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

.github/workflows/ReleaseNotes.md

@@ -1,3 +1 @@
-* [Core] Fixed several dead locks by implementing a new async signal (#1552)
-* [Server] Fixed duplicated invocation of the event _ClientAcknowledgedPublishPacketAsync_ for QoS level 2 (#1550)
-* [Server] Fixed issue in upgrading and downgrading of QoS levels for subscriptions and retained messages (#1560)
+

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

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

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

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

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

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

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

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

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

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

Source/MQTTnet.AspNetCore.Tests/MqttConnectionContextTest.cs

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

Source/MQTTnet.AspNetCore.Tests/ReaderExtensionsTest.cs

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

Source/MQTTnet.Extensions.ManagedClient/IManagedMqttClient.cs

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

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs

@@ -62,6 +62,7 @@ namespace MQTTnet.Extensions.ManagedClient
         CancellationTokenSource _publishingCancellationToken;
 
         ManagedMqttClientStorageManager _storageManager;
+        bool _isCleanDisconnect;
 
         public ManagedMqttClient(IMqttClient mqttClient, IMqttNetLogger logger)
         {
@@ -160,7 +161,7 @@ namespace MQTTnet.Extensions.ManagedClient
                 throw new InvalidOperationException("call StartAsync before publishing messages");
             }
 
-            MqttTopicValidator.ThrowIfInvalid(applicationMessage.ApplicationMessage.Topic);
+            MqttTopicValidator.ThrowIfInvalid(applicationMessage.ApplicationMessage);
 
             ManagedMqttApplicationMessage removedMessage = null;
             ApplicationMessageSkippedEventArgs applicationMessageSkippedEventArgs = null;
@@ -255,10 +256,12 @@ namespace MQTTnet.Extensions.ManagedClient
             _logger.Info("Started");
         }
 
-        public async Task StopAsync()
+        public async Task StopAsync(bool cleanDisconnect = true)
         {
             ThrowIfDisposed();
 
+            _isCleanDisconnect = cleanDisconnect;
+            
             StopPublishing();
             StopMaintainingConnection();
 
@@ -382,9 +385,12 @@ namespace MQTTnet.Extensions.ManagedClient
                 {
                     try
                     {
-                        using (var disconnectTimeout = new CancellationTokenSource(Options.ClientOptions.Timeout))
+                        if (_isCleanDisconnect)
                         {
-                            await InternalClient.DisconnectAsync(new MqttClientDisconnectOptions(), disconnectTimeout.Token).ConfigureAwait(false);
+                            using (var disconnectTimeout = new CancellationTokenSource(Options.ClientOptions.Timeout))
+                            {
+                                await InternalClient.DisconnectAsync(new MqttClientDisconnectOptions(), disconnectTimeout.Token).ConfigureAwait(false);
+                            }
                         }
                     }
                     catch (OperationCanceledException)

Source/MQTTnet.Tests/Client/LowLevelMqttClient_Tests.cs

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

Source/MQTTnet.Tests/Client/ManagedMqttClient_Tests.cs

@@ -11,6 +11,7 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Diagnostics;
 using MQTTnet.Extensions.ManagedClient;
+using MQTTnet.Formatter;
 using MQTTnet.Internal;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
@@ -21,6 +22,65 @@ namespace MQTTnet.Tests.Client
     [TestClass]
     public sealed class ManagedMqttClient_Tests : BaseTestClass
     {
+        [TestMethod]
+        public async Task Receive_While_Not_Cleanly_Disconnected()
+        {
+            using (var testEnvironment = CreateTestEnvironment(MqttProtocolVersion.V500))
+            {
+                await testEnvironment.StartServer(o => o.WithPersistentSessions());
+
+                var senderClient = await testEnvironment.ConnectClient();
+
+                // Prepare managed client.
+                var managedClient = testEnvironment.Factory.CreateManagedMqttClient();
+                await managedClient.SubscribeAsync("#");
+                var receivedMessages = testEnvironment.CreateApplicationMessageHandler(managedClient);
+                
+                var managedClientOptions = new ManagedMqttClientOptions
+                {
+                    ClientOptions = testEnvironment.Factory.CreateClientOptionsBuilder()
+                        .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                        .WithClientId(nameof(Receive_While_Not_Cleanly_Disconnected) + "_managed")
+                        .WithCleanSession(false)
+                        .Build()
+                };
+
+                await managedClient.StartAsync(managedClientOptions);
+                await LongTestDelay();
+                
+                // Send test data.
+                await senderClient.PublishStringAsync("topic1");
+                await LongTestDelay();
+                
+                receivedMessages.AssertReceivedCountEquals(1);
+
+                // Stop the managed client but keep session at server (not clean disconnect required).
+                await managedClient.StopAsync(false);
+                await LongTestDelay();
+                
+                // Send new messages in the meantime.
+                await senderClient.PublishStringAsync("topic2", qualityOfServiceLevel: MqttQualityOfServiceLevel.ExactlyOnce);
+                await LongTestDelay();
+                
+                // Start the managed client, it should receive the new message.
+                await managedClient.StartAsync(managedClientOptions);
+                await LongTestDelay();
+                
+                receivedMessages.AssertReceivedCountEquals(2);
+                
+                // Stop and start again, no new message should be received.
+                for (var i = 0; i < 3; i++)
+                {
+                    await managedClient.StopAsync(false);
+                    await LongTestDelay();
+                    await managedClient.StartAsync(managedClientOptions);
+                    await LongTestDelay();
+                }
+
+                receivedMessages.AssertReceivedCountEquals(2);
+            }
+        }
+
         [TestMethod]
         public async Task Connect_To_Invalid_Server()
         {
@@ -33,7 +93,6 @@ namespace MQTTnet.Tests.Client
                 var managedClient = testEnvironment.Factory.CreateManagedMqttClient(mqttClient);
 
                 ConnectingFailedEventArgs connectingFailedEventArgs = null;
-
                 managedClient.ConnectingFailedAsync += args =>
                 {
                     connectingFailedEventArgs = args;

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V3_Binary_Tests.cs

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

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V3_Tests.cs

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

Source/MQTTnet.Tests/Formatter/MqttPacketSerialization_V5_Tests.cs

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

Source/MQTTnet.Tests/Internal/AsyncSignal_Tests.cs

@@ -52,12 +52,26 @@ namespace MQTTnet.Tests.Internal
         }
 
         [TestMethod]
-        public async Task Loop_Signal()
+        public async Task Reset_Signal()
         {
             var asyncSignal = new AsyncSignal();
 
+            // WaitAsync should fail because no signal is available.
             for (var i = 0; i < 10; i++)
             {
+                try
+                {
+                    using (var timeout = new CancellationTokenSource(TimeSpan.FromMilliseconds(100)))
+                    {
+                        await asyncSignal.WaitAsync(timeout.Token);
+                    }
+                    
+                    Assert.Fail("This must fail because the signal is not yet set.");
+                }
+                catch (OperationCanceledException)
+                {
+                }
+                
                 asyncSignal.Set();
                 
                 // WaitAsync should return directly because the signal is available.

Source/MQTTnet.Tests/Internal/MqttPacketBusItem_Tests.cs

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

Source/MQTTnet.Tests/Internal/MqttPacketBus_Tests.cs

@@ -13,7 +13,129 @@ namespace MQTTnet.Tests.Internal
 {
     [TestClass]
     public sealed class MqttPacketBus_Tests
-    {
+    {       
+        [TestMethod]
+        public void Alternate_Priorities()
+        {
+            var bus = new MqttPacketBus();
+
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
+
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttSubAckPacket()), MqttPacketBusPartition.Control);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttSubAckPacket()), MqttPacketBusPartition.Control);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttSubAckPacket()), MqttPacketBusPartition.Control);
+
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPingRespPacket()), MqttPacketBusPartition.Health);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPingRespPacket()), MqttPacketBusPartition.Health);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPingRespPacket()), MqttPacketBusPartition.Health);
+
+            Assert.AreEqual(9, bus.TotalItemsCount);
+
+            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPublishPacket));
+            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttSubAckPacket));
+            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPingRespPacket));
+            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPublishPacket));
+            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttSubAckPacket));
+            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPingRespPacket));
+            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPublishPacket));
+            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttSubAckPacket));
+            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPingRespPacket));
+
+            Assert.AreEqual(0, bus.TotalItemsCount);
+        }
+
+        [TestMethod]
+        public void Await_Single_Packet()
+        {
+            var bus = new MqttPacketBus();
+
+            var delivered = false;
+
+            var item1 = new MqttPacketBusItem(new MqttPublishPacket());
+            var item2 = new MqttPacketBusItem(new MqttPublishPacket());
+
+            var item3 = new MqttPacketBusItem(new MqttPublishPacket());
+            item3.Completed += (_, __) =>
+            {
+                delivered = true;
+            };
+
+            bus.EnqueueItem(item1, MqttPacketBusPartition.Data);
+            bus.EnqueueItem(item2, MqttPacketBusPartition.Data);
+            bus.EnqueueItem(item3, MqttPacketBusPartition.Data);
+
+            Assert.IsFalse(delivered);
+
+            bus.DequeueItemAsync(CancellationToken.None).Result.Complete();
+
+            Assert.IsFalse(delivered);
+
+            bus.DequeueItemAsync(CancellationToken.None).Result.Complete();
+
+            Assert.IsFalse(delivered);
+
+            bus.DequeueItemAsync(CancellationToken.None).Result.Complete();
+
+            // The third packet has the event attached.
+            Assert.IsTrue(delivered);
+        }
+
+        [TestMethod]
+        public void Export_Packets_Without_Dequeue()
+        {
+            var bus = new MqttPacketBus();
+
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
+            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
+
+            Assert.AreEqual(3, bus.TotalItemsCount);
+
+            var exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Control);
+            Assert.AreEqual(0, exportedPackets.Count);
+
+            exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Health);
+            Assert.AreEqual(0, exportedPackets.Count);
+
+            exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Data);
+            Assert.AreEqual(3, exportedPackets.Count);
+
+            Assert.AreEqual(3, bus.TotalItemsCount);
+        }
+
+        [TestMethod]
+        public async Task Fill_From_Different_Task()
+        {
+            const int MessageCount = 500;
+
+            var delayRandom = new Random();
+
+            var bus = new MqttPacketBus();
+
+            _ = Task.Run(
+                () =>
+                {
+                    for (var i = 0; i < MessageCount; i++)
+                    {
+                        bus.EnqueueItem(new MqttPacketBusItem(MqttPingReqPacket.Instance), MqttPacketBusPartition.Health);
+
+                        Thread.Sleep(delayRandom.Next(0, 10));
+                    }
+                });
+
+            for (var i = 0; i < MessageCount; i++)
+            {
+                using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(10)))
+                {
+                    await bus.DequeueItemAsync(timeout.Token);
+                }
+            }
+
+            Assert.AreEqual(0, bus.TotalItemsCount);
+        }
+
         [TestMethod]
         [ExpectedException(typeof(TaskCanceledException))]
         public async Task Wait_With_Empty_Bus()
@@ -25,127 +147,5 @@ namespace MQTTnet.Tests.Internal
                 await bus.DequeueItemAsync(timeout.Token);
             }
         }
-        
-        [TestMethod]
-        public async Task Fill_From_Different_Task()
-        {
-            const int MessageCount = 500;
-
-            var delayRandom = new Random();
-            
-            var bus = new MqttPacketBus();
-
-            _ = Task.Run(
-                () =>
-                {
-                    for (int i = 0; i < MessageCount; i++)
-                    {
-                        bus.EnqueueItem(new MqttPacketBusItem(MqttPingReqPacket.Instance), MqttPacketBusPartition.Health);
-                        
-                        Thread.Sleep(delayRandom.Next(0, 10));
-                    }
-                });
-            
-            for (var i = 0; i < MessageCount; i++)
-            {
-                using (var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(10)))
-                {
-                    await bus.DequeueItemAsync(timeout.Token);
-                }    
-            }
-            
-            Assert.AreEqual(0, bus.TotalItemsCount);
-        }
-        
-        [TestMethod]
-        public void Alternate_Priorities()
-        {
-            var bus = new MqttPacketBus();
-            
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttSubAckPacket()), MqttPacketBusPartition.Control);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttSubAckPacket()), MqttPacketBusPartition.Control);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttSubAckPacket()), MqttPacketBusPartition.Control);
-            
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPingRespPacket()), MqttPacketBusPartition.Health);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPingRespPacket()), MqttPacketBusPartition.Health);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPingRespPacket()), MqttPacketBusPartition.Health);
-
-            Assert.AreEqual(9, bus.TotalItemsCount);
-            
-            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPublishPacket));
-            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttSubAckPacket));
-            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPingRespPacket));
-            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPublishPacket));
-            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttSubAckPacket));
-            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPingRespPacket));
-            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPublishPacket));
-            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttSubAckPacket));
-            Assert.IsInstanceOfType(bus.DequeueItemAsync(CancellationToken.None).Result.Packet, typeof(MqttPingRespPacket));
-            
-            Assert.AreEqual(0, bus.TotalItemsCount);
-        }
-        
-        [TestMethod]
-        public void Export_Packets_Without_Dequeue()
-        {
-            var bus = new MqttPacketBus();
-
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            bus.EnqueueItem(new MqttPacketBusItem(new MqttPublishPacket()), MqttPacketBusPartition.Data);
-            
-            Assert.AreEqual(3, bus.TotalItemsCount);
-
-            var exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Control);
-            Assert.AreEqual(0, exportedPackets.Count);
-            
-            exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Health);
-            Assert.AreEqual(0, exportedPackets.Count);
-            
-            exportedPackets = bus.ExportPackets(MqttPacketBusPartition.Data);
-            Assert.AreEqual(3, exportedPackets.Count);
-            
-            Assert.AreEqual(3, bus.TotalItemsCount);
-        }
-        
-        [TestMethod]
-        public void Await_Single_Packet()
-        {
-            var bus = new MqttPacketBus();
-
-            var delivered = false;
-
-            var item1 = new MqttPacketBusItem(new MqttPublishPacket());
-            var item2 = new MqttPacketBusItem(new MqttPublishPacket());
-            
-            var item3 = new MqttPacketBusItem(new MqttPublishPacket());
-            item3.Delivered += (_, __) =>
-            {
-                delivered = true;
-            };
-            
-            bus.EnqueueItem(item1, MqttPacketBusPartition.Data);
-            bus.EnqueueItem(item2, MqttPacketBusPartition.Data);
-            bus.EnqueueItem(item3, MqttPacketBusPartition.Data);
-            
-            Assert.IsFalse(delivered);
-
-            bus.DequeueItemAsync(CancellationToken.None).Result.MarkAsDelivered();
-            
-            Assert.IsFalse(delivered);
-            
-            bus.DequeueItemAsync(CancellationToken.None).Result.MarkAsDelivered();
-            
-            Assert.IsFalse(delivered);
-            
-            bus.DequeueItemAsync(CancellationToken.None).Result.MarkAsDelivered();
-            
-            // The third packet has the event attached.
-            Assert.IsTrue(delivered);
-        }
     }
 }

Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs

@@ -154,7 +154,7 @@ namespace MQTTnet.Tests.MQTTv5
                 var result = await client.PublishStringAsync("a", "b", MqttQualityOfServiceLevel.AtLeastOnce);
                 await client.DisconnectAsync();
 
-                Assert.AreEqual(MqttClientPublishReasonCode.Success, result.ReasonCode);
+                Assert.AreEqual(MqttClientPublishReasonCode.NoMatchingSubscribers, result.ReasonCode);
             }
         }
 
@@ -169,7 +169,7 @@ namespace MQTTnet.Tests.MQTTv5
                 var result = await client.PublishStringAsync("a", "b", MqttQualityOfServiceLevel.ExactlyOnce);
                 await client.DisconnectAsync();
 
-                Assert.AreEqual(MqttClientPublishReasonCode.Success, result.ReasonCode);
+                Assert.AreEqual(MqttClientPublishReasonCode.NoMatchingSubscribers, result.ReasonCode);
             }
         }
 

Source/MQTTnet.Tests/Mockups/TestApplicationMessageReceivedHandler.cs

@@ -2,8 +2,10 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Text;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
@@ -17,7 +19,12 @@ namespace MQTTnet.Tests.Mockups
 
         public TestApplicationMessageReceivedHandler(IMqttClient mqttClient)
         {
-            mqttClient.ApplicationMessageReceivedAsync += MqttClientOnApplicationMessageReceivedAsync;
+            if (mqttClient == null)
+            {
+                throw new ArgumentNullException(nameof(mqttClient));
+            }
+
+            mqttClient.ApplicationMessageReceivedAsync += OnApplicationMessageReceivedAsync;
         }
 
         public List<MqttApplicationMessageReceivedEventArgs> ReceivedEventArgs
@@ -39,7 +46,25 @@ namespace MQTTnet.Tests.Mockups
             }
         }
 
-        Task MqttClientOnApplicationMessageReceivedAsync(MqttApplicationMessageReceivedEventArgs eventArgs)
+        public string GeneratePayloadSequence()
+        {
+            var sequence = new StringBuilder();
+
+            lock (_receivedEventArgs)
+            {
+                foreach (var receivedEventArg in _receivedEventArgs)
+                {
+                    var payload = receivedEventArg.ApplicationMessage.ConvertPayloadToString();
+
+                    // An empty payload is not part of the sequence!
+                    sequence.Append(payload);
+                }
+            }
+
+            return sequence.ToString();
+        }
+
+        Task OnApplicationMessageReceivedAsync(MqttApplicationMessageReceivedEventArgs eventArgs)
         {
             lock (_receivedEventArgs)
             {

Source/MQTTnet.Tests/Mockups/TestEnvironment.cs

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

Source/MQTTnet.Tests/Server/Load_Tests.cs

@@ -55,7 +55,7 @@ namespace MQTTnet.Tests.Server
                             });
                     }
 
-                    SpinWait.SpinUntil(() => receivedMessages == 100000, TimeSpan.FromSeconds(90));
+                    SpinWait.SpinUntil(() => receivedMessages == 100000, TimeSpan.FromSeconds(120));
 
                     Assert.AreEqual(100000, receivedMessages);
                 }

Source/MQTTnet.Tests/Server/MqttSubscriptionsManager_Tests.cs

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

Source/MQTTnet.Tests/Server/Publish_Tests.cs

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

Source/MQTTnet.Tests/Server/Publishing_Tests.cs

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

Source/MQTTnet.Tests/Server/QoS_Tests.cs

@@ -15,6 +15,57 @@ namespace MQTTnet.Tests.Server
     [TestClass]
     public sealed class QoS_Tests : BaseTestClass
     {
+        [TestMethod]
+        public async Task Preserve_Message_Order_For_Queued_Messages()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer(o => o.WithPersistentSessions());
+
+                // Create a session which will contain the messages.
+                var dummyClient = await testEnvironment.ConnectClient(o => o.WithClientId("A").WithCleanSession(false));
+                await dummyClient.SubscribeAsync("#", MqttQualityOfServiceLevel.AtLeastOnce);
+                dummyClient.Dispose();
+                
+                // Now inject messages which are appended to the queue of the client.
+                await server.InjectApplicationMessage("T", "0", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "2", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                await server.InjectApplicationMessage("T", "1", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "4", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                await server.InjectApplicationMessage("T", "3", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "6", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                await server.InjectApplicationMessage("T", "5", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "8", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                await server.InjectApplicationMessage("T", "7", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await server.InjectApplicationMessage("T", "9", qualityOfServiceLevel: MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await LongTestDelay();
+                
+                // Create a new client for the existing message.
+                var client = await testEnvironment.ConnectClient(o => o.WithClientId("A").WithCleanSession(false));
+                var messages = testEnvironment.CreateApplicationMessageHandler(client);
+                
+                await LongTestDelay();
+                
+                var payloadSequence = messages.GeneratePayloadSequence();
+                Assert.AreEqual("0214365879", payloadSequence);
+
+                // Disconnect and reconnect to make sure that the server will not send the messages twice.
+                await client.DisconnectAsync();
+                await LongTestDelay();
+                await client.ReconnectAsync();
+                await LongTestDelay();
+                
+                payloadSequence = messages.GeneratePayloadSequence();
+                Assert.AreEqual("0214365879", payloadSequence);
+            }
+        }
+        
         [TestMethod]
         public async Task Fire_Event_On_Client_Acknowledges_QoS_0()
         {

Source/MQTTnet.Tests/Server/Session_Tests.cs

@@ -150,7 +150,7 @@ namespace MQTTnet.Tests.Server
                 // Try to connect 50 clients at the same time.
                 var clients = await Task.WhenAll(Enumerable.Range(0, 50).Select(i => ConnectAndSubscribe(testEnvironment, options, OnReceive)));
 
-                await Task.Delay(5000);
+                await Task.Delay(TimeSpan.FromSeconds(10));
                 
                 var connectedClients = clients.Where(c => c?.TryPingAsync().GetAwaiter().GetResult() == true).ToList();
 
@@ -162,7 +162,7 @@ namespace MQTTnet.Tests.Server
                 var sendClient = await testEnvironment.ConnectClient(option2);
                 await sendClient.PublishStringAsync("aaa", "1");
 
-                await Task.Delay(3000);
+                await Task.Delay(TimeSpan.FromSeconds(5));
 
                 Assert.AreEqual(true, hasReceive);
             }

Source/MQTTnet.Tests/Server/Subscribe_Tests.cs

@@ -46,6 +46,22 @@ namespace MQTTnet.Tests.Server
             }
         }
 
+        [TestMethod]
+        [ExpectedException(typeof(MqttClientDisconnectedException))]
+        public async Task Disconnect_While_Subscribing()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServer();
+
+                // The client will be disconnect directly after subscribing!
+                server.ClientSubscribedTopicAsync += ev => server.DisconnectClientAsync(ev.ClientId, MqttDisconnectReasonCode.NormalDisconnection);
+
+                var client = await testEnvironment.ConnectClient();
+                await client.SubscribeAsync("#");
+            }
+        }
+        
         [TestMethod]
         public async Task Enqueue_Message_After_Subscription()
         {

Source/MQTTnet.Tests/Server/Subscription_TopicHash_Tests.cs

@@ -1,9 +1,11 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
 using MQTTnet.Tests.Mockups;
@@ -13,43 +15,299 @@ namespace MQTTnet.Tests.Server
     [TestClass]
     public sealed class SubscriptionTopicHashTests
     {
-        enum TopicHashSelector {  NoWildcard, SingleWildcard, MultiWildcard };
-
         MqttSession _clientSession;
 
+        /// <summary>
+        ///     Even fairly regularly named topics as generated by the topic generator should result in shallow hash buckets
+        /// </summary>
+        /// <returns></returns>
         [TestMethod]
-        public void Match_Hash_Test_SingleWildCard()
+        public void Check_Hash_Bucket_Depth()
         {
-            var l0 = "pub0";
-            var l1 = "topic1";
-            var l2 = "+";
-            var l3 = "prop1";
-            var topic = $"{l0}/{l1}/{l2}/{l3}";
+            const int numPublishers = 5000;
+            const int numTopicsPerPublisher = 10;
+
+            TopicGenerator.Generate(
+                numPublishers,
+                numTopicsPerPublisher,
+                out var topicsByPublisher,
+                out _,
+                out _);
+
+            // There will be many 'similar' topics ending with, i.e. "sensor100", "sensor101", ...
+            // Hash bucket depths should remain low.
+            var bucketDepths = new Dictionary<ulong, int>();
+            var maxBucketDepth = 0;
+            ulong maxBucketDepthHash = 0;
+
+            var topicsByHash = new Dictionary<ulong, List<string>>();
+
+            foreach (var t in topicsByPublisher)
+            {
+                var topics = t.Value;
+                foreach (var topic in topics)
+                {
+                    MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var hashMask, out var hasWildcard);
+
+                    bucketDepths.TryGetValue(topicHash, out var currentValue);
+                    ++currentValue;
+                    bucketDepths[topicHash] = currentValue;
+
+                    if (currentValue > maxBucketDepth)
+                    {
+                        maxBucketDepth = currentValue;
+                        maxBucketDepthHash = topicHash;
+                    }
+
+                    if (!topicsByHash.TryGetValue(topicHash, out var topicList))
+                    {
+                        topicList = new List<string>();
+                        topicsByHash.Add(topicHash, topicList);
+                    }
+
+                    topicList.Add(topic);
+                }
+            }
+
+            var maxDepthTopics = topicsByHash[maxBucketDepthHash];
+
+            Console.Write("Max bucket depth is " + maxBucketDepth);
+
+            // for the test case the bucket depth should be less than 100
+            Assert.IsTrue(maxBucketDepth < 100, "Unexpected high topic hash bucket depth");
+        }
+
+
+        [TestMethod]
+        public void Check_Selected_Topic_Hashes()
+        {
+            CheckTopicHash("client1/building1/level1/sensor1", 0x655D4AF100000000, 0xFFFFFFFFFFFFFFFF);
+            CheckTopicHash("client1/building1/+/sensor1", 0x655D00F100000000, 0xFFFF00FFFFFFFFFF);
+            CheckTopicHash("client1/+/level1/+", 0x65004A0000000000, 0xFF00FF00FFFFFFFF);
+            CheckTopicHash("client1/building1/level1/#", 0x655D4A0000000000, 0xFFFFFF0000000000);
+            CheckTopicHash("client1/+/level1/#", 0x65004A0000000000, 0xFF00FF0000000000);
+        }
+
+        /// <summary>
+        ///     Test long topic name with last level being #
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_DetectMultiLevelWildcard()
+        {
+            var topic = "asdfasdf/asdfasdf/asdfasdf/asdfasdf/asdfas/dfaf/assfdgsdfgdf/#";
+
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
+
+            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
+        }
+
+
+        /// <summary>
+        ///     Test long topic name with last level being +
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_DetectSingleLevelWildcard()
+        {
+            var topic = "asdfasdf/asdfasdf/asdfasdf/asdfasdf/asdfas/dfaf/assfdgsdfgdf/+";
+
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
+
+            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
+        }
+
+        /// <summary>
+        ///     Test long topic name with last level being #
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_MultiWildcard()
+        {
+            var sb = new StringBuilder();
+            const int numLevels = 8;
+            var levelNames = new string[numLevels];
+            for (var i = 0; i < numLevels; ++i)
+            {
+                if (i > 0)
+                {
+                    sb.Append("/");
+                }
+
+                string levelName;
+                if (i == numLevels - 1)
+                {
+                    // last one is #
+                    levelName = "#";
+                }
+                else
+                {
+                    levelName = "level" + i;
+                }
+
+                levelNames[i] = levelName;
+                sb.Append(levelName);
+            }
+
+            var topic = sb.ToString();
+
+            // UInt64 is limited to 8 levels
 
             MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
 
             Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
 
             var hashBytes = GetBytes(topicHash);
-            Assert.AreNotEqual(hashBytes[0], 0, "checksum 0 mismatch");
-            Assert.AreNotEqual(hashBytes[1], 0, "checksum 1 mismatch");
-            Assert.AreEqual(hashBytes[2], 0, "checksum 2 mismatch");
-            Assert.AreNotEqual(hashBytes[3], 0, "checksum 3 mismatch");
-            Assert.AreEqual(hashBytes[4], 0, "checksum 4 mismatch");
-            Assert.AreEqual(hashBytes[5], 0, "checksum 5 mismatch");
-            Assert.AreEqual(hashBytes[6], 0, "checksum 6 mismatch");
-            Assert.AreEqual(hashBytes[7], 0, "checksum 7 mismatch");
+            // all bytes should contain checksum
+            var count = 0;
+            foreach (var h in hashBytes)
+            {
+                if (count < 7)
+                {
+                    Assert.AreNotEqual(h, 0, "checksum mismatch");
+                }
+                else
+                {
+                    Assert.AreEqual(h, 0, "checksum mismatch");
+                }
 
-            // The mask should have zeroes where the wildcard and ff at the end
+                ++count;
+            }
+
+            // The mask should have ff except for last level
             var hashMaskBytes = GetBytes(topicHashMask);
-            Assert.AreEqual(hashMaskBytes[0], 0xff, "mask 0 mismatch");
-            Assert.AreEqual(hashMaskBytes[1], 0xff, "mask 1 mismatch");
-            Assert.AreEqual(hashMaskBytes[2], 0, "mask 2 mismatch");
-            Assert.AreEqual(hashMaskBytes[3], 0xff, "mask 3 mismatch");
-            Assert.AreEqual(hashMaskBytes[4], 0xff, "mask 4 mismatch");
-            Assert.AreEqual(hashMaskBytes[5], 0xff, "mask 5 mismatch");
-            Assert.AreEqual(hashMaskBytes[6], 0xff, "mask 6 mismatch");
-            Assert.AreEqual(hashMaskBytes[7], 0xff, "mask 7 mismatch");
+            count = 0;
+            foreach (var h in hashMaskBytes)
+            {
+                if (count < 7)
+                {
+                    Assert.AreEqual(h, 0xff, "mask mismatch");
+                }
+                else
+                {
+                    Assert.AreEqual(h, 0, "last mask mismatch");
+                }
+
+                ++count;
+            }
+        }
+
+        /// <summary>
+        ///     Test long topic name exceeding 8 levels
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_NoWildCard()
+        {
+            var sb = new StringBuilder();
+            const int numLevels = 9;
+            var levelNames = new string[numLevels];
+            for (var i = 0; i < numLevels; ++i)
+            {
+                if (i > 0)
+                {
+                    sb.Append("/");
+                }
+
+                var levelName = "level" + i;
+                levelNames[i] = levelName;
+                sb.Append(levelName);
+            }
+
+            var topic = sb.ToString();
+
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
+
+            Assert.IsFalse(topicHasWildcard, "Wildcard detected when not present");
+
+
+            var hashBytes = GetBytes(topicHash);
+            // all bytes should contain checksum
+            var count = 0;
+            foreach (var h in hashBytes)
+            {
+                Assert.AreNotEqual(h, 0, "checksum mismatch");
+                ++count;
+            }
+
+            // The mask should have ff 
+            var hashMaskBytes = GetBytes(topicHashMask);
+            count = 0;
+            foreach (var h in hashMaskBytes)
+            {
+                Assert.AreEqual(h, 0xff, "mask mismatch");
+                ++count;
+            }
+        }
+
+        /// <summary>
+        ///     Test long topic name with last level being +
+        /// </summary>
+        [TestMethod]
+        public void Match_Hash_Test_LongTopic_SingleWildCard()
+        {
+            var sb = new StringBuilder();
+            const int numLevels = 8;
+            var levelNames = new string[numLevels];
+            for (var i = 0; i < numLevels; ++i)
+            {
+                if (i > 0)
+                {
+                    sb.Append("/");
+                }
+
+                string levelName;
+                if (i == numLevels - 1)
+                {
+                    // last one is +
+                    levelName = "+";
+                }
+                else
+                {
+                    levelName = "level" + i;
+                }
+
+                levelNames[i] = levelName;
+                sb.Append(levelName);
+            }
+
+            var topic = sb.ToString();
+
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
+
+            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
+
+
+            var hashBytes = GetBytes(topicHash);
+            // all bytes should contain checksum
+            var count = 0;
+            foreach (var h in hashBytes)
+            {
+                if (count < 7)
+                {
+                    Assert.AreNotEqual(h, 0, "checksum mismatch");
+                }
+                else
+                {
+                    // wildcard position
+                    Assert.AreEqual(h, 0, "checksum mismatch");
+                }
+
+                ++count;
+            }
+
+            // The mask should have ff 
+            var hashMaskBytes = GetBytes(topicHashMask);
+            count = 0;
+            foreach (var h in hashMaskBytes)
+            {
+                if (count < 7)
+                {
+                    Assert.AreEqual(h, 0xff, "mask mismatch");
+                }
+                else
+                {
+                    Assert.AreEqual(h, 0, "last mask mismatch");
+                }
+
+                ++count;
+            }
         }
 
         [TestMethod]
@@ -123,205 +381,13 @@ namespace MQTTnet.Tests.Server
             Assert.AreEqual(hashMaskBytes[7], 0xff, "mask 7 mismatch");
         }
 
-        /// <summary>
-        /// Test long topic name with last level being #
-        /// </summary>
         [TestMethod]
-        public void Match_Hash_Test_LongTopic_MultiWildcard()
+        public async Task Match_Hash_Test_Search_MultiWildcard()
         {
-            var sb = new StringBuilder();
-            const int numLevels = 8;
-            var levelNames = new string[numLevels];
-            for (var i = 0; i < numLevels; ++i)
-            {
-                if (i > 0)
-                    sb.Append("/");
-                string levelName;
-                if (i == numLevels - 1)
-                {
-                    // last one is #
-                    levelName = "#";
-                }
-                else
-                {
-                   levelName = "level" + i;
-                }
-                levelNames[i] = levelName;
-                sb.Append(levelName);
-            }
-            var topic = sb.ToString();
-            
-            // UInt64 is limited to 8 levels
-
-            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
-
-            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
-
-            var hashBytes = GetBytes(topicHash);
-            // all bytes should contain checksum
-            int count = 0;
-            foreach (var h in hashBytes)
-            {
-                if (count < 7)
-                {
-                    Assert.AreNotEqual(h, 0, "checksum mismatch");
-                }
-                else
-                {
-                    Assert.AreEqual(h, 0, "checksum mismatch");
-                }
-                ++count;
-            }
-            // The mask should have ff except for last level
-            var hashMaskBytes = GetBytes(topicHashMask);
-            count = 0;
-            foreach (var h in hashMaskBytes)
-            {
-                if (count < 7)
-                {
-                    Assert.AreEqual(h, 0xff, "mask mismatch");
-                }
-                else
-                {
-                    Assert.AreEqual(h, 0, "last mask mismatch");
-                }
-                ++count;
-            }
-        }
-
-        /// <summary>
-        /// Test long topic name with last level being +
-        /// </summary>
-        [TestMethod]
-        public void Match_Hash_Test_LongTopic_SingleWildCard()
-        {
-            var sb = new StringBuilder();
-            const int numLevels = 8;
-            var levelNames = new string[numLevels];
-            for (var i = 0; i < numLevels; ++i)
-            {
-                if (i > 0)
-                    sb.Append("/");
-                string levelName;
-                if (i == numLevels - 1)
-                {
-                    // last one is +
-                    levelName = "+";
-                }
-                else
-                {
-                    levelName = "level" + i;
-                }
-                levelNames[i] = levelName;
-                sb.Append(levelName);
-            }
-            var topic = sb.ToString();
-
-            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
-
-            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
-
-
-            var hashBytes = GetBytes(topicHash);
-            // all bytes should contain checksum
-            int count = 0;
-            foreach (var h in hashBytes)
-            {
-                if (count < 7)
-                {
-                    Assert.AreNotEqual(h, 0, "checksum mismatch");
-                }
-                else
-                {
-                    // wildcard position
-                    Assert.AreEqual(h, 0, "checksum mismatch");
-                }
-                ++count;
-            }
-            // The mask should have ff 
-            var hashMaskBytes = GetBytes(topicHashMask);
-            count = 0;
-            foreach (var h in hashMaskBytes)
-            {
-                if (count < 7)
-                {
-                    Assert.AreEqual(h, 0xff, "mask mismatch");
-                }
-                else
-                {
-                    Assert.AreEqual(h, 0, "last mask mismatch");
-                }
-                ++count;
-            }
-        }
-
-        /// <summary>
-        /// Test long topic name exceeding 8 levels
-        /// </summary>
-        [TestMethod]
-        public void Match_Hash_Test_LongTopic_NoWildCard()
-        {
-            var sb = new StringBuilder();
-            const int numLevels = 9;
-            var levelNames = new string[numLevels];
-            for(var i=0; i < numLevels; ++i)
-            {
-                if (i > 0)
-                    sb.Append("/");
-                var levelName = "level" + i;
-                levelNames[i] = levelName;
-                sb.Append(levelName);
-            }
-            var topic = sb.ToString();
-
-            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
-
-            Assert.IsFalse(topicHasWildcard, "Wildcard detected when not present");
-
-
-            var hashBytes = GetBytes(topicHash);
-            // all bytes should contain checksum
-            int count = 0;
-            foreach(var h in hashBytes)
-            {
-                Assert.AreNotEqual(h, 0, "checksum mismatch");
-                ++count;
-            }
-            // The mask should have ff 
-            var hashMaskBytes = GetBytes(topicHashMask);
-            count = 0;
-            foreach (var h in hashMaskBytes)
-            {
-                Assert.AreEqual(h, 0xff, "mask mismatch");
-                ++count;
-            }
-        }
-
-
-        /// <summary>
-        /// Test long topic name with last level being +
-        /// </summary>
-        [TestMethod]
-        public void Match_Hash_Test_LongTopic_DetectSingleLevelWildcard()
-        {
-            var topic = "asdfasdf/asdfasdf/asdfasdf/asdfasdf/asdfas/dfaf/assfdgsdfgdf/+";
-
-            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
-
-            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
-        }
-
-        /// <summary>
-        /// Test long topic name with last level being #
-        /// </summary>
-        [TestMethod]
-        public void Match_Hash_Test_LongTopic_DetectMultiLevelWildcard()
-        {
-            var topic = "asdfasdf/asdfasdf/asdfasdf/asdfasdf/asdfas/dfaf/assfdgsdfgdf/#";
-
-            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
-
-            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
+            var topics = await PrepareTopicHashSubscriptions(TopicHashSelector.MultiWildcard);
+            var matchCount = CheckTopicSubscriptions(_clientSession, topics, "Multi Wildcard");
+            // Should match all topics
+            Assert.AreEqual(topics.Count, matchCount, "Topics not matched");
         }
 
         [TestMethod]
@@ -373,77 +439,38 @@ namespace MQTTnet.Tests.Server
         }
 
         [TestMethod]
-        public async Task Match_Hash_Test_Search_MultiWildcard()
+        public void Match_Hash_Test_SingleWildCard()
         {
-            var topics = await PrepareTopicHashSubscriptions(TopicHashSelector.MultiWildcard);
-            var matchCount = CheckTopicSubscriptions(_clientSession, topics, "Multi Wildcard");
-            // Should match all topics
-            Assert.AreEqual(topics.Count, matchCount, "Topics not matched");
-        }
+            var l0 = "pub0";
+            var l1 = "topic1";
+            var l2 = "+";
+            var l3 = "prop1";
+            var topic = $"{l0}/{l1}/{l2}/{l3}";
 
-        /// <summary>
-        /// Even fairly regularly named topics as generated by the topic generator should result in shallow hash buckets
-        /// </summary>
-        /// <returns></returns>
-        [TestMethod]
-        public void Check_Hash_Bucket_Depth()
-        {
-            const int numPublishers = 5000;
-            const int numTopicsPerPublisher = 10;
+            MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var topicHashMask, out var topicHasWildcard);
 
-            TopicGenerator.Generate(numPublishers, numTopicsPerPublisher, out var topicsByPublisher, out var singleWildcardTopicsByPublisher, out var multiWildcardTopicsByPublisher);
+            Assert.IsTrue(topicHasWildcard, "Wildcard not detected");
 
-            // There will be many 'similar' topics ending with, i.e. "sensor100", "sensor101", ...
-            // Hash bucket depths should remain low.
-            var bucketDepths = new Dictionary<UInt64, int>();
-            int maxBucketDepth = 0;
-            UInt64 maxBucketDepthHash = 0;
+            var hashBytes = GetBytes(topicHash);
+            Assert.AreNotEqual(hashBytes[0], 0, "checksum 0 mismatch");
+            Assert.AreNotEqual(hashBytes[1], 0, "checksum 1 mismatch");
+            Assert.AreEqual(hashBytes[2], 0, "checksum 2 mismatch");
+            Assert.AreNotEqual(hashBytes[3], 0, "checksum 3 mismatch");
+            Assert.AreEqual(hashBytes[4], 0, "checksum 4 mismatch");
+            Assert.AreEqual(hashBytes[5], 0, "checksum 5 mismatch");
+            Assert.AreEqual(hashBytes[6], 0, "checksum 6 mismatch");
+            Assert.AreEqual(hashBytes[7], 0, "checksum 7 mismatch");
 
-            var topicsByHash = new Dictionary<UInt64, List<string>>();
-
-            foreach (var t in topicsByPublisher)
-            {
-                var topics = t.Value;
-                foreach (var topic in topics)
-                {
-                    MqttSubscription.CalculateTopicHash(topic, out var topicHash, out var hashMask, out var hasWildcard);
-
-                    bucketDepths.TryGetValue(topicHash, out var currentValue);
-                    ++currentValue;
-                    bucketDepths[topicHash] = currentValue;
-
-                    if (currentValue > maxBucketDepth)
-                    {
-                        maxBucketDepth = currentValue;
-                        maxBucketDepthHash = topicHash;
-                    }
-
-                    if (!topicsByHash.TryGetValue(topicHash, out var topicList))
-                    {
-                        topicList = new List<string>();
-                        topicsByHash.Add(topicHash, topicList);
-                    }
-                    topicList.Add(topic);
-                }
-            }
-
-            var maxDepthTopics = topicsByHash[maxBucketDepthHash];
-
-            Console.Write("Max bucket depth is " + maxBucketDepth);
-
-            // for the test case the bucket depth should be less than 100
-            Assert.IsTrue(maxBucketDepth < 100, "Unexpected high topic hash bucket depth");
-        }
-
-
-        [TestMethod]
-        public void Check_Selected_Topic_Hashes()
-        {
-            CheckTopicHash("client1/building1/level1/sensor1", 0x655D4AF100000000, 0xFFFFFFFFFFFFFFFF);
-            CheckTopicHash("client1/building1/+/sensor1", 0x655D00F100000000, 0xFFFF00FFFFFFFFFF);
-            CheckTopicHash("client1/+/level1/+", 0x65004A0000000000, 0xFF00FF00FFFFFFFF);
-            CheckTopicHash("client1/building1/level1/#", 0x655D4A0000000000, 0xFFFFFF0000000000);
-            CheckTopicHash("client1/+/level1/#", 0x65004A0000000000, 0xFF00FF0000000000);
+            // The mask should have zeroes where the wildcard and ff at the end
+            var hashMaskBytes = GetBytes(topicHashMask);
+            Assert.AreEqual(hashMaskBytes[0], 0xff, "mask 0 mismatch");
+            Assert.AreEqual(hashMaskBytes[1], 0xff, "mask 1 mismatch");
+            Assert.AreEqual(hashMaskBytes[2], 0, "mask 2 mismatch");
+            Assert.AreEqual(hashMaskBytes[3], 0xff, "mask 3 mismatch");
+            Assert.AreEqual(hashMaskBytes[4], 0xff, "mask 4 mismatch");
+            Assert.AreEqual(hashMaskBytes[5], 0xff, "mask 5 mismatch");
+            Assert.AreEqual(hashMaskBytes[6], 0xff, "mask 6 mismatch");
+            Assert.AreEqual(hashMaskBytes[7], 0xff, "mask 7 mismatch");
         }
 
         void CheckTopicHash(string topic, ulong expectedHash, ulong expectedHashMask)
@@ -459,13 +486,66 @@ namespace MQTTnet.Tests.Server
             Assert.AreEqual(expectedHashMask, hashMask, "Topic hash mask not as expected");
         }
 
+        int CheckTopicSubscriptions(MqttSession clientSession, List<string> topicsToFind, string subject)
+        {
+            var matchCount = 0;
+
+            {
+                var resultCount = 0;
+
+                var stopWatch = new Stopwatch();
+                stopWatch.Start();
+                var countUp = 0;
+                var countDown = topicsToFind.Count - 1;
+                for (; countUp < topicsToFind.Count; ++countUp, --countDown)
+                {
+                    var topicToFind = topicsToFind[countDown];
+
+                    MqttSubscription.CalculateTopicHash(topicToFind, out var topicHash, out _, out _);
+
+                    clientSession.TryCheckSubscriptions(topicToFind, topicHash, MqttQualityOfServiceLevel.AtMostOnce, "OtherClient", out var result);
+                    if (result.IsSubscribed)
+                    {
+                        ++matchCount;
+                    }
+
+                    ++resultCount;
+                }
+
+                stopWatch.Stop();
+
+                Console.Write("Match count: " + matchCount + "; ");
+
+                Console.WriteLine(subject + " lookup milliseconds: " + stopWatch.ElapsedMilliseconds);
+            }
+
+            return matchCount;
+        }
+
+        byte[] GetBytes(ulong value)
+        {
+            var bytes = BitConverter.GetBytes(value);
+            // Ensure that highest byte comes first for comparison left to right
+            if (BitConverter.IsLittleEndian)
+            {
+                return bytes.Reverse().ToArray();
+            }
+
+            return bytes;
+        }
+
 
         async Task<List<string>> PrepareTopicHashSubscriptions(TopicHashSelector selector)
         {
             const int numPublishers = 1;
             const int numTopicsPerPublisher = 10000;
 
-            TopicGenerator.Generate(numPublishers, numTopicsPerPublisher, out var topicsByPublisher, out var singleWildcardTopicsByPublisher, out var multiWildcardTopicsByPublisher);
+            TopicGenerator.Generate(
+                numPublishers,
+                numTopicsPerPublisher,
+                out var topicsByPublisher,
+                out var singleWildcardTopicsByPublisher,
+                out var multiWildcardTopicsByPublisher);
 
             var topics = topicsByPublisher.FirstOrDefault().Value;
             var singleWildcardTopics = singleWildcardTopicsByPublisher.FirstOrDefault().Value;
@@ -477,15 +557,7 @@ namespace MQTTnet.Tests.Server
             var eventContainer = new MqttServerEventContainer();
             var retainedMessagesManager = new MqttRetainedMessagesManager(eventContainer, logger);
             var sessionManager = new MqttClientSessionsManager(serverOptions, retainedMessagesManager, eventContainer, logger);
-            _clientSession = new MqttSession(
-                        clientId,
-                        false,
-                        new Dictionary<object, object>(),
-                        serverOptions,
-                        eventContainer,
-                        retainedMessagesManager,
-                        sessionManager
-                        );
+            _clientSession = new MqttSession(clientId, false, new Dictionary<object, object>(), serverOptions, eventContainer, retainedMessagesManager, sessionManager);
 
             List<string> topicsToSubscribe;
 
@@ -501,62 +573,26 @@ namespace MQTTnet.Tests.Server
                     topicsToSubscribe = topics;
                     break;
             }
+
             foreach (var t in topicsToSubscribe)
             {
-                var subPacket = new Packets.MqttSubscribePacket();
-                var filter = new Packets.MqttTopicFilter
+                var subPacket = new MqttSubscribePacket();
+                var filter = new MqttTopicFilter
                 {
                     Topic = t
                 };
                 subPacket.TopicFilters.Add(filter);
-                await _clientSession.SubscriptionsManager.Subscribe(subPacket, default);
+                await _clientSession.Subscribe(subPacket, default);
             }
 
             return topics;
         }
 
-        int CheckTopicSubscriptions(MqttSession clientSession, List<string> topicsToFind, string subject)
+        enum TopicHashSelector
         {
-            var matchCount = 0;
-
-            {
-                int resultCount = 0;
-
-                var stopWatch = new System.Diagnostics.Stopwatch();
-                stopWatch.Start();
-                var countUp = 0;
-                var countDown = topicsToFind.Count - 1;
-                for (; countUp < topicsToFind.Count; ++countUp, --countDown)
-                {
-                    var topicToFind = topicsToFind[countDown];
-
-                    MqttSubscription.CalculateTopicHash(topicToFind, out var topicHash, out var hashMask, out var hasWildcard);
-
-                    var result = clientSession.SubscriptionsManager.CheckSubscriptions(topicToFind, topicHash, MqttQualityOfServiceLevel.AtMostOnce, "OtherClient");
-                    if (result.IsSubscribed)
-                    {
-                        ++matchCount;
-                    }
-                    ++resultCount;
-                }
-
-                stopWatch.Stop();
-
-                Console.Write("Match count: " + matchCount + "; ");
-
-                Console.WriteLine(subject + " lookup milliseconds: " + stopWatch.ElapsedMilliseconds);
-            }
-
-            return matchCount;
-        }
-
-        byte[] GetBytes(UInt64 value)
-        {
-            var bytes = BitConverter.GetBytes(value);
-            // Ensure that highest byte comes first for comparison left to right
-            if (BitConverter.IsLittleEndian)
-                return bytes.Reverse().ToArray();
-            return bytes;
+            NoWildcard,
+            SingleWildcard,
+            MultiWildcard
         }
     }
-}
+}

Source/MQTTnet.Tests/Server/Unsubscribe_Tests.cs

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

Source/MQTTnet.Tests/TopicGenerator.cs

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

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -86,7 +86,7 @@ namespace MQTTnet.Adapter
                     await Task.WhenAny(connectTask, timeout.Task).ConfigureAwait(false);
                     if (timeout.Task.IsCompleted && !connectTask.IsCompleted)
                     {
-                        throw new OperationCanceledException("MQTT connect cancelled.", cancellationToken);
+                        throw new OperationCanceledException("MQTT connect canceled.", cancellationToken);
                     }
 
                     // Make sure that the exception from the connect task gets thrown.
@@ -298,7 +298,7 @@ namespace MQTTnet.Adapter
                 // Check two times for cancellation because the call to _ReadAsync_ might block for some time.
                 if (cancellationToken.IsCancellationRequested)
                 {
-                    return ReadFixedHeaderResult.Cancelled;
+                    return ReadFixedHeaderResult.Canceled;
                 }
 
                 int bytesRead;
@@ -308,7 +308,7 @@ namespace MQTTnet.Adapter
                 }
                 catch (OperationCanceledException)
                 {
-                    return ReadFixedHeaderResult.Cancelled;
+                    return ReadFixedHeaderResult.Canceled;
                 }
                 catch (SocketException)
                 {
@@ -317,7 +317,7 @@ namespace MQTTnet.Adapter
                
                 if (cancellationToken.IsCancellationRequested)
                 {
-                    return ReadFixedHeaderResult.Cancelled;
+                    return ReadFixedHeaderResult.Canceled;
                 }
 
                 if (bytesRead == 0)

Source/MQTTnet/Client/Exceptions/MqttClientDisconnectedException.cs

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

Source/MQTTnet/Client/Exceptions/MqttClientUnexpectedDisconnectReceivedException.cs

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

Source/MQTTnet/Client/MqttClient.cs

@@ -32,25 +32,21 @@ namespace MQTTnet.Client
         readonly object _disconnectLock = new object();
         readonly AsyncEvent<InspectMqttPacketEventArgs> _inspectPacketEvent = new AsyncEvent<InspectMqttPacketEventArgs>();
         readonly MqttNetSourceLogger _logger;
-        readonly MqttPacketDispatcher _packetDispatcher = new MqttPacketDispatcher();
         readonly MqttPacketFactories _packetFactories = new MqttPacketFactories();
 
         readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new MqttPacketIdentifierProvider();
         readonly IMqttNetLogger _rootLogger;
 
         IMqttChannelAdapter _adapter;
-
         CancellationTokenSource _backgroundCancellationTokenSource;
         bool _cleanDisconnectInitiated;
         volatile int _connectionStatus;
-
         MqttClientDisconnectReason _disconnectReason;
         string _disconnectReasonString;
         Task _keepAlivePacketsSenderTask;
-
         DateTime _lastPacketSentTimestamp;
+        MqttPacketDispatcher _packetDispatcher;
         Task _packetReceiverTask;
-
         AsyncQueue<MqttPublishPacket> _publishPacketReceiverQueue;
         Task _publishPacketReceiverTask;
 
@@ -128,7 +124,7 @@ namespace MQTTnet.Client
                 }
 
                 _packetIdentifierProvider.Reset();
-                _packetDispatcher.CancelAll();
+                _packetDispatcher = new MqttPacketDispatcher();
 
                 _backgroundCancellationTokenSource = new CancellationTokenSource();
                 var backgroundCancellationToken = _backgroundCancellationTokenSource.Token;
@@ -414,6 +410,10 @@ namespace MQTTnet.Client
                 _publishPacketReceiverQueue = null;
 
                 _adapter?.Dispose();
+                _adapter = null;
+                
+                _packetDispatcher?.Dispose();
+                _packetDispatcher = null;
             }
         }
 
@@ -447,6 +447,8 @@ namespace MQTTnet.Client
 
             try
             {
+                _packetDispatcher.Dispose(new MqttClientDisconnectedException(exception));
+                
                 var receiverTask = WaitForTaskAsync(_packetReceiverTask, sender);
                 var publishPacketReceiverTask = WaitForTaskAsync(_publishPacketReceiverTask, sender);
                 var keepAliveTask = WaitForTaskAsync(_keepAlivePacketsSenderTask, sender);
@@ -558,7 +560,7 @@ namespace MQTTnet.Client
             _disconnectReasonString = disconnectPacket.ReasonString;
 
             // Also dispatch disconnect to waiting threads to generate a proper exception.
-            _packetDispatcher.FailAll(new MqttUnexpectedDisconnectReceivedException(disconnectPacket));
+            _packetDispatcher.Dispose(new MqttClientUnexpectedDisconnectReceivedException(disconnectPacket));
 
             return DisconnectInternalAsync(_packetReceiverTask, null, null);
         }
@@ -776,7 +778,7 @@ namespace MQTTnet.Client
                 }
                 else
                 {
-                    _logger.Error(exception, "Error while receiving packets.");
+                    _logger.Error(exception, $"Error while processing received packet ({packet.GetType().Name}).");
                 }
 
                 _packetDispatcher.FailAll(exception);

Source/MQTTnet/Client/MqttClientExtensions.cs

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

Source/MQTTnet/Client/Publishing/MqttClientPublishResult.cs

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

Source/MQTTnet/Exceptions/MqttCommunicationException.cs

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

Source/MQTTnet/Formatter/MqttPubAckPacketFactory.cs

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

Source/MQTTnet/Formatter/MqttPubRecPacketFactory.cs

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

Source/MQTTnet/Formatter/MqttUnsubAckPacketFactory.cs

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

Source/MQTTnet/Formatter/ReadFixedHeaderResult.cs

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

Source/MQTTnet/Internal/AsyncEvent.cs

@@ -123,7 +123,7 @@ namespace MQTTnet.Internal
             }
             catch (Exception exception)
             {
-                logger.Warning(exception, $"Error while invoking event ({typeof(TEventArgs)}).");
+                logger.Warning(exception, $"Error while invoking event with arguments of type {typeof(TEventArgs)}.");
             }
         }
     }

Source/MQTTnet/Internal/AsyncQueue.cs

@@ -29,7 +29,7 @@ namespace MQTTnet.Internal
         {
             lock (_syncRoot)
             {
-                _signal?.Dispose();
+                _signal.Dispose();
 
                 _isDisposed = true;
             }
@@ -48,10 +48,10 @@ namespace MQTTnet.Internal
         {
             if (_queue.TryDequeue(out var item))
             {
-                return new AsyncQueueDequeueResult<TItem>(true, item);
+                return AsyncQueueDequeueResult<TItem>.Success(item);
             }
 
-            return new AsyncQueueDequeueResult<TItem>(false, default);
+            return AsyncQueueDequeueResult<TItem>.NonSuccess;
         }
 
         public async Task<AsyncQueueDequeueResult<TItem>> TryDequeueAsync(CancellationToken cancellationToken)
@@ -60,36 +60,42 @@ namespace MQTTnet.Internal
             {
                 try
                 {
-                    Task task;
+                    Task task = null;
                     lock (_syncRoot)
                     {
                         if (_isDisposed)
                         {
-                            return new AsyncQueueDequeueResult<TItem>(false, default);
+                            return AsyncQueueDequeueResult<TItem>.NonSuccess;
                         }
 
-                        task = _signal.WaitAsync(cancellationToken);
+                        if (_queue.IsEmpty)
+                        {
+                            task = _signal.WaitAsync(cancellationToken);
+                        }
                     }
 
-                    await task.ConfigureAwait(false);
-
+                    if (task != null)
+                    {
+                        await task.ConfigureAwait(false);    
+                    }
+                    
                     if (cancellationToken.IsCancellationRequested)
                     {
-                        return new AsyncQueueDequeueResult<TItem>(false, default);
+                        return AsyncQueueDequeueResult<TItem>.NonSuccess;
                     }
 
                     if (_queue.TryDequeue(out var item))
                     {
-                        return new AsyncQueueDequeueResult<TItem>(true, item);
+                        return AsyncQueueDequeueResult<TItem>.Success(item);
                     }
                 }
                 catch (OperationCanceledException)
                 {
-                    return new AsyncQueueDequeueResult<TItem>(false, default);
+                    return AsyncQueueDequeueResult<TItem>.NonSuccess;
                 }
             }
 
-            return new AsyncQueueDequeueResult<TItem>(false, default);
+            return AsyncQueueDequeueResult<TItem>.NonSuccess;
         }
     }
 }

Source/MQTTnet/Internal/AsyncQueueDequeueResult.cs

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

Source/MQTTnet/Internal/AsyncSignal.cs

@@ -33,9 +33,17 @@ namespace MQTTnet.Internal
             {
                 _isSignaled = true;
 
+                Cleanup();
+                
                 // If there is already a waiting task let it run.
-                _waiter?.Approve();
-                _waiter = null;
+                if (_waiter != null)
+                {
+                    _waiter.Approve();
+                    _waiter = null;
+
+                    // Since we already got a waiter the signal must be reset right now!
+                    _isSignaled = false;
+                }
             }
         }
 
@@ -47,8 +55,11 @@ namespace MQTTnet.Internal
             {
                 ThrowIfDisposed();
 
+                Cleanup();
+
                 if (_isSignaled)
                 {
+                    _isSignaled = false;
                     return CompletedTask.Instance;
                 }
 
@@ -58,7 +69,7 @@ namespace MQTTnet.Internal
                     {
                         throw new InvalidOperationException("Only one waiting task is permitted per async signal.");
                     }
-                    
+
                     _waiter.Dispose();
                 }
 
@@ -67,6 +78,15 @@ namespace MQTTnet.Internal
             }
         }
 
+        void Cleanup()
+        {
+            // Cleanup if the previous waiter was cancelled.
+            if (_waiter != null && _waiter.Task.IsCanceled)
+            {
+                _waiter = null;
+            }
+        }
+
         void ThrowIfDisposed()
         {
             if (_isDisposed)

Source/MQTTnet/Internal/MqttPacketBusItem.cs

@@ -10,40 +10,36 @@ namespace MQTTnet.Internal
 {
     public sealed class MqttPacketBusItem
     {
-        readonly AsyncTaskCompletionSource<int> _promise = new AsyncTaskCompletionSource<int>();
-
+        readonly AsyncTaskCompletionSource<bool> _promise = new AsyncTaskCompletionSource<bool>();
+        
         public MqttPacketBusItem(MqttPacket packet)
         {
             Packet = packet ?? throw new ArgumentNullException(nameof(packet));
         }
-        
+
+        public event EventHandler Completed;
+
         public MqttPacket Packet { get; }
 
-        public event EventHandler Delivered;
-
-        public Task WaitForDeliveryAsync()
-        {
-            return _promise.Task;
-        }
-        
-        public void MarkAsDelivered()
-        {
-            if (_promise.TrySetResult(0))
-            {
-                Delivered?.Invoke(this, EventArgs.Empty);
-            }
-        }
-
-        public void MarkAsFailed(Exception exception)
-        {
-            if (exception == null) throw new ArgumentNullException(nameof(exception));
-            
-            _promise.TrySetException(exception);
-        }
-
-        public void MarkAsCancelled()
+        public void Cancel()
         {
             _promise.TrySetCanceled();
         }
+
+        public void Complete()
+        {
+            _promise.TrySetResult(true);
+            Completed?.Invoke(this, EventArgs.Empty);
+        }
+
+        public void Fail(Exception exception)
+        {
+            _promise.TrySetException(exception);
+        }
+
+        public Task WaitAsync()
+        {
+            return _promise.Task;
+        }
     }
 }

Source/MQTTnet/MQTTnet.csproj.DotSettings

@@ -2,6 +2,7 @@
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=client_005Cconnecting/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=client_005Cdiagnostics/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=client_005Cdisconnecting/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=client_005Cexceptions/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=client_005Cextendedauthenticationexchange/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=client_005Coptions/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=client_005Cpublishing/@EntryIndexedValue">True</s:Boolean>

Source/MQTTnet/PacketDispatcher/MqttPacketDispatcher.cs

@@ -2,48 +2,103 @@
 // 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.Packets;
 using System;
 using System.Collections.Generic;
+using MQTTnet.Packets;
 
 namespace MQTTnet.PacketDispatcher
 {
-    public sealed class MqttPacketDispatcher
+    public sealed class MqttPacketDispatcher : IDisposable
     {
-        readonly List<IMqttPacketAwaitable> _awaitables = new List<IMqttPacketAwaitable>();
+        readonly List<IMqttPacketAwaitable> _waiters = new List<IMqttPacketAwaitable>();
 
-        public void FailAll(Exception exception)
+        bool _isDisposed;
+
+        public MqttPacketAwaitable<TResponsePacket> AddAwaitable<TResponsePacket>(ushort packetIdentifier) where TResponsePacket : MqttPacket
         {
-            if (exception == null) throw new ArgumentNullException(nameof(exception));
+            var awaitable = new MqttPacketAwaitable<TResponsePacket>(packetIdentifier, this);
 
-            lock (_awaitables)
+            lock (_waiters)
             {
-                foreach (var awaitable in _awaitables)
-                {
-                    awaitable.Fail(exception);
-                }
-
-                _awaitables.Clear();
+                _waiters.Add(awaitable);
             }
+
+            return awaitable;
         }
 
         public void CancelAll()
         {
-            lock (_awaitables)
+            lock (_waiters)
             {
-                foreach (var awaitable in _awaitables)
+                foreach (var awaitable in _waiters)
                 {
                     awaitable.Cancel();
                 }
 
-                _awaitables.Clear();
+                _waiters.Clear();
             }
         }
-        
+
+        public void Dispose()
+        {
+            Dispose(new ObjectDisposedException(nameof(MqttPacketDispatcher)));
+        }
+
+        public void Dispose(Exception exception)
+        {
+            if (exception == null)
+            {
+                throw new ArgumentNullException(nameof(exception));
+            }
+
+            lock (_waiters)
+            {
+                FailAll(exception);
+                
+                // Make sure that no task can start waiting after this instance is already disposed.
+                // This will prevent unexpected freezes.
+                _isDisposed = true;
+            }
+        }
+
+        public void FailAll(Exception exception)
+        {
+            if (exception == null)
+            {
+                throw new ArgumentNullException(nameof(exception));
+            }
+
+            lock (_waiters)
+            {
+                foreach (var awaitable in _waiters)
+                {
+                    awaitable.Fail(exception);
+                }
+
+                _waiters.Clear();
+            }
+        }
+
+        public void RemoveAwaitable(IMqttPacketAwaitable awaitable)
+        {
+            if (awaitable == null)
+            {
+                throw new ArgumentNullException(nameof(awaitable));
+            }
+
+            lock (_waiters)
+            {
+                _waiters.Remove(awaitable);
+            }
+        }
+
         public bool TryDispatch(MqttPacket packet)
         {
-            if (packet == null) throw new ArgumentNullException(nameof(packet));
-            
+            if (packet == null)
+            {
+                throw new ArgumentNullException(nameof(packet));
+            }
+
             ushort identifier = 0;
             if (packet is MqttPacketWithIdentifier packetWithIdentifier)
             {
@@ -51,13 +106,15 @@ namespace MQTTnet.PacketDispatcher
             }
 
             var packetType = packet.GetType();
-            var awaitables = new List<IMqttPacketAwaitable>();
-            
-            lock (_awaitables)
+            var waiters = new List<IMqttPacketAwaitable>();
+
+            lock (_waiters)
             {
-                for (var i = _awaitables.Count - 1; i >= 0; i--)
+                ThrowIfDisposed();
+                
+                for (var i = _waiters.Count - 1; i >= 0; i--)
                 {
-                    var entry = _awaitables[i];
+                    var entry = _waiters[i];
 
                     // Note: The PingRespPacket will also arrive here and has NO identifier but there
                     // is code which waits for it. So the code must be able to deal with filters which
@@ -66,39 +123,25 @@ namespace MQTTnet.PacketDispatcher
                     {
                         continue;
                     }
-                    
-                    awaitables.Add(entry);
-                    _awaitables.RemoveAt(i);
+
+                    waiters.Add(entry);
+                    _waiters.RemoveAt(i);
                 }
             }
-            
-            foreach (var matchingEntry in awaitables)
+
+            foreach (var matchingEntry in waiters)
             {
                 matchingEntry.Complete(packet);
             }
 
-            return awaitables.Count > 0;
-        }
-        
-        public MqttPacketAwaitable<TResponsePacket> AddAwaitable<TResponsePacket>(ushort packetIdentifier) where TResponsePacket : MqttPacket
-        {
-            var awaitable = new MqttPacketAwaitable<TResponsePacket>(packetIdentifier, this);
-
-            lock (_awaitables)
-            {
-                _awaitables.Add(awaitable);
-            }
-            
-            return awaitable;
+            return waiters.Count > 0;
         }
 
-        public void RemoveAwaitable(IMqttPacketAwaitable awaitable)
+        void ThrowIfDisposed()
         {
-            if (awaitable == null) throw new ArgumentNullException(nameof(awaitable));
-            
-            lock (_awaitables)
+            if (_isDisposed)
             {
-                _awaitables.Remove(awaitable);
+                throw new ObjectDisposedException(nameof(MqttPacketDispatcher));
             }
         }
     }

Source/MQTTnet/Protocol/MqttPubAckReasonCode.cs

@@ -7,7 +7,12 @@ namespace MQTTnet.Protocol
     public enum MqttPubAckReasonCode
     {
         Success = 0,
+        
+        /// <summary>
+        /// The message is accepted but there are no subscribers. This is sent only by the Server. If the Server knows that there are no matching subscribers, it MAY use this Reason Code instead of 0x00 (Success).
+        /// </summary>
         NoMatchingSubscribers = 16,
+        
         UnspecifiedError = 128,
         ImplementationSpecificError = 131,
         NotAuthorized = 135,

Source/MQTTnet/Server/Internal/DispatchApplicationMessageResult.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace MQTTnet.Server
+{
+    public sealed class DispatchApplicationMessageResult
+    {
+        public DispatchApplicationMessageResult(int matchingSubscribersCount)
+        {
+            MatchingSubscribersCount = matchingSubscribersCount;
+        }
+
+        public int MatchingSubscribersCount { get; }
+    }
+}

Source/MQTTnet/Server/Internal/MqttClient.cs

@@ -33,6 +33,7 @@ namespace MQTTnet.Server
         readonly Dictionary<ushort, string> _topicAlias = new Dictionary<ushort, string>();
 
         CancellationTokenSource _cancellationToken;
+        bool _disconnectPacketSent;
 
         public MqttClient(
             MqttConnectPacket connectPacket,
@@ -146,19 +147,22 @@ namespace MQTTnet.Server
         {
             IsRunning = false;
 
-            // Sending DISCONNECT packets from the server to the client is only supported when using MQTTv5+.
-            if (ChannelAdapter.PacketFormatterAdapter.ProtocolVersion == MqttProtocolVersion.V500)
+            if (!_disconnectPacketSent)
             {
-                // 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)
+                // Sending DISCONNECT packets from the server to the client is only supported when using MQTTv5+.
+                if (ChannelAdapter.PacketFormatterAdapter.ProtocolVersion == MqttProtocolVersion.V500)
                 {
-                    // 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);
+                    // 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)
+                    {
+                        // 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);
+                    }
                 }
             }
 
@@ -233,9 +237,10 @@ namespace MQTTnet.Server
                 return;
             }
 
+            DispatchApplicationMessageResult dispatchResult = null;
             if (processPublish && applicationMessage != null)
             {
-                await _sessionsManager.DispatchApplicationMessage(Id, applicationMessage).ConfigureAwait(false);
+                dispatchResult = await _sessionsManager.DispatchApplicationMessage(Id, applicationMessage).ConfigureAwait(false);
             }
 
             switch (publishPacket.QualityOfServiceLevel)
@@ -247,13 +252,13 @@ namespace MQTTnet.Server
                 }
                 case MqttQualityOfServiceLevel.AtLeastOnce:
                 {
-                    var pubAckPacket = _packetFactories.PubAck.Create(publishPacket, interceptingPublishEventArgs);
+                    var pubAckPacket = _packetFactories.PubAck.Create(publishPacket, interceptingPublishEventArgs, dispatchResult);
                     Session.EnqueueControlPacket(new MqttPacketBusItem(pubAckPacket));
                     break;
                 }
                 case MqttQualityOfServiceLevel.ExactlyOnce:
                 {
-                    var pubRecPacket = _packetFactories.PubRec.Create(publishPacket, interceptingPublishEventArgs);
+                    var pubRecPacket = _packetFactories.PubRec.Create(publishPacket, interceptingPublishEventArgs, dispatchResult);
                     Session.EnqueueControlPacket(new MqttPacketBusItem(pubRecPacket));
                     break;
                 }
@@ -282,7 +287,7 @@ namespace MQTTnet.Server
 
         async Task HandleIncomingSubscribePacket(MqttSubscribePacket subscribePacket, CancellationToken cancellationToken)
         {
-            var subscribeResult = await Session.SubscriptionsManager.Subscribe(subscribePacket, cancellationToken).ConfigureAwait(false);
+            var subscribeResult = await Session.Subscribe(subscribePacket, cancellationToken).ConfigureAwait(false);
 
             var subAckPacket = _packetFactories.SubAck.Create(subscribePacket, subscribeResult);
 
@@ -306,7 +311,7 @@ namespace MQTTnet.Server
 
         async Task HandleIncomingUnsubscribePacket(MqttUnsubscribePacket unsubscribePacket, CancellationToken cancellationToken)
         {
-            var unsubscribeResult = await Session.SubscriptionsManager.Unsubscribe(unsubscribePacket, cancellationToken).ConfigureAwait(false);
+            var unsubscribeResult = await Session.Unsubscribe(unsubscribePacket, cancellationToken).ConfigureAwait(false);
 
             var unsubAckPacket = _packetFactories.UnsubAck.Create(unsubscribePacket, unsubscribeResult);
 
@@ -484,7 +489,7 @@ namespace MQTTnet.Server
 
             try
             {
-                while (!cancellationToken.IsCancellationRequested)
+                while (!cancellationToken.IsCancellationRequested && !IsTakenOver && IsRunning)
                 {
                     packetBusItem = await Session.DequeuePacketAsync(cancellationToken).ConfigureAwait(false);
 
@@ -502,15 +507,15 @@ namespace MQTTnet.Server
                     try
                     {
                         await SendPacketAsync(packetBusItem.Packet, cancellationToken).ConfigureAwait(false);
-                        packetBusItem.MarkAsDelivered();
+                        packetBusItem.Complete();
                     }
                     catch (OperationCanceledException)
                     {
-                        packetBusItem.MarkAsCancelled();
+                        packetBusItem.Cancel();
                     }
                     catch (Exception exception)
                     {
-                        packetBusItem.MarkAsFailed(exception);
+                        packetBusItem.Fail(exception);
                     }
                     finally
                     {
@@ -558,7 +563,7 @@ namespace MQTTnet.Server
             catch (ObjectDisposedException)
             {
                 // This can happen when connections are created and dropped very quickly.
-                // It is not an issue if the cancellation token cannot be cancelled multiple times.
+                // It is not an issue if the cancellation token cannot be canceled multiple times.
             }
         }
 
@@ -566,6 +571,9 @@ namespace MQTTnet.Server
         {
             try
             {
+                // This also indicates that it was tried at least!
+                _disconnectPacketSent = true;
+                
                 var disconnectPacket = _packetFactories.Disconnect.Create(reasonCode);
 
                 using (var timeout = new CancellationTokenSource(_serverOptions.DefaultCommunicationTimeout))

Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs

@@ -127,21 +127,20 @@ namespace MQTTnet.Server
 
             _logger.Verbose("Session for client '{0}' deleted.", clientId);
         }
-
-        public async Task DispatchApplicationMessage(string senderId, MqttApplicationMessage applicationMessage)
+        
+        public async Task<DispatchApplicationMessageResult> DispatchApplicationMessage(string senderId, MqttApplicationMessage applicationMessage)
         {
+            var matchingSubscribersCount = 0;
             try
             {
                 if (applicationMessage.Retain)
                 {
                     await _retainedMessagesManager.UpdateMessage(senderId, applicationMessage).ConfigureAwait(false);
                 }
-
-                var deliveryCount = 0;
+                
                 List<MqttSession> subscriberSessions;
                 lock (_sessionsManagementLock)
                 {
-                    // only subscriber clients are of interest here.
                     subscriberSessions = _subscriberSessions.ToList();
                 }
 
@@ -150,11 +149,17 @@ namespace MQTTnet.Server
 
                 foreach (var session in subscriberSessions)
                 {
-                    var checkSubscriptionsResult = session.SubscriptionsManager.CheckSubscriptions(
+                    if(!session.TryCheckSubscriptions(
                         applicationMessage.Topic,
                         topicHash,
                         applicationMessage.QualityOfServiceLevel,
-                        senderId);
+                        senderId,
+                        out var checkSubscriptionsResult))
+                    {
+                        // Checking the subscriptions has failed for the session. The session
+                        // will be ignored.
+                        continue;
+                    }
 
                     if (!checkSubscriptionsResult.IsSubscribed)
                     {
@@ -181,17 +186,22 @@ namespace MQTTnet.Server
                     }
 
                     session.EnqueueDataPacket(new MqttPacketBusItem(newPublishPacket));
-                    deliveryCount++;
+                    matchingSubscribersCount++;
 
                     _logger.Verbose("Client '{0}': Queued PUBLISH packet with topic '{1}'.", session.Id, applicationMessage.Topic);
                 }
 
-                await FireApplicationMessageNotConsumedEvent(applicationMessage, deliveryCount, senderId);
+                if (matchingSubscribersCount == 0)
+                {
+                    await FireApplicationMessageNotConsumedEvent(applicationMessage, senderId).ConfigureAwait(false);
+                }
             }
             catch (Exception exception)
             {
                 _logger.Error(exception, "Unhandled exception while processing next queued application message.");
             }
+
+            return new DispatchApplicationMessageResult(matchingSubscribersCount);
         }
 
         public void Dispose()
@@ -301,7 +311,7 @@ namespace MQTTnet.Server
                         channelAdapter.Endpoint,
                         client.Session.Items);
 
-                    await _eventContainer.ClientConnectedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
+                    await _eventContainer.ClientConnectedEvent.TryInvokeAsync(eventArgs, _logger).ConfigureAwait(false);
                 }
 
                 await client.RunAsync().ConfigureAwait(false);
@@ -414,7 +424,7 @@ namespace MQTTnet.Server
 
             var clientSession = GetClientSession(clientId);
 
-            var subscribeResult = await clientSession.SubscriptionsManager.Subscribe(fakeSubscribePacket, CancellationToken.None).ConfigureAwait(false);
+            var subscribeResult = await clientSession.Subscribe(fakeSubscribePacket, CancellationToken.None).ConfigureAwait(false);
 
             if (subscribeResult.RetainedMessages != null)
             {
@@ -441,7 +451,7 @@ namespace MQTTnet.Server
             var fakeUnsubscribePacket = new MqttUnsubscribePacket();
             fakeUnsubscribePacket.TopicFilters.AddRange(topicFilters);
 
-            return GetClientSession(clientId).SubscriptionsManager.Unsubscribe(fakeUnsubscribePacket, CancellationToken.None);
+            return GetClientSession(clientId).Unsubscribe(fakeUnsubscribePacket, CancellationToken.None);
         }
 
         MqttClient CreateClient(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter, MqttSession session)
@@ -484,10 +494,14 @@ namespace MQTTnet.Server
 
             using (await _createConnectionSyncRoot.EnterAsync().ConfigureAwait(false))
             {
-                MqttSession session;
                 MqttSession oldSession;
+                MqttClient oldClient;
+                
                 lock (_sessionsManagementLock)
                 {
+                    MqttSession session;
+                    
+                    // Create a new session (if required).
                     if (!_sessions.TryGetValue(connectPacket.ClientId, out oldSession))
                     {
                         session = CreateSession(connectPacket.ClientId, validatingConnectionEventArgs.SessionItems, sessionShouldPersist);
@@ -505,6 +519,7 @@ namespace MQTTnet.Server
                             _logger.Verbose("Reusing existing session of client '{0}'.", connectPacket.ClientId);
                             session = oldSession;
                             oldSession = null;
+                            
                             // Session persistence could change for MQTT 5 clients that reconnect with different SessionExpiryInterval
                             session.IsPersistent = sessionShouldPersist;
                             connAckPacket.IsSessionPresent = true;
@@ -513,39 +528,41 @@ namespace MQTTnet.Server
                     }
 
                     _sessions[connectPacket.ClientId] = session;
+                    
+                    // Create a new client (always required).
+                    _clients.TryGetValue(connectPacket.ClientId, out oldClient);
+                    if (oldClient != null)
+                    {
+                        // This will stop the current client from sending and receiving but remains the connection active
+                        // for a later DISCONNECT packet.
+                        oldClient.IsTakenOver = true;
+                    }
+                    
+                    client = CreateClient(connectPacket, channelAdapter, session);
+
+                    _clients[connectPacket.ClientId] = client;
                 }
 
                 if (!connAckPacket.IsSessionPresent)
                 {
                     // TODO: This event is not yet final. It can already be used but restoring sessions from storage will be added later!
                     var preparingSessionEventArgs = new PreparingSessionEventArgs();
-                    await _eventContainer.PreparingSessionEvent.InvokeAsync(preparingSessionEventArgs).ConfigureAwait(false);
+                    await _eventContainer.PreparingSessionEvent.TryInvokeAsync(preparingSessionEventArgs, _logger).ConfigureAwait(false);
                 }
-
-                MqttClient existingClient;
-
-                lock (_clients)
+                
+                if (oldClient != null)
                 {
-                    _clients.TryGetValue(connectPacket.ClientId, out existingClient);
-                    client = CreateClient(connectPacket, channelAdapter, session);
-
-                    _clients[connectPacket.ClientId] = client;
-                }
-
-                if (existingClient != null)
-                {
-                    existingClient.IsTakenOver = true;
-                    await existingClient.StopAsync(MqttDisconnectReasonCode.SessionTakenOver).ConfigureAwait(false);
+                    await oldClient.StopAsync(MqttDisconnectReasonCode.SessionTakenOver).ConfigureAwait(false);
 
                     if (_eventContainer.ClientDisconnectedEvent.HasHandlers)
                     {
                         var eventArgs = new ClientDisconnectedEventArgs(
-                            existingClient.Id,
+                            oldClient.Id,
                             MqttClientDisconnectType.Takeover,
-                            existingClient.Endpoint,
-                            existingClient.Session.Items);
+                            oldClient.Endpoint,
+                            oldClient.Session.Items);
                         
-                        await _eventContainer.ClientDisconnectedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
+                        await _eventContainer.ClientDisconnectedEvent.TryInvokeAsync(eventArgs, _logger).ConfigureAwait(false);
                     }
                 }
 
@@ -562,13 +579,8 @@ namespace MQTTnet.Server
             return new MqttSession(clientId, isPersistent, sessionItems, _options, _eventContainer, _retainedMessagesManager, this);
         }
 
-        async Task FireApplicationMessageNotConsumedEvent(MqttApplicationMessage applicationMessage, int deliveryCount, string senderId)
+        async Task FireApplicationMessageNotConsumedEvent(MqttApplicationMessage applicationMessage, string senderId)
         {
-            if (deliveryCount > 0)
-            {
-                return;
-            }
-
             if (!_eventContainer.ApplicationMessageNotConsumedEvent.HasHandlers)
             {
                 return;

Source/MQTTnet/Server/Internal/MqttClientSubscriptionsManager.cs

@@ -7,6 +7,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
@@ -30,7 +31,7 @@ namespace MQTTnet.Server
         readonly Dictionary<string, MqttSubscription> _subscriptions = new Dictionary<string, MqttSubscription>();
 
         // Use subscription lock to maintain consistency across subscriptions and topic hash dictionaries
-        readonly SemaphoreSlim _subscriptionsLock = new SemaphoreSlim(1);
+        readonly AsyncLock _subscriptionsLock = new AsyncLock();
         readonly Dictionary<ulong, TopicHashMaskSubscriptions> _wildcardSubscriptionsByTopicHash = new Dictionary<ulong, TopicHashMaskSubscriptions>();
 
         public MqttClientSubscriptionsManager(
@@ -45,13 +46,12 @@ namespace MQTTnet.Server
             _subscriptionChangedNotification = subscriptionChangedNotification;
         }
 
-        public CheckSubscriptionsResult CheckSubscriptions(string topic, ulong topicHash, MqttQualityOfServiceLevel applicationMessageQoSLevel, string senderClientId)
+        public CheckSubscriptionsResult CheckSubscriptions(string topic, ulong topicHash, MqttQualityOfServiceLevel qualityOfServiceLevel, string senderId)
         {
             var possibleSubscriptions = new List<MqttSubscription>();
 
             // Check for possible subscriptions. They might have collisions but this is fine.
-            _subscriptionsLock.Wait();
-            try
+            using (_subscriptionsLock.EnterAsync(CancellationToken.None).GetAwaiter().GetResult())
             {
                 if (_noWildcardSubscriptionsByTopicHash.TryGetValue(topicHash, out var noWildcardSubscriptions))
                 {
@@ -63,18 +63,14 @@ namespace MQTTnet.Server
                     var wildcardSubscriptions = wcs.Value;
                     var subscriptionHash = wcs.Key;
                     var subscriptionHashMask = wildcardSubscriptions.HashMask;
-                    
+
                     if ((topicHash & subscriptionHashMask) == subscriptionHash)
                     {
                         possibleSubscriptions.AddRange(wildcardSubscriptions.Subscriptions.ToList());
                     }
                 }
             }
-            finally
-            {
-                _subscriptionsLock.Release();
-            }
-            
+
             // The pre check has evaluated that nothing is subscribed.
             // If there were some possible candidates they get checked below
             // again to avoid collisions.
@@ -83,7 +79,7 @@ namespace MQTTnet.Server
                 return CheckSubscriptionsResult.NotSubscribed;
             }
 
-            var senderIsReceiver = string.Equals(senderClientId, _session.Id);
+            var senderIsReceiver = string.Equals(senderId, _session.Id);
             var maxQoSLevel = -1; // Not subscribed.
 
             HashSet<uint> subscriptionIdentifiers = null;
@@ -136,7 +132,7 @@ namespace MQTTnet.Server
                 SubscriptionIdentifiers = subscriptionIdentifiers?.ToList() ?? EmptySubscriptionIdentifiers,
 
                 // Start with the same QoS as the publisher.
-                QualityOfServiceLevel = applicationMessageQoSLevel
+                QualityOfServiceLevel = qualityOfServiceLevel
             };
 
             // Now downgrade if required.
@@ -148,7 +144,7 @@ namespace MQTTnet.Server
             // Subscribing to a Topic Filter at QoS 2 is equivalent to saying "I would like to receive Messages matching this filter at the QoS with which they were published".
             // This means a publisher is responsible for determining the maximum QoS a Message can be delivered at, but a subscriber is able to require that the Server
             // downgrades the QoS to one more suitable for its usage.
-            if (maxQoSLevel < (int)applicationMessageQoSLevel)
+            if (maxQoSLevel < (int)qualityOfServiceLevel)
             {
                 result.QualityOfServiceLevel = (MqttQualityOfServiceLevel)maxQoSLevel;
             }
@@ -209,7 +205,7 @@ namespace MQTTnet.Server
             // This call will add the new subscription to the internal storage.
             // So the event _ClientSubscribedTopicEvent_ must be called afterwards.
             _subscriptionChangedNotification?.OnSubscriptionsAdded(_session, addedSubscriptions);
-            
+
             if (_eventContainer.ClientSubscribedTopicEvent.HasHandlers)
             {
                 foreach (var finalTopicFilter in finalTopicFilters)
@@ -218,23 +214,22 @@ namespace MQTTnet.Server
                     await _eventContainer.ClientSubscribedTopicEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
                 }
             }
-            
+
             return result;
         }
 
-        public async Task<MqttUnsubscribeResult> Unsubscribe(MqttUnsubscribePacket unsubscribePacket, CancellationToken cancellationToken)
+        public async Task<UnsubscribeResult> Unsubscribe(MqttUnsubscribePacket unsubscribePacket, CancellationToken cancellationToken)
         {
             if (unsubscribePacket == null)
             {
                 throw new ArgumentNullException(nameof(unsubscribePacket));
             }
 
-            var result = new MqttUnsubscribeResult();
+            var result = new UnsubscribeResult();
 
             var removedSubscriptions = new List<string>();
 
-            await _subscriptionsLock.WaitAsync(cancellationToken).ConfigureAwait(false);
-            try
+            using (await _subscriptionsLock.EnterAsync(cancellationToken).ConfigureAwait(false))
             {
                 foreach (var topicFilter in unsubscribePacket.TopicFilters)
                 {
@@ -265,7 +260,7 @@ namespace MQTTnet.Server
                         if (existingSubscription != null)
                         {
                             var topicHash = existingSubscription.TopicHash;
-                            
+
                             if (existingSubscription.TopicHasWildcard)
                             {
                                 if (_wildcardSubscriptionsByTopicHash.TryGetValue(topicHash, out var subscriptions))
@@ -294,11 +289,8 @@ namespace MQTTnet.Server
                     }
                 }
             }
-            finally
-            {
-                _subscriptionsLock.Release();
-                _subscriptionChangedNotification?.OnSubscriptionsRemoved(_session, removedSubscriptions);
-            }
+
+            _subscriptionChangedNotification?.OnSubscriptionsRemoved(_session, removedSubscriptions);
 
             if (_eventContainer.ClientUnsubscribedTopicEvent.HasHandlers)
             {
@@ -345,8 +337,7 @@ namespace MQTTnet.Server
 
             // Add to subscriptions and maintain topic hash dictionaries
 
-            _subscriptionsLock.Wait();
-            try
+            using (_subscriptionsLock.EnterAsync(CancellationToken.None).GetAwaiter().GetResult())
             {
                 MqttSubscription.CalculateTopicHash(topicFilter.Topic, out var topicHash, out var topicHashMask, out var hasWildcard);
 
@@ -396,10 +387,6 @@ namespace MQTTnet.Server
                     subscriptions.Add(subscription);
                 }
             }
-            finally
-            {
-                _subscriptionsLock.Release();
-            }
 
             return new CreateSubscriptionResult
             {
@@ -493,23 +480,20 @@ namespace MQTTnet.Server
 
         async Task<InterceptingUnsubscriptionEventArgs> InterceptUnsubscribe(string topicFilter, MqttSubscription mqttSubscription, CancellationToken cancellationToken)
         {
-            var clientUnsubscribingTopicEventArgs = new InterceptingUnsubscriptionEventArgs(cancellationToken, topicFilter, _session.Items, topicFilter);
-
-            if (mqttSubscription == null)
+            var clientUnsubscribingTopicEventArgs = new InterceptingUnsubscriptionEventArgs(cancellationToken, topicFilter, _session.Items, topicFilter)
             {
-                clientUnsubscribingTopicEventArgs.Response.ReasonCode = MqttUnsubscribeReasonCode.NoSubscriptionExisted;
-            }
-            else
-            {
-                clientUnsubscribingTopicEventArgs.Response.ReasonCode = MqttUnsubscribeReasonCode.Success;
-            }
+                Response =
+                {
+                    ReasonCode = mqttSubscription == null ? MqttUnsubscribeReasonCode.NoSubscriptionExisted : MqttUnsubscribeReasonCode.Success
+                }
+            };
 
             await _eventContainer.InterceptingUnsubscriptionEvent.InvokeAsync(clientUnsubscribingTopicEventArgs).ConfigureAwait(false);
 
             return clientUnsubscribingTopicEventArgs;
         }
 
-        public sealed class CreateSubscriptionResult
+        sealed class CreateSubscriptionResult
         {
             public bool IsNewSubscription { get; set; }
 

Source/MQTTnet/Server/Internal/MqttSession.cs

@@ -4,7 +4,6 @@
 
 using System;
 using System.Collections;
-using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
@@ -23,8 +22,10 @@ namespace MQTTnet.Server
         readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new MqttPacketIdentifierProvider();
 
         readonly MqttServerOptions _serverOptions;
+        readonly MqttClientSubscriptionsManager _subscriptionsManager;
 
-        readonly ConcurrentDictionary<ushort, MqttPublishPacket> _unacknowledgedPublishPackets = new ConcurrentDictionary<ushort, MqttPublishPacket>();
+        // Do not use a dictionary in order to keep the ordering of the messages.
+        readonly List<MqttPublishPacket> _unacknowledgedPublishPackets = new List<MqttPublishPacket>();
 
         // Bookkeeping to know if this is a subscribing client; lazy initialize later.
         HashSet<string> _subscribedTopics;
@@ -45,7 +46,7 @@ namespace MQTTnet.Server
             _serverOptions = serverOptions ?? throw new ArgumentNullException(nameof(serverOptions));
             _clientSessionsManager = clientSessionsManager ?? throw new ArgumentNullException(nameof(clientSessionsManager));
 
-            SubscriptionsManager = new MqttClientSubscriptionsManager(this, eventContainer, retainedMessagesManager, clientSessionsManager);
+            _subscriptionsManager = new MqttClientSubscriptionsManager(this, eventContainer, retainedMessagesManager, clientSessionsManager);
         }
 
         public DateTime CreatedTimestamp { get; } = DateTime.UtcNow;
@@ -67,21 +68,18 @@ namespace MQTTnet.Server
 
         public long PendingDataPacketsCount => _packetBus.PartitionItemsCount(MqttPacketBusPartition.Data);
 
-        public MqttClientSubscriptionsManager SubscriptionsManager { get; }
-
         public bool WillMessageSent { get; set; }
 
-        public MqttPublishPacket PeekAcknowledgePublishPacket(ushort packetIdentifier)
-        {
-            // This will only return the matching PUBLISH packet but does not remove it.
-            // This is required for QoS 2.
-            _unacknowledgedPublishPackets.TryGetValue(packetIdentifier, out var publishPacket);
-            return publishPacket;
-        }
-        
         public MqttPublishPacket AcknowledgePublishPacket(ushort packetIdentifier)
         {
-            _unacknowledgedPublishPackets.TryRemove(packetIdentifier, out var publishPacket);
+            MqttPublishPacket publishPacket;
+
+            lock (_unacknowledgedPublishPackets)
+            {
+                publishPacket = _unacknowledgedPublishPackets.FirstOrDefault(p => p.PacketIdentifier.Equals(packetIdentifier));
+                _unacknowledgedPublishPackets.Remove(publishPacket);
+            }
+
             return publishPacket;
         }
 
@@ -107,8 +105,8 @@ namespace MQTTnet.Server
 
         public void Dispose()
         {
-            _packetBus?.Dispose();
-            SubscriptionsManager.Dispose();
+            _packetBus.Dispose();
+            _subscriptionsManager.Dispose();
         }
 
         public void EnqueueControlPacket(MqttPacketBusItem packetBusItem)
@@ -139,7 +137,10 @@ namespace MQTTnet.Server
             {
                 publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
 
-                _unacknowledgedPublishPackets[publishPacket.PacketIdentifier] = publishPacket;
+                lock (_unacknowledgedPublishPackets)
+                {
+                    _unacknowledgedPublishPackets.Add(publishPacket);
+                }
             }
 
             _packetBus.EnqueueItem(packetBusItem, MqttPacketBusPartition.Data);
@@ -150,14 +151,48 @@ namespace MQTTnet.Server
             _packetBus.EnqueueItem(packetBusItem, MqttPacketBusPartition.Health);
         }
 
+        public MqttPublishPacket PeekAcknowledgePublishPacket(ushort packetIdentifier)
+        {
+            // This will only return the matching PUBLISH packet but does not remove it.
+            // This is required for QoS 2.
+            lock (_unacknowledgedPublishPackets)
+            {
+                return _unacknowledgedPublishPackets.FirstOrDefault(p => p.PacketIdentifier.Equals(packetIdentifier));
+            }
+        }
+
         public void Recover()
         {
             // TODO: Keep the bus and only insert pending items again.
             // TODO: Check if packet identifier must be restarted or not.
             // TODO: Recover package identifier.
+            
+            /*
+                The Session state in the Client consists of:
+                ·         QoS 1 and QoS 2 messages which have been sent to the Server, but have not been completely acknowledged.
+                ·         QoS 2 messages which have been received from the Server, but have not been completely acknowledged. 
+
+                The Session state in the Server consists of:
+                ·         The existence of a Session, even if the rest of the Session state is empty.
+                ·         The Client’s subscriptions.
+                ·         QoS 1 and QoS 2 messages which have been sent to the Client, but have not been completely acknowledged.
+                ·         QoS 1 and QoS 2 messages pending transmission to the Client.
+                ·         QoS 2 messages which have been received from the Client, but have not been completely acknowledged.
+                ·         Optionally, QoS 0 messages pending transmission to the Client. 
+             */
+
+            // Create a copy of all currently unacknowledged publish packets and clear the storage.
+            // We must re-enqueue them in order to trigger other code.
+            List<MqttPublishPacket> unacknowledgedPublishPackets;
+            lock (_unacknowledgedPublishPackets)
+            {
+                unacknowledgedPublishPackets = _unacknowledgedPublishPackets.ToList();
+                _unacknowledgedPublishPackets.Clear();    
+            }
+
             _packetBus.Clear();
 
-            foreach (var publishPacket in _unacknowledgedPublishPackets.Values.ToList())
+            foreach (var publishPacket in unacknowledgedPublishPackets)
             {
                 EnqueueDataPacket(new MqttPacketBusItem(publishPacket));
             }
@@ -167,5 +202,30 @@ namespace MQTTnet.Server
         {
             _subscribedTopics?.Remove(topic);
         }
+
+        public Task<SubscribeResult> Subscribe(MqttSubscribePacket subscribePacket, CancellationToken cancellationToken)
+        {
+            return _subscriptionsManager.Subscribe(subscribePacket, cancellationToken);
+        }
+
+        public bool TryCheckSubscriptions(string topic, ulong topicHash, MqttQualityOfServiceLevel qualityOfServiceLevel, string senderId, out CheckSubscriptionsResult result)
+        {
+            result = null;
+
+            try
+            {
+                result = _subscriptionsManager.CheckSubscriptions(topic, topicHash, qualityOfServiceLevel, senderId);
+                return true;
+            }
+            catch
+            {
+                return false;
+            }
+        }
+
+        public Task<UnsubscribeResult> Unsubscribe(MqttUnsubscribePacket unsubscribePacket, CancellationToken cancellationToken)
+        {
+            return _subscriptionsManager.Unsubscribe(unsubscribePacket, cancellationToken);
+        }
     }
 }

Source/MQTTnet/Server/Internal/MqttSubscription.cs

@@ -179,7 +179,7 @@ namespace MQTTnet.Server
             RetainAsPublished = retainAsPublished;
             GrantedQualityOfServiceLevel = qualityOfServiceLevel;
             Identifier = identifier;
-            
+
             CalculateTopicHash(Topic, out var hash, out var hashMask, out var hasWildcard);
             TopicHash = hash;
             TopicHashMask = hashMask;

Source/MQTTnet/Server/Internal/UnsubscribeResult.cs

@@ -7,7 +7,7 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttUnsubscribeResult
+    public sealed class UnsubscribeResult
     {
         public List<MqttUnsubscribeReasonCode> ReasonCodes { get; } = new List<MqttUnsubscribeReasonCode>(128);
         

Source/MQTTnet/Server/MqttServerExtensions.cs

@@ -3,29 +3,89 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Text;
 using System.Threading.Tasks;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
+using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
     public static class MqttServerExtensions
     {
+        public static Task InjectApplicationMessage(
+            this MqttServer server,
+            string topic,
+            string payload = null,
+            MqttQualityOfServiceLevel qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce,
+            bool retain = false)
+        {
+            if (server == null)
+            {
+                throw new ArgumentNullException(nameof(server));
+            }
+
+            if (topic == null)
+            {
+                throw new ArgumentNullException(nameof(topic));
+            }
+
+            var payloadBuffer = EmptyBuffer.Array;
+            if (payload is string stringPayload)
+            {
+                payloadBuffer = Encoding.UTF8.GetBytes(stringPayload);
+            }
+
+            return server.InjectApplicationMessage(
+                new InjectedMqttApplicationMessage(
+                    new MqttApplicationMessage
+                    {
+                        Topic = topic,
+                        Payload = payloadBuffer,
+                        QualityOfServiceLevel = qualityOfServiceLevel,
+                        Retain = retain
+                    }));
+        }
+
         public static Task SubscribeAsync(this MqttServer server, string clientId, params MqttTopicFilter[] topicFilters)
         {
-            if (server == null) throw new ArgumentNullException(nameof(server));
-            if (clientId == null) throw new ArgumentNullException(nameof(clientId));
-            if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
+            if (server == null)
+            {
+                throw new ArgumentNullException(nameof(server));
+            }
+
+            if (clientId == null)
+            {
+                throw new ArgumentNullException(nameof(clientId));
+            }
+
+            if (topicFilters == null)
+            {
+                throw new ArgumentNullException(nameof(topicFilters));
+            }
 
             return server.SubscribeAsync(clientId, topicFilters);
         }
-        
+
         public static Task SubscribeAsync(this MqttServer server, string clientId, string topic)
         {
-            if (server == null) throw new ArgumentNullException(nameof(server));
-            if (clientId == null) throw new ArgumentNullException(nameof(clientId));
-            if (topic == null) throw new ArgumentNullException(nameof(topic));
+            if (server == null)
+            {
+                throw new ArgumentNullException(nameof(server));
+            }
 
-            return server.SubscribeAsync(clientId, new MqttTopicFilterBuilder().WithTopic(topic).Build());
+            if (clientId == null)
+            {
+                throw new ArgumentNullException(nameof(clientId));
+            }
+
+            if (topic == null)
+            {
+                throw new ArgumentNullException(nameof(topic));
+            }
+
+            var topicFilters = new MqttTopicFilterBuilder().WithTopic(topic).Build();
+            return server.SubscribeAsync(clientId, topicFilters);
         }
     }
-}
+}

Source/MQTTnet/Server/Status/MqttSessionStatus.cs

@@ -45,7 +45,7 @@ namespace MQTTnet.Server
             var packetBusItem = new MqttPacketBusItem(publishPacketFactory.Create(applicationMessage));
             _session.EnqueueDataPacket(packetBusItem);
 
-            return packetBusItem.WaitForDeliveryAsync();
+            return packetBusItem.WaitAsync();
         }
         
         public Task DeleteAsync()