFactory _clientAdapterFactory;
         readonly AsyncEvent<InspectMqttPacketEventArgs> _inspectPacketEvent = new AsyncEvent<InspectMqttPacketEventArgs>();
         readonly MqttNetSourceLogger _logger;
-
         readonly IMqttNetLogger _rootLogger;
 
         IMqttChannelAdapter _adapter;

Source/MQTTnet/MQTTnet.csproj

@@ -23,6 +23,7 @@
         <Description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker) and supports v3.1.0, v3.1.1 and v5.0.0 of the MQTT protocol.</Description>
         <Authors>The contributors of MQTTnet</Authors>
         <PackageId>MQTTnet</PackageId>
+        <PackageReadmeFile>README.md</PackageReadmeFile>
         <DelaySign>false</DelaySign>
         <SignAssembly>false</SignAssembly>
         <PublishRepositoryUrl>true</PublishRepositoryUrl>
@@ -69,10 +70,8 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <None Include="..\..\Images\nuget.png">
-            <Pack>True</Pack>
-            <PackagePath>\</PackagePath>
-        </None>
+        <None Include="..\..\Images\nuget.png" Pack="True" PackagePath="\" />
+        <None Include="..\..\README.md" Pack="true" PackagePath="\" />
     </ItemGroup>
 
     <ItemGroup>

Source/MQTTnet/MQTTnet.csproj.DotSettings

@@ -17,4 +17,5 @@
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=server_005Cmanagement/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=server_005Coptions/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=server_005Cstatus/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=server_005Cstopping/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=server_005Cstorage/@EntryIndexedValue">True</s:Boolean></wpf:ResourceDictionary>

Source/MQTTnet/MqttFactory.cs

@@ -11,6 +11,7 @@ using MQTTnet.Diagnostics;
 using MQTTnet.Implementations;
 using MQTTnet.LowLevelClient;
 using MQTTnet.Server;
+using MQTTnet.Server.Disconnecting;
 using MqttClient = MQTTnet.Client.MqttClient;
 
 namespace MQTTnet
@@ -175,6 +176,16 @@ namespace MQTTnet
             return new MqttServer(options, serverAdapters, DefaultLogger);
         }
 
+        public MqttServerClientDisconnectOptionsBuilder CreateMqttServerClientDisconnectOptionsBuilder()
+        {
+            return new MqttServerClientDisconnectOptionsBuilder();
+        }
+
+        public MqttServerStopOptionsBuilder CreateMqttServerStopOptionsBuilder()
+        {
+            return new MqttServerStopOptionsBuilder();
+        }
+
         public MqttServerOptionsBuilder CreateServerOptionsBuilder()
         {
             return new MqttServerOptionsBuilder();

Source/MQTTnet/Protocol/MqttPorts.cs

@@ -0,0 +1,13 @@
+// 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.
+
+namespace MQTTnet.Protocol
+{
+    public static class MqttPorts
+    {
+        public const int Default = 1883;
+
+        public const int Secure = 8883;
+    }
+}

Source/MQTTnet/Server/Disconnecting/MqttServerClientDisconnectOptions.cs

@@ -0,0 +1,33 @@
+// 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 MQTTnet.Packets;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Server.Disconnecting
+{
+    public sealed class MqttServerClientDisconnectOptions
+    {
+        public MqttDisconnectReasonCode ReasonCode { get; set; } = MqttDisconnectReasonCode.NormalDisconnection;
+
+        /// <summary>
+        ///     The reason string is sent to every client via a DISCONNECT packet.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+        /// </summary>
+        public string ReasonString { get; set; }
+
+        /// <summary>
+        ///     The server reference is sent to every client via a DISCONNECT packet.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+        /// </summary>
+        public string ServerReference { get; set; }
+
+        /// <summary>
+        ///     These user properties are sent to every client via a DISCONNECT packet.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+        /// </summary>
+        public List<MqttUserProperty> UserProperties { get; set; }
+    }
+}

Source/MQTTnet/Server/Disconnecting/MqttServerClientDisconnectOptionsBuilder.cs

@@ -0,0 +1,55 @@
+// 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 MQTTnet.Packets;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Server.Disconnecting
+{
+    public sealed class MqttServerClientDisconnectOptionsBuilder
+    {
+        readonly MqttServerClientDisconnectOptions _options = new MqttServerClientDisconnectOptions();
+
+        public MqttServerClientDisconnectOptions Build()
+        {
+            return _options;
+        }
+
+        public MqttServerClientDisconnectOptionsBuilder WithReasonCode(MqttDisconnectReasonCode value)
+        {
+            _options.ReasonCode = value;
+            return this;
+        }
+
+        public MqttServerClientDisconnectOptionsBuilder WithReasonString(string value)
+        {
+            _options.ReasonString = value;
+            return this;
+        }
+
+        public MqttServerClientDisconnectOptionsBuilder WithServerReference(string value)
+        {
+            _options.ServerReference = value;
+            return this;
+        }
+
+        public MqttServerClientDisconnectOptionsBuilder WithUserProperties(List<MqttUserProperty> value)
+        {
+            _options.UserProperties = value;
+            return this;
+        }
+
+        public MqttServerClientDisconnectOptionsBuilder WithUserProperty(string name, string value)
+        {
+            if (_options.UserProperties == null)
+            {
+                _options.UserProperties = new List<MqttUserProperty>();
+            }
+
+            _options.UserProperties.Add(new MqttUserProperty(name, value));
+            return this;
+        }
+    }
+}

Source/MQTTnet/Server/Events/ApplicationMessageEnqueuedEventArgs.cs

@@ -0,0 +1,27 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace MQTTnet.Server
+{
+    public sealed class ApplicationMessageEnqueuedEventArgs : EventArgs
+    {
+        public ApplicationMessageEnqueuedEventArgs(string senderClientId, string receiverClientId, MqttApplicationMessage applicationMessage, bool isDropped)
+        {
+            SenderClientId = senderClientId ?? throw new ArgumentNullException( nameof(senderClientId));
+            ReceiverClientId = receiverClientId ?? throw new ArgumentNullException(nameof(receiverClientId));
+            ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
+            IsDropped = isDropped;
+        }
+
+        public string SenderClientId { get; }
+
+        public string ReceiverClientId { get; }
+
+        public bool IsDropped { get; }
+
+        public MqttApplicationMessage ApplicationMessage { get; }
+    }
+}

Source/MQTTnet/Server/Events/QueueMessageOverwrittenEventArgs.cs

@@ -0,0 +1,22 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using MQTTnet.Packets;
+
+namespace MQTTnet.Server
+{
+    public sealed class QueueMessageOverwrittenEventArgs : EventArgs
+    {
+        public QueueMessageOverwrittenEventArgs(string receiverClientId, MqttPacket packet)
+        {
+            ReceiverClientId = receiverClientId ?? throw new ArgumentNullException(nameof(receiverClientId));
+            Packet = packet ?? throw new ArgumentNullException(nameof(packet));
+        }
+
+        public MqttPacket Packet { get; }
+
+        public string ReceiverClientId { get; }
+    }
+}

Source/MQTTnet/Server/Internal/EnqueueDataPacketResult.cs

@@ -0,0 +1,12 @@
+// 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.
+
+namespace MQTTnet.Server
+{
+    public enum EnqueueDataPacketResult
+    {
+        Enqueued,
+        Dropped
+    }
+}

Source/MQTTnet/Server/Internal/MqttClient.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
@@ -14,6 +15,7 @@ using MQTTnet.Formatter;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using MQTTnet.Server.Disconnecting;
 
 namespace MQTTnet.Server
 {
@@ -95,15 +97,16 @@ namespace MQTTnet.Server
             {
                 var cancellationToken = _cancellationToken.Token;
                 IsRunning = true;
-                
-                _ = Task.Factory.StartNew(() => SendPacketsLoop(cancellationToken), cancellationToken, TaskCreationOptions.PreferFairness, TaskScheduler.Default).ConfigureAwait(false);
+
+                _ = Task.Factory.StartNew(() => SendPacketsLoop(cancellationToken), cancellationToken, TaskCreationOptions.PreferFairness, TaskScheduler.Default)
+                    .ConfigureAwait(false);
 
                 await ReceivePackagesLoop(cancellationToken).ConfigureAwait(false);
             }
             finally
             {
                 IsRunning = false;
-                
+
                 Session.DisconnectedTimestamp = DateTime.UtcNow;
 
                 _cancellationToken?.TryCancel();
@@ -141,7 +144,7 @@ namespace MQTTnet.Server
             Statistics.HandleSentPacket(packet);
         }
 
-        public async Task StopAsync(MqttDisconnectReasonCode reason)
+        public async Task StopAsync(MqttServerClientDisconnectOptions disconnectOptions)
         {
             IsRunning = false;
 
@@ -150,16 +153,20 @@ namespace MQTTnet.Server
                 // Sending DISCONNECT packets from the server to the client is only supported when using MQTTv5+.
                 if (ChannelAdapter.PacketFormatterAdapter.ProtocolVersion == MqttProtocolVersion.V500)
                 {
-                    // The Client or Server MAY send a DISCONNECT packet before closing the Network Connection.
-                    // This library does not sent a DISCONNECT packet for a normal disconnection. Maybe adding
-                    // a configuration option is requested in the future.
-                    if (reason != MqttDisconnectReasonCode.NormalDisconnection)
+                    // From RFC: The Client or Server MAY send a DISCONNECT packet before closing the Network Connection.
+                    // This library does not sent a DISCONNECT packet for a normal disconnection.
+                    // TODO: Maybe adding a configuration option is requested in the future.
+                    if (disconnectOptions != null)
                     {
-                        // Is is very important to send the DISCONNECT packet here BEFORE cancelling the
-                        // token because the entire connection is closed (disposed) as soon as the cancellation
-                        // token is cancelled. To there is no chance that the DISCONNECT packet will ever arrive
-                        // at the client!
-                        await TrySendDisconnectPacket(reason).ConfigureAwait(false);
+                        if (disconnectOptions.ReasonCode != MqttDisconnectReasonCode.NormalDisconnection || disconnectOptions.UserProperties?.Any() == true ||
+                            !string.IsNullOrEmpty(disconnectOptions.ReasonString) || !string.IsNullOrEmpty(disconnectOptions.ServerReference))
+                        {
+                            // Is is very important to send the DISCONNECT packet here BEFORE cancelling the
+                            // token because the entire connection is closed (disposed) as soon as the cancellation
+                            // token is cancelled. To there is no chance that the DISCONNECT packet will ever arrive
+                            // at the client!
+                            await TrySendDisconnectPacket(disconnectOptions).ConfigureAwait(false);
+                        }
                     }
                 }
             }
@@ -219,7 +226,7 @@ namespace MQTTnet.Server
 
             if (dispatchApplicationMessageResult.CloseConnection)
             {
-                await StopAsync(MqttDisconnectReasonCode.UnspecifiedError);
+                await StopAsync(new MqttServerClientDisconnectOptions { ReasonCode = MqttDisconnectReasonCode.UnspecifiedError });
                 return;
             }
 
@@ -550,14 +557,14 @@ namespace MQTTnet.Server
             _cancellationToken?.TryCancel();
         }
 
-        async Task TrySendDisconnectPacket(MqttDisconnectReasonCode reasonCode)
+        async Task TrySendDisconnectPacket(MqttServerClientDisconnectOptions options)
         {
             try
             {
                 // This also indicates that it was tried at least!
                 _disconnectPacketSent = true;
 
-                var disconnectPacket = MqttPacketFactories.Disconnect.Create(reasonCode);
+                var disconnectPacket = MqttPacketFactories.Disconnect.Create(options);
 
                 using (var timeout = new CancellationTokenSource(_serverOptions.DefaultCommunicationTimeout))
                 {
@@ -566,7 +573,7 @@ namespace MQTTnet.Server
             }
             catch (Exception exception)
             {
-                _logger.Warning(exception, "Client '{0}': Error while sending DISCONNECT packet (ReasonCode = {1})", Id, reasonCode);
+                _logger.Warning(exception, "Client '{0}': Error while sending DISCONNECT packet (ReasonCode = {1})", Id, options.ReasonCode);
             }
         }
     }

Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs

@@ -16,6 +16,7 @@ using MQTTnet.Formatter;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using MQTTnet.Server.Disconnecting;
 
 namespace MQTTnet.Server
 {
@@ -32,11 +33,11 @@ namespace MQTTnet.Server
         readonly MqttRetainedMessagesManager _retainedMessagesManager;
         readonly IMqttNetLogger _rootLogger;
 
+        readonly ReaderWriterLockSlim _sessionsManagementLock = new ReaderWriterLockSlim();
+
         // The _sessions dictionary contains all session, the _subscriberSessions hash set contains subscriber sessions only.
         // See the MqttSubscription object for a detailed explanation.
         readonly MqttSessionsStorage _sessionsStorage = new MqttSessionsStorage();
-
-        readonly ReaderWriterLockSlim _sessionsManagementLock = new ReaderWriterLockSlim();
         readonly HashSet<MqttSession> _subscriberSessions = new HashSet<MqttSession>();
 
         public MqttClientSessionsManager(
@@ -58,8 +59,13 @@ namespace MQTTnet.Server
             _eventContainer = eventContainer ?? throw new ArgumentNullException(nameof(eventContainer));
         }
 
-        public async Task CloseAllConnections()
+        public async Task CloseAllConnections(MqttServerClientDisconnectOptions options)
         {
+            if (options == null)
+            {
+                throw new ArgumentNullException(nameof(options));
+            }
+
             List<MqttClient> connections;
             lock (_clients)
             {
@@ -69,7 +75,7 @@ namespace MQTTnet.Server
 
             foreach (var connection in connections)
             {
-                await connection.StopAsync(MqttDisconnectReasonCode.NormalDisconnection).ConfigureAwait(false);
+                await connection.StopAsync(options).ConfigureAwait(false);
             }
         }
 
@@ -101,7 +107,7 @@ namespace MQTTnet.Server
             {
                 if (connection != null)
                 {
-                    await connection.StopAsync(MqttDisconnectReasonCode.NormalDisconnection).ConfigureAwait(false);
+                    await connection.StopAsync(new MqttServerClientDisconnectOptions { ReasonCode = MqttDisconnectReasonCode.NormalDisconnection }).ConfigureAwait(false);
                 }
             }
             catch (Exception exception)
@@ -204,7 +210,7 @@ namespace MQTTnet.Server
                         {
                             continue;
                         }
-                        
+
                         if (_eventContainer.InterceptingClientEnqueueEvent.HasHandlers)
                         {
                             var eventArgs = new InterceptingClientApplicationMessageEnqueueEventArgs(senderId, session.Id, applicationMessage);
@@ -238,7 +244,14 @@ namespace MQTTnet.Server
 
                         matchingSubscribersCount++;
 
-                        session.EnqueueDataPacket(new MqttPacketBusItem(publishPacketCopy));
+                        var result = session.EnqueueDataPacket(new MqttPacketBusItem(publishPacketCopy));
+
+                        if (_eventContainer.ApplicationMessageEnqueuedOrDroppedEvent.HasHandlers)
+                        {
+                            var eventArgs = new ApplicationMessageEnqueuedEventArgs(senderId, session.Id, applicationMessage, result == EnqueueDataPacketResult.Dropped);
+                            await _eventContainer.ApplicationMessageEnqueuedOrDroppedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
+                        }
+
                         _logger.Verbose("Client '{0}': Queued PUBLISH packet with topic '{1}'", session.Id, applicationMessage.Topic);
                     }
 
@@ -366,7 +379,8 @@ namespace MQTTnet.Server
 
                 if (_eventContainer.ClientConnectedEvent.HasHandlers)
                 {
-                    var eventArgs = new ClientConnectedEventArgs(connectPacket,
+                    var eventArgs = new ClientConnectedEventArgs(
+                        connectPacket,
                         channelAdapter.PacketFormatterAdapter.ProtocolVersion,
                         channelAdapter.Endpoint,
                         client.Session.Items);
@@ -595,13 +609,13 @@ namespace MQTTnet.Server
                             session.IsPersistent = sessionShouldPersist;
                             session.DisconnectedTimestamp = null;
                             session.Recover();
-                            
+
                             connAckPacket.IsSessionPresent = true;
                         }
                     }
 
                     _sessionsStorage.UpdateSession(connectPacket.ClientId, session);
-                    
+
                     // Create a new client (always required).
                     lock (_clients)
                     {
@@ -631,7 +645,8 @@ namespace MQTTnet.Server
 
                 if (oldClient != null)
                 {
-                    await oldClient.StopAsync(MqttDisconnectReasonCode.SessionTakenOver).ConfigureAwait(false);
+                    // TODO: Consider event here for session takeover to allow manipulation of user properties etc.
+                    await oldClient.StopAsync(new MqttServerClientDisconnectOptions { ReasonCode = MqttDisconnectReasonCode.SessionTakenOver }).ConfigureAwait(false);
 
                     if (_eventContainer.ClientDisconnectedEvent.HasHandlers)
                     {
@@ -734,4 +749,4 @@ namespace MQTTnet.Server
             return eventArgs;
         }
     }
-}
+}

Source/MQTTnet/Server/Internal/MqttClientStatistics.cs

@@ -92,7 +92,7 @@ namespace MQTTnet.Server
             }
         }
 
-        private struct Statistics
+        struct Statistics
         {
             public long _receivedPacketsCount;
             public long _sentPacketsCount;

Source/MQTTnet/Server/Internal/MqttServerEventContainer.cs

@@ -22,6 +22,10 @@ namespace MQTTnet.Server
         public AsyncEvent<ClientUnsubscribedTopicEventArgs> ClientUnsubscribedTopicEvent { get; } = new AsyncEvent<ClientUnsubscribedTopicEventArgs>();
 
         public AsyncEvent<InterceptingClientApplicationMessageEnqueueEventArgs> InterceptingClientEnqueueEvent { get; } = new AsyncEvent<InterceptingClientApplicationMessageEnqueueEventArgs>();
+        
+        public AsyncEvent<ApplicationMessageEnqueuedEventArgs> ApplicationMessageEnqueuedOrDroppedEvent { get; } = new AsyncEvent<ApplicationMessageEnqueuedEventArgs>();
+
+        public AsyncEvent<QueueMessageOverwrittenEventArgs> QueuedApplicationMessageOverwrittenEvent { get; } = new AsyncEvent<QueueMessageOverwrittenEventArgs>();
 
         public AsyncEvent<InterceptingPacketEventArgs> InterceptingInboundPacketEvent { get; } = new AsyncEvent<InterceptingPacketEventArgs>();
 

Source/MQTTnet/Server/Internal/MqttServerKeepAliveMonitor.cs

@@ -8,11 +8,20 @@ using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
 using MQTTnet.Internal;
 using MQTTnet.Protocol;
+using MQTTnet.Server.Disconnecting;
 
 namespace MQTTnet.Server
 {
     public sealed class MqttServerKeepAliveMonitor
     {
+        static readonly MqttServerClientDisconnectOptions DefaultDisconnectOptions = new MqttServerClientDisconnectOptions
+        {
+            ReasonCode = MqttDisconnectReasonCode.KeepAliveTimeout,
+            UserProperties = null,
+            ReasonString = null,
+            ServerReference = null
+        };
+
         readonly MqttNetSourceLogger _logger;
         readonly MqttServerOptions _options;
         readonly MqttClientSessionsManager _sessionsManager;
@@ -43,12 +52,12 @@ namespace MQTTnet.Server
         {
             try
             {
-                _logger.Info("Starting keep alive monitor.");
+                _logger.Info("Starting keep alive monitor");
 
                 while (!cancellationToken.IsCancellationRequested)
                 {
                     TryProcessClients();
-                    Sleep(_options.KeepAliveMonitorInterval);
+                    Sleep(_options.KeepAliveOptions.MonitorInterval);
                 }
             }
             catch (OperationCanceledException)
@@ -56,11 +65,11 @@ namespace MQTTnet.Server
             }
             catch (Exception exception)
             {
-                _logger.Error(exception, "Unhandled exception while checking keep alive timeouts.");
+                _logger.Error(exception, "Unhandled exception while checking keep alive timeouts");
             }
             finally
             {
-                _logger.Verbose("Stopped checking keep alive timeout.");
+                _logger.Verbose("Stopped checking keep alive timeout");
             }
         }
 
@@ -98,13 +107,6 @@ namespace MQTTnet.Server
                     return;
                 }
 
-                if (connection.ChannelAdapter.IsReadingPacket)
-                {
-                    // The connection is currently reading a (large) packet. So it is obviously 
-                    // doing something and thus "connected".
-                    return;
-                }
-
                 // Values described here: [MQTT-3.1.2-24].
                 // If the client sends 5 sec. the server will allow up to 7.5 seconds.
                 // If the client sends 1 sec. the server will allow up to 1.5 seconds.
@@ -117,18 +119,18 @@ namespace MQTTnet.Server
                     return;
                 }
 
-                _logger.Warning("Client '{0}': Did not receive any packet or keep alive signal.", connection.Id);
+                _logger.Warning("Client '{0}': Did not receive any packet or keep alive signal", connection.Id);
 
                 // Execute the disconnection in background so that the keep alive monitor can continue
                 // with checking other connections.
                 // We do not need to wait for the task so no await is needed.
                 // Also the internal state of the connection must be swapped to "Finalizing" because the
                 // next iteration of the keep alive timer happens.
-                var _ = connection.StopAsync(MqttDisconnectReasonCode.KeepAliveTimeout);
+                _ = connection.StopAsync(DefaultDisconnectOptions);
             }
             catch (Exception exception)
             {
-                _logger.Error(exception, "Client {0}: Unhandled exception while checking keep alive timeouts.", connection.Id);
+                _logger.Error(exception, "Client {0}: Unhandled exception while checking keep alive timeouts", connection.Id);
             }
         }
 

Source/MQTTnet/Server/Internal/MqttSession.cs

@@ -18,6 +18,7 @@ namespace MQTTnet.Server
     public sealed class MqttSession : IDisposable
     {
         readonly MqttClientSessionsManager _clientSessionsManager;
+        readonly MqttServerEventContainer _eventContainer;
         readonly MqttPacketBus _packetBus = new MqttPacketBus();
         readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new MqttPacketIdentifierProvider();
 
@@ -44,6 +45,7 @@ namespace MQTTnet.Server
             _connectPacket = connectPacket ?? throw new ArgumentNullException(nameof(connectPacket));
             _serverOptions = serverOptions ?? throw new ArgumentNullException(nameof(serverOptions));
             _clientSessionsManager = clientSessionsManager ?? throw new ArgumentNullException(nameof(clientSessionsManager));
+            _eventContainer = eventContainer ?? throw new ArgumentNullException(nameof(eventContainer));
 
             _subscriptionsManager = new MqttClientSubscriptionsManager(this, eventContainer, retainedMessagesManager, clientSessionsManager);
         }
@@ -117,20 +119,25 @@ namespace MQTTnet.Server
             _packetBus.EnqueueItem(packetBusItem, MqttPacketBusPartition.Control);
         }
 
-        public void EnqueueDataPacket(MqttPacketBusItem packetBusItem)
+        public EnqueueDataPacketResult EnqueueDataPacket(MqttPacketBusItem packetBusItem)
         {
             if (_packetBus.ItemsCount(MqttPacketBusPartition.Data) >= _serverOptions.MaxPendingMessagesPerClient)
             {
                 if (_serverOptions.PendingMessagesOverflowStrategy == MqttPendingMessagesOverflowStrategy.DropNewMessage)
                 {
-                    return;
+                    return EnqueueDataPacketResult.Dropped;
                 }
 
                 if (_serverOptions.PendingMessagesOverflowStrategy == MqttPendingMessagesOverflowStrategy.DropOldestQueuedMessage)
                 {
                     // Only drop from the data partition. Dropping from control partition might break the connection
                     // because the client does not receive PINGREQ packets etc. any longer.
-                    _packetBus.DropFirstItem(MqttPacketBusPartition.Data);
+                    var firstItem = _packetBus.DropFirstItem(MqttPacketBusPartition.Data);
+                    if (firstItem != null && _eventContainer.QueuedApplicationMessageOverwrittenEvent.HasHandlers)
+                    {
+                        var eventArgs = new QueueMessageOverwrittenEventArgs(Id, firstItem.Packet);
+                        _eventContainer.QueuedApplicationMessageOverwrittenEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
+                    }
                 }
             }
 
@@ -147,6 +154,7 @@ namespace MQTTnet.Server
             }
 
             _packetBus.EnqueueItem(packetBusItem, MqttPacketBusPartition.Data);
+            return EnqueueDataPacketResult.Enqueued;
         }
 
         public void EnqueueHealthPacket(MqttPacketBusItem packetBusItem)

Source/MQTTnet/Server/MqttServer.cs

@@ -14,6 +14,7 @@ using MQTTnet.Diagnostics;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using MQTTnet.Server.Disconnecting;
 
 namespace MQTTnet.Server
 {
@@ -29,6 +30,7 @@ namespace MQTTnet.Server
         readonly IMqttNetLogger _rootLogger;
 
         CancellationTokenSource _cancellationTokenSource;
+        bool _isStopping;
 
         public MqttServer(MqttServerOptions options, IEnumerable<IMqttServerAdapter> adapters, IMqttNetLogger logger)
         {
@@ -49,6 +51,12 @@ namespace MQTTnet.Server
             _keepAliveMonitor = new MqttServerKeepAliveMonitor(options, _clientSessionsManager, _rootLogger);
         }
 
+        public event Func<ApplicationMessageEnqueuedEventArgs, Task> ApplicationMessageEnqueuedOrDroppedAsync
+        {
+            add => _eventContainer.ApplicationMessageEnqueuedOrDroppedEvent.AddHandler(value);
+            remove => _eventContainer.ApplicationMessageEnqueuedOrDroppedEvent.RemoveHandler(value);
+        }
+
         public event Func<ApplicationMessageNotConsumedEventArgs, Task> ApplicationMessageNotConsumedAsync
         {
             add => _eventContainer.ApplicationMessageNotConsumedEvent.AddHandler(value);
@@ -133,6 +141,12 @@ namespace MQTTnet.Server
             remove => _eventContainer.PreparingSessionEvent.RemoveHandler(value);
         }
 
+        public event Func<QueueMessageOverwrittenEventArgs, Task> QueuedApplicationMessageOverwrittenAsync
+        {
+            add => _eventContainer.QueuedApplicationMessageOverwrittenEvent.AddHandler(value);
+            remove => _eventContainer.QueuedApplicationMessageOverwrittenEvent.RemoveHandler(value);
+        }
+
         public event Func<RetainedMessageChangedEventArgs, Task> RetainedMessageChangedAsync
         {
             add => _eventContainer.RetainedMessageChangedEvent.AddHandler(value);
@@ -169,6 +183,13 @@ namespace MQTTnet.Server
             remove => _eventContainer.ValidatingConnectionEvent.RemoveHandler(value);
         }
 
+        /// <summary>
+        ///     Gets or sets whether the server will accept new connections.
+        ///     If not, the server will close the connection without any notification (DISCONNECT packet).
+        ///     This feature can be used when the server is shutting down.
+        /// </summary>
+        public bool AcceptNewConnections { get; set; } = true;
+
         public bool IsStarted => _cancellationTokenSource != null;
 
         /// <summary>
@@ -184,16 +205,21 @@ namespace MQTTnet.Server
             return _retainedMessagesManager?.ClearMessages() ?? CompletedTask.Instance;
         }
 
-        public Task DisconnectClientAsync(string id, MqttDisconnectReasonCode reasonCode)
+        public Task DisconnectClientAsync(string id, MqttServerClientDisconnectOptions options)
         {
             if (id == null)
             {
                 throw new ArgumentNullException(nameof(id));
             }
 
+            if (options == null)
+            {
+                throw new ArgumentNullException(nameof(options));
+            }
+
             ThrowIfNotStarted();
 
-            return _clientSessionsManager.GetClient(id).StopAsync(reasonCode);
+            return _clientSessionsManager.GetClient(id).StopAsync(options);
         }
 
         public Task<IList<MqttClientStatus>> GetClientsAsync()
@@ -203,13 +229,6 @@ namespace MQTTnet.Server
             return _clientSessionsManager.GetClientsStatus();
         }
 
-        public Task<IList<MqttApplicationMessage>> GetRetainedMessagesAsync()
-        {
-            ThrowIfNotStarted();
-
-            return _retainedMessagesManager.GetMessages();
-        }
-
         public Task<MqttApplicationMessage> GetRetainedMessageAsync(string topic)
         {
             if (topic == null)
@@ -222,6 +241,13 @@ namespace MQTTnet.Server
             return _retainedMessagesManager.GetMessage(topic);
         }
 
+        public Task<IList<MqttApplicationMessage>> GetRetainedMessagesAsync()
+        {
+            ThrowIfNotStarted();
+
+            return _retainedMessagesManager.GetMessages();
+        }
+
         public Task<IList<MqttSessionStatus>> GetSessionsAsync()
         {
             ThrowIfNotStarted();
@@ -247,7 +273,7 @@ namespace MQTTnet.Server
 
             if (string.IsNullOrEmpty(injectedApplicationMessage.ApplicationMessage.Topic))
             {
-                throw new NotSupportedException("Injected application messages must contain a topic. Topic alias is not supported.");
+                throw new NotSupportedException("Injected application messages must contain a topic (topic alias is not supported)");
             }
 
             var sessionItems = injectedApplicationMessage.CustomSessionItems ?? ServerSessionItems;
@@ -263,6 +289,8 @@ namespace MQTTnet.Server
         {
             ThrowIfStarted();
 
+            _isStopping = false;
+            
             _cancellationTokenSource = new CancellationTokenSource();
             var cancellationToken = _cancellationTokenSource.Token;
 
@@ -278,11 +306,16 @@ namespace MQTTnet.Server
 
             await _eventContainer.StartedEvent.InvokeAsync(EventArgs.Empty).ConfigureAwait(false);
 
-            _logger.Info("Started.");
+            _logger.Info("Started");
         }
 
-        public async Task StopAsync()
+        public async Task StopAsync(MqttServerStopOptions options)
         {
+            if (options == null)
+            {
+                throw new ArgumentNullException(nameof(options));
+            }
+
             try
             {
                 if (_cancellationTokenSource == null)
@@ -290,9 +323,11 @@ namespace MQTTnet.Server
                     return;
                 }
 
+                _isStopping = true;
+
                 _cancellationTokenSource.Cancel(false);
 
-                await _clientSessionsManager.CloseAllConnections().ConfigureAwait(false);
+                await _clientSessionsManager.CloseAllConnections(options.DefaultClientDisconnectOptions).ConfigureAwait(false);
 
                 foreach (var adapter in _adapters)
                 {
@@ -308,7 +343,7 @@ namespace MQTTnet.Server
 
             await _eventContainer.StoppedEvent.InvokeAsync(EventArgs.Empty).ConfigureAwait(false);
 
-            _logger.Info("Stopped.");
+            _logger.Info("Stopped");
         }
 
         public Task SubscribeAsync(string clientId, ICollection<MqttTopicFilter> topicFilters)
@@ -362,14 +397,14 @@ namespace MQTTnet.Server
             ThrowIfDisposed();
             ThrowIfNotStarted();
 
-            return _retainedMessagesManager?.UpdateMessage(null, retainedMessage);
+            return _retainedMessagesManager?.UpdateMessage(string.Empty, retainedMessage);
         }
 
         protected override void Dispose(bool disposing)
         {
             if (disposing)
             {
-                StopAsync().GetAwaiter().GetResult();
+                StopAsync(new MqttServerStopOptions()).GetAwaiter().GetResult();
 
                 foreach (var adapter in _adapters)
                 {
@@ -382,6 +417,11 @@ namespace MQTTnet.Server
 
         Task OnHandleClient(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
         {
+            if (_isStopping || !AcceptNewConnections)
+            {
+                return CompletedTask.Instance;
+            }
+
             return _clientSessionsManager.HandleClientConnectionAsync(channelAdapter, cancellationToken);
         }
 

Source/MQTTnet/Server/MqttServerExtensions.cs

@@ -8,11 +8,22 @@ using System.Threading.Tasks;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using MQTTnet.Server.Disconnecting;
 
 namespace MQTTnet.Server
 {
     public static class MqttServerExtensions
     {
+        public static Task DisconnectClientAsync(this MqttServer server, string id, MqttDisconnectReasonCode reasonCode = MqttDisconnectReasonCode.NormalDisconnection)
+        {
+            if (server == null)
+            {
+                throw new ArgumentNullException(nameof(server));
+            }
+
+            return server.DisconnectClientAsync(id, new MqttServerClientDisconnectOptions { ReasonCode = reasonCode });
+        }
+
         public static Task InjectApplicationMessage(
             this MqttServer server,
             string topic,
@@ -47,6 +58,16 @@ namespace MQTTnet.Server
                     }));
         }
 
+        public static Task StopAsync(this MqttServer server)
+        {
+            if (server == null)
+            {
+                throw new ArgumentNullException(nameof(server));
+            }
+
+            return server.StopAsync(new MqttServerStopOptions());
+        }
+
         public static Task SubscribeAsync(this MqttServer server, string clientId, params MqttTopicFilter[] topicFilters)
         {
             if (server == null)

Source/MQTTnet/Server/Options/MqttServerKeepAliveOptions.cs

@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace MQTTnet.Server
+{
+    public sealed class MqttServerKeepAliveOptions
+    {
+        /// <summary>
+        ///     When this mode is enabled the MQTT server will not close a connection when the
+        ///     client is currently sending a (large) payload. This may lead to "dead" connections
+        ///     When this mode is disabled the MQTT server will disconnect a client when the keep
+        ///     alive timeout is reached even if the client is currently sending a (large) payload.
+        /// </summary>
+        public bool DisconnectClientWhenReadingPayload { get; set; }
+
+        public TimeSpan MonitorInterval { get; set; } = TimeSpan.FromMilliseconds(500);
+    }
+}

Source/MQTTnet/Server/Options/MqttServerOptions.cs

@@ -9,12 +9,19 @@ namespace MQTTnet.Server
     public sealed class MqttServerOptions
     {
         public TimeSpan DefaultCommunicationTimeout { get; set; } = TimeSpan.FromSeconds(100);
-        
+
         public MqttServerTcpEndpointOptions DefaultEndpointOptions { get; } = new MqttServerTcpEndpointOptions();
 
         public bool EnablePersistentSessions { get; set; }
 
-        public TimeSpan KeepAliveMonitorInterval { get; set; } = TimeSpan.FromMilliseconds(500);
+        [Obsolete("Use KeepAliveOptions instead.")]
+        public TimeSpan KeepAliveMonitorInterval
+        {
+            get => KeepAliveOptions.MonitorInterval;
+            set => KeepAliveOptions.MonitorInterval = value;
+        }
+
+        public MqttServerKeepAliveOptions KeepAliveOptions { get; } = new MqttServerKeepAliveOptions();
 
         public int MaxPendingMessagesPerClient { get; set; } = 250;
 

Source/MQTTnet/Server/Status/MqttClientStatus.cs

@@ -5,7 +5,7 @@
 using System;
 using System.Threading.Tasks;
 using MQTTnet.Formatter;
-using MQTTnet.Protocol;
+using MQTTnet.Server.Disconnecting;
 
 namespace MQTTnet.Server
 {
@@ -18,41 +18,46 @@ namespace MQTTnet.Server
             _client = client ?? throw new ArgumentNullException(nameof(client));
         }
 
-        /// <summary>
-        /// Gets or sets 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 Id => _client.Id;
+        public long BytesReceived => _client.ChannelAdapter.BytesReceived;
+
+        public long BytesSent => _client.ChannelAdapter.BytesSent;
+
+        public DateTime ConnectedTimestamp => _client.Statistics.ConnectedTimestamp;
 
         public string Endpoint => _client.Endpoint;
 
-        public MqttProtocolVersion ProtocolVersion => _client.ChannelAdapter.PacketFormatterAdapter.ProtocolVersion;
+        /// <summary>
+        ///     Gets or sets 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 Id => _client.Id;
 
-        public DateTime ConnectedTimestamp => _client.Statistics.ConnectedTimestamp;
+        public DateTime LastNonKeepAlivePacketReceivedTimestamp => _client.Statistics.LastNonKeepAlivePacketReceivedTimestamp;
 
         public DateTime LastPacketReceivedTimestamp => _client.Statistics.LastPacketReceivedTimestamp;
 
         public DateTime LastPacketSentTimestamp => _client.Statistics.LastPacketSentTimestamp;
 
-        public DateTime LastNonKeepAlivePacketReceivedTimestamp => _client.Statistics.LastNonKeepAlivePacketReceivedTimestamp;
+        public MqttProtocolVersion ProtocolVersion => _client.ChannelAdapter.PacketFormatterAdapter.ProtocolVersion;
 
         public long ReceivedApplicationMessagesCount => _client.Statistics.ReceivedApplicationMessagesCount;
 
-        public long SentApplicationMessagesCount => _client.Statistics.SentApplicationMessagesCount;
-
         public long ReceivedPacketsCount => _client.Statistics.ReceivedPacketsCount;
 
+        public long SentApplicationMessagesCount => _client.Statistics.SentApplicationMessagesCount;
+
         public long SentPacketsCount => _client.Statistics.SentPacketsCount;
 
         public MqttSessionStatus Session { get; set; }
 
-        public long BytesSent => _client.ChannelAdapter.BytesSent;
-
-        public long BytesReceived => _client.ChannelAdapter.BytesReceived;
-        
-        public Task DisconnectAsync()
+        public Task DisconnectAsync(MqttServerClientDisconnectOptions options)
         {
-            return _client.StopAsync(MqttDisconnectReasonCode.NormalDisconnection);
+            if (options == null)
+            {
+                throw new ArgumentNullException(nameof(options));
+            }
+
+            return _client.StopAsync(options);
         }
 
         public void ResetStatistics()
@@ -60,4 +65,4 @@ namespace MQTTnet.Server
             _client.ResetStatistics();
         }
     }
-}
+}

Source/MQTTnet/Server/Status/MqttClientStatusExtensions.cs

@@ -0,0 +1,32 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Threading.Tasks;
+using MQTTnet.Protocol;
+using MQTTnet.Server.Disconnecting;
+
+namespace MQTTnet.Server
+{
+    public static class MqttClientStatusExtensions
+    {
+        static readonly MqttServerClientDisconnectOptions DefaultDisconnectOptions = new MqttServerClientDisconnectOptions
+        {
+            ReasonCode = MqttDisconnectReasonCode.NormalDisconnection,
+            ReasonString = null,
+            UserProperties = null,
+            ServerReference = null
+        };
+
+        public static Task DisconnectAsync(this MqttClientStatus clientStatus)
+        {
+            if (clientStatus == null)
+            {
+                throw new ArgumentNullException(nameof(clientStatus));
+            }
+
+            return clientStatus.DisconnectAsync(DefaultDisconnectOptions);
+        }
+    }
+}

Source/MQTTnet/Server/Stopping/MqttServerStopOptions.cs

@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using MQTTnet.Protocol;
+using MQTTnet.Server.Disconnecting;
+
+namespace MQTTnet.Server
+{
+    public sealed class MqttServerStopOptions
+    {
+        /// <summary>
+        ///     These disconnect options are sent to every connected client via a DISCONNECT packet.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+        /// </summary>
+        public MqttServerClientDisconnectOptions DefaultClientDisconnectOptions { get; set; } = new MqttServerClientDisconnectOptions
+        {
+            ReasonCode = MqttDisconnectReasonCode.ServerShuttingDown,
+            UserProperties = null,
+            ReasonString = null,
+            ServerReference = null
+        };
+    }
+}

Source/MQTTnet/Server/Stopping/MqttServerStopOptionsBuilder.cs

@@ -0,0 +1,39 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using MQTTnet.Server.Disconnecting;
+
+namespace MQTTnet.Server
+{
+    public sealed class MqttServerStopOptionsBuilder
+    {
+        readonly MqttServerStopOptions _options = new MqttServerStopOptions();
+
+        public MqttServerStopOptionsBuilder WithDefaultClientDisconnectOptions(MqttServerClientDisconnectOptions value)
+        {
+            _options.DefaultClientDisconnectOptions = value;
+            return this;
+        }
+        
+        public MqttServerStopOptionsBuilder WithDefaultClientDisconnectOptions(Action<MqttServerClientDisconnectOptionsBuilder> builder)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            var optionsBuilder = new MqttServerClientDisconnectOptionsBuilder();
+            builder.Invoke(optionsBuilder);
+            
+            _options.DefaultClientDisconnectOptions = optionsBuilder.Build();
+            return this;
+        }
+        
+        public MqttServerStopOptions Build()
+        {
+            return _options;
+        }
+    }
+}