Refactor subscribe and unsubscribe to support MQTTv5.

Build/MQTTnet.nuspec

@@ -13,6 +13,7 @@
     <releaseNotes> * [Core] Added support for MQTTv5 packages.
 * [Client] Added new MQTTv5 features to options builder.
 * [Client] Added uniform API across all supported MQTT versions (BREAKING CHANGE!)
+* [Client] The client will now avoid sending an ACK if an exception has been thrown in message handler (thanks to @ramonsmits).
 * [Server] Added support for MQTTv5 clients. The server will still return _success_ for all cases at the moment even if more granular codes are available.
 
 * [Note] Due to MQTTv5 a lot of new classes were introduced. This required adding new namespaces as well. Most classes are backward compatible but new namespaces must be added.

MQTTnet.noUWP.sln

@@ -3,8 +3,6 @@ Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio 15
 VisualStudioVersion = 15.0.27004.2010
 MinimumVisualStudioVersion = 10.0.40219.1
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "MQTTnet.Core.Tests", "Tests\MQTTnet.Core.Tests\MQTTnet.Core.Tests.csproj", "{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}"
-EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Tests", "Tests", "{9248C2E1-B9D6-40BF-81EC-86004D7765B4}"
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Source", "Source", "{32A630A7-2598-41D7-B625-204CD906F5FB}"
@@ -45,6 +43,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "MQTTnet.Extensions.ManagedC
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "MQTTnet.AspNetCore.Tests", "Tests\MQTTnet.AspNetCore.Tests\MQTTnet.AspNetCore.Tests.csproj", "{61B62223-F5D0-48E4-BBD6-2CBA9353CB5E}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "MQTTnet.Tests", "Tests\MQTTnet.Core.Tests\MQTTnet.Tests.csproj", "{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -57,22 +57,6 @@ Global
 		Release|x86 = Release|x86
 	EndGlobalSection
 	GlobalSection(ProjectConfigurationPlatforms) = postSolution
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Debug|ARM.ActiveCfg = Debug|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Debug|ARM.Build.0 = Debug|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Debug|x64.ActiveCfg = Debug|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Debug|x64.Build.0 = Debug|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Debug|x86.ActiveCfg = Debug|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Debug|x86.Build.0 = Debug|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Release|Any CPU.Build.0 = Release|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Release|ARM.ActiveCfg = Release|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Release|ARM.Build.0 = Release|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Release|x64.ActiveCfg = Release|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Release|x64.Build.0 = Release|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Release|x86.ActiveCfg = Release|Any CPU
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC}.Release|x86.Build.0 = Release|Any CPU
 		{3587E506-55A2-4EB3-99C7-DC01E42D25D2}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{3587E506-55A2-4EB3-99C7-DC01E42D25D2}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{3587E506-55A2-4EB3-99C7-DC01E42D25D2}.Debug|ARM.ActiveCfg = Debug|Any CPU
@@ -201,12 +185,27 @@ Global
 		{61B62223-F5D0-48E4-BBD6-2CBA9353CB5E}.Release|x64.Build.0 = Release|Any CPU
 		{61B62223-F5D0-48E4-BBD6-2CBA9353CB5E}.Release|x86.ActiveCfg = Release|Any CPU
 		{61B62223-F5D0-48E4-BBD6-2CBA9353CB5E}.Release|x86.Build.0 = Release|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Debug|ARM.ActiveCfg = Debug|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Debug|ARM.Build.0 = Debug|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Debug|x64.Build.0 = Debug|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Debug|x86.Build.0 = Debug|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Release|Any CPU.Build.0 = Release|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Release|ARM.ActiveCfg = Release|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Release|ARM.Build.0 = Release|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Release|x64.ActiveCfg = Release|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Release|x64.Build.0 = Release|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Release|x86.ActiveCfg = Release|Any CPU
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
 	EndGlobalSection
 	GlobalSection(NestedProjects) = preSolution
-		{A7FF0C91-25DE-4BA6-B39E-F54E8DADF1CC} = {9248C2E1-B9D6-40BF-81EC-86004D7765B4}
 		{3587E506-55A2-4EB3-99C7-DC01E42D25D2} = {32A630A7-2598-41D7-B625-204CD906F5FB}
 		{3D283AAD-AAA8-4339-8394-52F80B6304DB} = {9248C2E1-B9D6-40BF-81EC-86004D7765B4}
 		{C6FF8AEA-0855-41EC-A1F3-AC262225BAB9} = {9248C2E1-B9D6-40BF-81EC-86004D7765B4}
@@ -215,6 +214,7 @@ Global
 		{998D04DD-7CB0-45F5-A393-E2495C16399E} = {9248C2E1-B9D6-40BF-81EC-86004D7765B4}
 		{C400533A-8EBA-4F0B-BF4D-295C3708604B} = {12816BCC-AF9E-44A9-9AE5-C246AF2A0587}
 		{61B62223-F5D0-48E4-BBD6-2CBA9353CB5E} = {9248C2E1-B9D6-40BF-81EC-86004D7765B4}
+		{9C7106CA-96B8-4ABE-B3B4-9357AB8ACB41} = {9248C2E1-B9D6-40BF-81EC-86004D7765B4}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {07536672-5CBC-4BE3-ACE0-708A431A7894}

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs

@@ -5,6 +5,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Client;
 using MQTTnet.Client.Publishing;
+using MQTTnet.Client.Receiving;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
 using MQTTnet.Internal;
@@ -52,6 +53,12 @@ namespace MQTTnet.Extensions.ManagedClient
         public event EventHandler<MqttClientConnectedEventArgs> Connected;
         public event EventHandler<MqttClientDisconnectedEventArgs> Disconnected;
 
+        public IMqttApplicationMessageHandler ReceivedApplicationMessageHandler
+        {
+            get => _mqttClient.ReceivedApplicationMessageHandler;
+            set => _mqttClient.ReceivedApplicationMessageHandler = value;
+        }
+
         public event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived;
         public event EventHandler<ApplicationMessageProcessedEventArgs> ApplicationMessageProcessed;
         public event EventHandler<ApplicationMessageSkippedEventArgs> ApplicationMessageSkipped;
@@ -372,12 +379,12 @@ namespace MQTTnet.Extensions.ManagedClient
             {
                 if (unsubscriptions.Any())
                 {
-                    await _mqttClient.UnsubscribeAsync(unsubscriptions).ConfigureAwait(false);
+                    await _mqttClient.UnsubscribeAsync(unsubscriptions.ToArray()).ConfigureAwait(false);
                 }
 
                 if (subscriptions.Any())
                 {
-                    await _mqttClient.SubscribeAsync(subscriptions).ConfigureAwait(false);
+                    await _mqttClient.SubscribeAsync(subscriptions.ToArray()).ConfigureAwait(false);
                 }
             }
             catch (Exception exception)

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -174,12 +174,18 @@ namespace MQTTnet.Adapter
 
         private async Task<ReceivedMqttPacket> ReceiveAsync(CancellationToken cancellationToken)
         {
-            var fixedHeader = await _packetReader.ReadFixedHeaderAsync(_fixedHeaderBuffer, cancellationToken).ConfigureAwait(false);
+            var readFixedHeaderResult = await _packetReader.ReadFixedHeaderAsync(_fixedHeaderBuffer, cancellationToken).ConfigureAwait(false);
 
             try
             {
+                if (readFixedHeaderResult.ConnectionClosed)
+                {
+                    return null;
+                }
+
                 ReadingPacketStarted?.Invoke(this, EventArgs.Empty);
 
+                var fixedHeader = readFixedHeaderResult.FixedHeader;
                 if (fixedHeader.RemainingLength == 0)
                 {
                     return new ReceivedMqttPacket(fixedHeader.Flags, null, 2);
@@ -205,9 +211,16 @@ namespace MQTTnet.Adapter
                     var readBytes = _channel.ReadAsync(body, bodyOffset, chunkSize, cancellationToken).ConfigureAwait(false).GetAwaiter().GetResult();
 #endif
 
-                    cancellationToken.ThrowIfCancellationRequested();
-                    ExceptionHelper.ThrowIfGracefulSocketClose(readBytes);
+                    if (cancellationToken.IsCancellationRequested)
+                    {
+                        return null;
+                    }
 
+                    if (readBytes == 0)
+                    {
+                        return null;
+                    }
+                    
                     bodyOffset += readBytes;
                 } while (bodyOffset < body.Length);
 

Source/MQTTnet/Client/IMqttClient.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using System.Threading.Tasks;
 using MQTTnet.Client.Connecting;
 using MQTTnet.Client.Disconnecting;
@@ -20,7 +19,7 @@ namespace MQTTnet.Client
         Task<MqttClientConnectResult> ConnectAsync(IMqttClientOptions options);
         Task DisconnectAsync(MqttClientDisconnectOptions options);
 
-        Task<MqttClientSubscribeResult> SubscribeAsync(IEnumerable<TopicFilter> topicFilters);
-        Task<MqttClientUnsubscribeResult> UnsubscribeAsync(IEnumerable<string> topics);
+        Task<MqttClientSubscribeResult> SubscribeAsync(MqttClientSubscribeOptions options);
+        Task<MqttClientUnsubscribeResult> UnsubscribeAsync(MqttClientUnsubscribeOptions options);
     }
 }

Source/MQTTnet/Client/MqttClient.cs

@@ -1,20 +1,19 @@
 using System;
-using System.Collections.Generic;
 using System.Diagnostics;
-using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
 using MQTTnet.Client.Connecting;
 using MQTTnet.Client.Disconnecting;
 using MQTTnet.Client.Options;
-using MQTTnet.Client.PacketDispatcher;
 using MQTTnet.Client.Publishing;
+using MQTTnet.Client.Receiving;
 using MQTTnet.Client.Subscribing;
 using MQTTnet.Client.Unsubscribing;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
+using MQTTnet.PacketDispatcher;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
@@ -23,16 +22,18 @@ namespace MQTTnet.Client
     public class MqttClient : IMqttClient
     {
         private readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new MqttPacketIdentifierProvider();
+        private readonly MqttPacketDispatcher _packetDispatcher = new MqttPacketDispatcher();
         private readonly Stopwatch _sendTracker = new Stopwatch();
         private readonly object _disconnectLock = new object();
-        private readonly MqttPacketDispatcher _packetDispatcher = new MqttPacketDispatcher();
 
         private readonly IMqttClientAdapterFactory _adapterFactory;
         private readonly IMqttNetChildLogger _logger;
 
         private CancellationTokenSource _cancellationTokenSource;
-        internal Task _packetReceiverTask;
-        internal Task _keepAliveMessageSenderTask;
+        private Task _packetReceiverTask;
+        private Task _keepAlivePacketsSenderTask;
+        private Task _backgroundWorkerTask;
+
         private IMqttChannelAdapter _adapter;
         private bool _cleanDisconnectInitiated;
         private long _disconnectGate;
@@ -47,6 +48,8 @@ namespace MQTTnet.Client
 
         public event EventHandler<MqttClientConnectedEventArgs> Connected;
         public event EventHandler<MqttClientDisconnectedEventArgs> Disconnected;
+
+        public IMqttApplicationMessageHandler ReceivedApplicationMessageHandler { get; set; }
         public event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived;
 
         public bool IsConnected { get; private set; }
@@ -66,28 +69,31 @@ namespace MQTTnet.Client
                 _packetIdentifierProvider.Reset();
                 _packetDispatcher.Reset();
 
-                var cancellationTokenSource = new CancellationTokenSource();
-                _cancellationTokenSource = cancellationTokenSource;
+                _cancellationTokenSource = new CancellationTokenSource();
+                var cancellationToken = _cancellationTokenSource.Token;
 
                 _disconnectGate = 0;
-                _adapter = _adapterFactory.CreateClientAdapter(options, _logger);
+                var adapter = _adapterFactory.CreateClientAdapter(options, _logger);
+                _adapter = adapter;
 
                 _logger.Verbose($"Trying to connect with server ({Options.ChannelOptions}).");
-                await _adapter.ConnectAsync(Options.CommunicationTimeout, cancellationTokenSource.Token).ConfigureAwait(false);
+                await _adapter.ConnectAsync(Options.CommunicationTimeout, cancellationToken).ConfigureAwait(false);
                 _logger.Verbose("Connection with server established.");
 
-                StartReceivingPackets(cancellationTokenSource.Token);
+                _packetReceiverTask = Task.Run(() => TryReceivePacketsAsync(cancellationToken), cancellationToken);
 
-                var connectResult = await AuthenticateAsync(options.WillMessage, cancellationTokenSource.Token).ConfigureAwait(false);
+                var connectResult = await AuthenticateAsync(adapter, options.WillMessage, cancellationToken).ConfigureAwait(false);
                 _logger.Verbose("MQTT connection with server established.");
 
                 _sendTracker.Restart();
 
                 if (Options.KeepAlivePeriod != TimeSpan.Zero)
                 {
-                    StartSendingKeepAliveMessages(cancellationTokenSource.Token);
+                    _keepAlivePacketsSenderTask = Task.Run(() => TrySendKeepAliveMessagesAsync(cancellationToken), cancellationToken);
                 }
 
+                _backgroundWorkerTask = Task.Run(() => TryProcessReceivedPacketsAsync(cancellationToken), cancellationToken);
+
                 IsConnected = true;
                 Connected?.Invoke(this, new MqttClientConnectedEventArgs(connectResult));
 
@@ -129,35 +135,27 @@ namespace MQTTnet.Client
             }
         }
 
-        public async Task<MqttClientSubscribeResult> SubscribeAsync(IEnumerable<TopicFilter> topicFilters)
+        public async Task<MqttClientSubscribeResult> SubscribeAsync(MqttClientSubscribeOptions options)
         {
-            if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
+            if (options == null) throw new ArgumentNullException(nameof(options));
 
             ThrowIfNotConnected();
 
-            var subscribePacket = new MqttSubscribePacket
-            {
-                PacketIdentifier = _packetIdentifierProvider.GetNewPacketIdentifier()
-            };
-
-            subscribePacket.TopicFilters.AddRange(topicFilters);
+            var subscribePacket = _adapter.PacketFormatterAdapter.DataConverter.CreateSubscribePacket(options);
+            subscribePacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
 
             var subAckPacket = await SendAndReceiveAsync<MqttSubAckPacket>(subscribePacket, _cancellationTokenSource.Token).ConfigureAwait(false);
             return _adapter.PacketFormatterAdapter.DataConverter.CreateClientSubscribeResult(subscribePacket, subAckPacket);
         }
 
-        public async Task<MqttClientUnsubscribeResult> UnsubscribeAsync(IEnumerable<string> topicFilters)
+        public async Task<MqttClientUnsubscribeResult> UnsubscribeAsync(MqttClientUnsubscribeOptions options)
         {
-            if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
+            if (options == null) throw new ArgumentNullException(nameof(options));
 
             ThrowIfNotConnected();
 
-            var unsubscribePacket = new MqttUnsubscribePacket
-            {
-                PacketIdentifier = _packetIdentifierProvider.GetNewPacketIdentifier()
-            };
-
-            unsubscribePacket.TopicFilters.AddRange(topicFilters);
+            var unsubscribePacket = _adapter.PacketFormatterAdapter.DataConverter.CreateUnsubscribePacket(options);
+            unsubscribePacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
 
             var unsubAckPacket = await SendAndReceiveAsync<MqttUnsubAckPacket>(unsubscribePacket, _cancellationTokenSource.Token).ConfigureAwait(false);
             return _adapter.PacketFormatterAdapter.DataConverter.CreateClientUnsubscribeResult(unsubscribePacket, unsubAckPacket);
@@ -181,7 +179,7 @@ namespace MQTTnet.Client
                     }
                 case MqttQualityOfServiceLevel.AtLeastOnce:
                     {
-                        publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNewPacketIdentifier();
+                        publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
                         var response = await SendAndReceiveAsync<MqttPubAckPacket>(publishPacket, _cancellationTokenSource.Token).ConfigureAwait(false);
 
                         var result = new MqttClientPublishResult();
@@ -213,15 +211,14 @@ namespace MQTTnet.Client
             _adapter = null;
         }
 
-        private async Task<MqttClientConnectResult> AuthenticateAsync(MqttApplicationMessage willApplicationMessage, CancellationToken cancellationToken)
+        private async Task<MqttClientConnectResult> AuthenticateAsync(IMqttChannelAdapter channelAdapter, MqttApplicationMessage willApplicationMessage, CancellationToken cancellationToken)
         {
-            var connectPacket = _adapter.PacketFormatterAdapter.DataConverter.CreateConnectPacket(
+            var connectPacket = channelAdapter.PacketFormatterAdapter.DataConverter.CreateConnectPacket(
                 willApplicationMessage,
                 Options);
 
             var connAckPacket = await SendAndReceiveAsync<MqttConnAckPacket>(connectPacket, cancellationToken).ConfigureAwait(false);
-
-            var result = _adapter.PacketFormatterAdapter.DataConverter.CreateClientConnectResult(connAckPacket);
+            var result = channelAdapter.PacketFormatterAdapter.DataConverter.CreateClientConnectResult(connAckPacket);
 
             if (result.ResultCode != MqttClientConnectResultCode.Success)
             {
@@ -257,7 +254,8 @@ namespace MQTTnet.Client
                 }
 
                 await WaitForTaskAsync(_packetReceiverTask, sender).ConfigureAwait(false);
-                await WaitForTaskAsync(_keepAliveMessageSenderTask, sender).ConfigureAwait(false);
+                await WaitForTaskAsync(_keepAlivePacketsSenderTask, sender).ConfigureAwait(false);
+                await WaitForTaskAsync(_backgroundWorkerTask, sender).ConfigureAwait(false);
 
                 _logger.Verbose("Disconnected from adapter.");
             }
@@ -316,26 +314,22 @@ namespace MQTTnet.Client
                 identifier = packetWithIdentifier.PacketIdentifier.Value;
             }
 
-            var packetAwaiter = _packetDispatcher.AddPacketAwaiter<TResponsePacket>(identifier);
-            try
+            using (var packetAwaiter = _packetDispatcher.AddPacketAwaiter<TResponsePacket>(identifier))
             {
-                await _adapter.SendPacketAsync(requestPacket, cancellationToken).ConfigureAwait(false);
-                return await packetAwaiter.WaitOneAsync(Options.CommunicationTimeout);
-
-                //return (TResponsePacket)await Internal.TaskExtensions.TimeoutAfterAsync(ct => packetAwaiter.Task, Options.CommunicationTimeout, cancellationToken).ConfigureAwait(false);
-            }
-            catch (MqttCommunicationTimedOutException)
-            {
-                _logger.Warning(null, "Timeout while waiting for packet of type '{0}'.", typeof(TResponsePacket).Namespace);
-                throw;
-            }
-            finally
-            {
-                _packetDispatcher.RemovePacketAwaiter<TResponsePacket>(identifier);
+                try
+                {
+                    await _adapter.SendPacketAsync(requestPacket, cancellationToken).ConfigureAwait(false);
+                    return await packetAwaiter.WaitOneAsync(Options.CommunicationTimeout).ConfigureAwait(false);
+                }
+                catch (MqttCommunicationTimedOutException)
+                {
+                    _logger.Warning(null, "Timeout while waiting for packet of type '{0}'.", typeof(TResponsePacket).Namespace);
+                    throw;
+                }
             }
         }
 
-        private async Task SendKeepAliveMessagesAsync(CancellationToken cancellationToken)
+        private async Task TrySendKeepAliveMessagesAsync(CancellationToken cancellationToken)
         {
             try
             {
@@ -380,7 +374,7 @@ namespace MQTTnet.Client
 
                 if (!DisconnectIsPending())
                 {
-                    await DisconnectInternalAsync(_keepAliveMessageSenderTask, exception).ConfigureAwait(false);
+                    await DisconnectInternalAsync(_keepAlivePacketsSenderTask, exception).ConfigureAwait(false);
                 }
             }
             finally
@@ -389,7 +383,7 @@ namespace MQTTnet.Client
             }
         }
 
-        private async Task ReceivePacketsAsync(CancellationToken cancellationToken)
+        private async Task TryReceivePacketsAsync(CancellationToken cancellationToken)
         {
             try
             {
@@ -399,10 +393,17 @@ namespace MQTTnet.Client
                 {
                     var packet = await _adapter.ReceivePacketAsync(TimeSpan.Zero, cancellationToken).ConfigureAwait(false);
 
-                    if (packet != null && !cancellationToken.IsCancellationRequested)
+                    if (cancellationToken.IsCancellationRequested)
+                    {
+                        return;
+                    }
+
+                    if (packet == null && !DisconnectIsPending())
                     {
-                        await ProcessReceivedPacketAsync(packet, cancellationToken).ConfigureAwait(false);
+                        await DisconnectInternalAsync(_packetReceiverTask, null).ConfigureAwait(false);
                     }
+
+                    _packetDispatcher.Dispatch(packet);
                 }
             }
             catch (Exception exception)
@@ -437,79 +438,146 @@ namespace MQTTnet.Client
             }
         }
 
-        private Task ProcessReceivedPacketAsync(MqttBasePacket packet, CancellationToken cancellationToken)
+        private async Task TryProcessReceivedPacketAsync(MqttBasePacket packet, CancellationToken cancellationToken)
         {
-            if (packet is MqttPublishPacket publishPacket)
+            try
             {
-                return ProcessReceivedPublishPacketAsync(publishPacket, cancellationToken);
+                if (packet is MqttPublishPacket publishPacket)
+                {
+                    await TryProcessReceivedPublishPacketAsync(publishPacket, cancellationToken).ConfigureAwait(false);
+                }
+                else if (packet is MqttPingReqPacket)
+                {
+                    await SendAsync(new MqttPingRespPacket(), cancellationToken).ConfigureAwait(false);
+                }
+                else if (packet is MqttDisconnectPacket)
+                {
+                    await DisconnectAsync(null).ConfigureAwait(false);
+                }
+                else
+                {
+                    throw new MqttProtocolViolationException($"Received packet '{packet}' at an unexpected time.");
+                }
             }
-
-            if (packet is MqttPingReqPacket)
+            catch (Exception exception)
             {
-                return SendAsync(new MqttPingRespPacket(), cancellationToken);
-            }
+                if (_cleanDisconnectInitiated)
+                {
+                    return;
+                }
 
-            if (packet is MqttDisconnectPacket)
-            {
-                return DisconnectAsync(null);
-            }
+                if (exception is OperationCanceledException)
+                {
+                }
+                else if (exception is MqttCommunicationException)
+                {
+                    _logger.Warning(exception, "MQTT communication exception while receiving packets.");
+                }
+                else
+                {
+                    _logger.Error(exception, "Unhandled exception while receiving packets.");
+                }
 
-            if (packet is MqttPubRelPacket pubRelPacket)
-            {
-                return ProcessReceivedPubRelPacket(pubRelPacket, cancellationToken);
-            }
+                _packetDispatcher.Dispatch(exception);
 
-            _packetDispatcher.Dispatch(packet);
-            return Task.FromResult(0);
+                if (!DisconnectIsPending())
+                {
+                    await DisconnectInternalAsync(_packetReceiverTask, exception).ConfigureAwait(false);
+                }
+            }
         }
 
-        private Task ProcessReceivedPublishPacketAsync(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
+        private async Task TryProcessReceivedPublishPacketAsync(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
         {
-            if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtMostOnce)
+            try
             {
-                FireApplicationMessageReceivedEvent(publishPacket);
-                return Task.FromResult(0);
-            }
+                if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtMostOnce)
+                {
+                    await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
+                }
+                else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtLeastOnce)
+                {
+                    await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
 
-            if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtLeastOnce)
-            {
-                FireApplicationMessageReceivedEvent(publishPacket);
-                return SendAsync(new MqttPubAckPacket { PacketIdentifier = publishPacket.PacketIdentifier, ReasonCode = MqttPubAckReasonCode.Success }, cancellationToken);
-            }
+                    await SendAsync(new MqttPubAckPacket
+                    {
+                        PacketIdentifier = publishPacket.PacketIdentifier,
+                        ReasonCode = MqttPubAckReasonCode.Success
+                    }, cancellationToken).ConfigureAwait(false);
+                }
+                else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.ExactlyOnce)
+                {
+                    var pubRecPacket = new MqttPubRecPacket
+                    {
+                        PacketIdentifier = publishPacket.PacketIdentifier,
+                        ReasonCode = MqttPubRecReasonCode.Success
+                    };
 
-            if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.ExactlyOnce)
+                    var pubRelPacket = await SendAndReceiveAsync<MqttPubRelPacket>(pubRecPacket, cancellationToken).ConfigureAwait(false);
+
+                    // QoS 2 is implement as method "B" (4.3.3 QoS 2: Exactly once delivery)
+                    await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
+
+                    await SendAsync(new MqttPubCompPacket
+                    {
+                        PacketIdentifier = pubRelPacket.PacketIdentifier,
+                        ReasonCode = MqttPubCompReasonCode.Success
+                    }, cancellationToken).ConfigureAwait(false);
+                }
+                else
+                {
+                    throw new MqttProtocolViolationException("Received a not supported QoS level.");
+                }
+            }
+            catch (Exception exception)
             {
-                // QoS 2 is implement as method "B" (4.3.3 QoS 2: Exactly once delivery)
-                FireApplicationMessageReceivedEvent(publishPacket);
-                return SendAsync(new MqttPubRecPacket { PacketIdentifier = publishPacket.PacketIdentifier, ReasonCode = MqttPubRecReasonCode.Success }, cancellationToken);
+                _logger.Error(exception, "Unhandled exception while handling application message.");
             }
-
-            throw new MqttCommunicationException("Received a not supported QoS level.");
         }
 
-        private Task ProcessReceivedPubRelPacket(MqttPubRelPacket pubRelPacket, CancellationToken cancellationToken)
+        private async Task TryProcessReceivedPacketsAsync(CancellationToken cancellationToken)
         {
-            var response = new MqttPubCompPacket
+            try
             {
-                PacketIdentifier = pubRelPacket.PacketIdentifier,
-                ReasonCode = MqttPubCompReasonCode.Success
-            };
+                while (!cancellationToken.IsCancellationRequested)
+                {
+                    var packet = _packetDispatcher.Take(cancellationToken);
+
+                    if (cancellationToken.IsCancellationRequested)
+                    {
+                        return;
+                    }
 
-            return SendAsync(response, cancellationToken);
+                    await TryProcessReceivedPacketAsync(packet, cancellationToken).ConfigureAwait(false);
+                }
+            }
+            catch (OperationCanceledException)
+            {
+            }
+            catch (Exception exception)
+            {
+                _logger.Error(exception, "Error while processing packet.");
+            }
         }
 
         private async Task<MqttClientPublishResult> PublishExactlyOnceAsync(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
         {
-            publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNewPacketIdentifier();
+            publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
 
             var pubRecPacket = await SendAndReceiveAsync<MqttPubRecPacket>(publishPacket, cancellationToken).ConfigureAwait(false);
+
+            // TODO: Check response code.
+
             var pubRelPacket = new MqttPubRelPacket
             {
-                PacketIdentifier = pubRecPacket.PacketIdentifier,
+                PacketIdentifier = publishPacket.PacketIdentifier,
                 ReasonCode = MqttPubRelReasonCode.Success
             };
 
             var pubCompPacket = await SendAndReceiveAsync<MqttPubCompPacket>(pubRelPacket, cancellationToken).ConfigureAwait(false);
+
+            // TODO: Check response code.
+
             var result = new MqttClientPublishResult();
 
             if (pubRecPacket.ReasonCode != null)
@@ -520,35 +588,38 @@ namespace MQTTnet.Client
             return result;
         }
 
-        private void StartReceivingPackets(CancellationToken cancellationToken)
+        ////private void StartReceivingPackets(CancellationToken cancellationToken)
+        ////{
+        ////    _packetReceiverTask = Task.Factory.StartNew(
+        ////        () => TryReceivePacketsAsync(cancellationToken),
+        ////        cancellationToken,
+        ////        TaskCreationOptions.LongRunning,
+        ////        TaskScheduler.Default).Unwrap();
+        ////}
+
+        ////private void StartSendingKeepAliveMessages(CancellationToken cancellationToken)
+        ////{
+        ////    _keepAlivePacketsSenderTask = Task.Factory.StartNew(
+        ////        () => TrySendKeepAliveMessagesAsync(cancellationToken),
+        ////        cancellationToken,
+        ////        TaskCreationOptions.LongRunning,
+        ////        TaskScheduler.Default).Unwrap();
+        ////}
+
+        private Task HandleReceivedApplicationMessageAsync(MqttPublishPacket publishPacket)
         {
-            _packetReceiverTask = Task.Factory.StartNew(
-                () => ReceivePacketsAsync(cancellationToken),
-                cancellationToken,
-                TaskCreationOptions.LongRunning,
-                TaskScheduler.Default).Unwrap();
-        }
+            var applicationMessage = _adapter.PacketFormatterAdapter.DataConverter.CreateApplicationMessage(publishPacket);
 
-        private void StartSendingKeepAliveMessages(CancellationToken cancellationToken)
-        {
-            _keepAliveMessageSenderTask = Task.Factory.StartNew(
-                () => SendKeepAliveMessagesAsync(cancellationToken),
-                cancellationToken,
-                TaskCreationOptions.LongRunning,
-                TaskScheduler.Default).Unwrap();
-        }
+            ApplicationMessageReceived?.Invoke(this, new MqttApplicationMessageReceivedEventArgs(Options.ClientId, applicationMessage));
 
-        private void FireApplicationMessageReceivedEvent(MqttPublishPacket publishPacket)
-        {
-            try
+            var handler = ReceivedApplicationMessageHandler;
+            if (handler != null)
             {
-                var applicationMessage = _adapter.PacketFormatterAdapter.DataConverter.CreateApplicationMessage(publishPacket);
-                ApplicationMessageReceived?.Invoke(this, new MqttApplicationMessageReceivedEventArgs(Options.ClientId, applicationMessage));
-            }
-            catch (Exception exception)
-            {
-                _logger.Error(exception, "Unhandled exception while handling application message.");
+                return handler.HandleApplicationMessageAsync(
+                    new MqttApplicationMessageHandlerContext(Options.ClientId, applicationMessage));
             }
+
+            return Task.FromResult(0);
         }
 
         private static async Task WaitForTaskAsync(Task task, Task sender)

Source/MQTTnet/Client/MqttClientExtensions.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Client.Receiving;
 using MQTTnet.Client.Subscribing;
 using MQTTnet.Client.Unsubscribing;
 using MQTTnet.Protocol;
@@ -8,6 +9,51 @@ namespace MQTTnet.Client
 {
     public static class MqttClientExtensions
     {
+        public static IMqttClient UseReceivedApplicationMessageHandler(this IMqttClient client, Func<MqttApplicationMessageHandlerContext, Task> handler)
+        {
+            if (handler == null)
+            {
+                client.ReceivedApplicationMessageHandler = null;
+                return client;
+            }
+
+            client.ReceivedApplicationMessageHandler = new MqttApplicationMessageHandlerDelegate(handler);
+
+            return client;
+        }
+
+        public static IMqttClient UseReceivedApplicationMessageHandler(this IMqttClient client, Action<MqttApplicationMessageHandlerContext> handler)
+        {
+            if (handler == null)
+            {
+                client.ReceivedApplicationMessageHandler = null;
+                return client;
+            }
+
+            client.ReceivedApplicationMessageHandler = new MqttApplicationMessageHandlerDelegate(handler);
+
+            return client;
+        }
+
+        public static IMqttClient UseReceivedApplicationMessageHandler(this IMqttClient client, IMqttApplicationMessageHandler handler)
+        {
+            client.ReceivedApplicationMessageHandler = handler;
+
+            return client;
+        }
+
+        public static Task ReconnectAsync(this IMqttClient client)
+        {
+            if (client == null) throw new ArgumentNullException(nameof(client));
+
+            if (client.Options == null)
+            {
+                throw new InvalidOperationException("_ReconnectAsync_ can be used only if _ConnectAsync_ was called before.");
+            }
+
+            return client.ConnectAsync(client.Options);
+        }
+
         public static Task DisconnectAsync(this IMqttClient client)
         {
             if (client == null) throw new ArgumentNullException(nameof(client));
@@ -20,7 +66,10 @@ namespace MQTTnet.Client
             if (client == null) throw new ArgumentNullException(nameof(client));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            return client.SubscribeAsync(topicFilters);
+            var options = new MqttClientSubscribeOptions();
+            options.TopicFilters.AddRange(topicFilters);
+
+            return client.SubscribeAsync(options);
         }
 
         public static Task<MqttClientSubscribeResult> SubscribeAsync(this IMqttClient client, string topic, MqttQualityOfServiceLevel qualityOfServiceLevel)
@@ -44,7 +93,10 @@ namespace MQTTnet.Client
             if (client == null) throw new ArgumentNullException(nameof(client));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            return client.UnsubscribeAsync(topicFilters);
+            var options = new MqttClientUnsubscribeOptions();
+            options.TopicFilters.AddRange(topicFilters);
+
+            return client.UnsubscribeAsync(options);
         }
     }
 }

Source/MQTTnet/Client/MqttPacketIdentifierProvider.cs

@@ -13,7 +13,7 @@
             }
         }
 
-        public ushort GetNewPacketIdentifier()
+        public ushort GetNextPacketIdentifier()
         {
             lock (_syncRoot)
             {

Source/MQTTnet/Client/Options/MqttClientOptions.cs

@@ -7,7 +7,7 @@ namespace MQTTnet.Client.Options
     {
         public string ClientId { get; set; } = Guid.NewGuid().ToString("N");
         public bool CleanSession { get; set; } = true;
-        public IMqttClientCredentials Credentials { get; set; } = new MqttClientCredentials();
+        public IMqttClientCredentials Credentials { get; set; }
         public MqttProtocolVersion ProtocolVersion { get; set; } = MqttProtocolVersion.V311;
 
         public IMqttClientChannelOptions ChannelOptions { get; set; }

Source/MQTTnet/Client/PacketDispatcher/MqttPacketAwaiter.cs

@@ -1,37 +0,0 @@
-using System;
-using System.Threading;
-using System.Threading.Tasks;
-using MQTTnet.Packets;
-
-namespace MQTTnet.Client.PacketDispatcher
-{
-    public sealed class MqttPacketAwaiter<TPacket> : IMqttPacketAwaiter where TPacket : MqttBasePacket
-    {
-        private readonly TaskCompletionSource<MqttBasePacket> _packet = new TaskCompletionSource<MqttBasePacket>();
-
-        public async Task<TPacket> WaitOneAsync(TimeSpan timeout)
-        {
-            using (var timeoutToken = new CancellationTokenSource(timeout))
-            {
-                timeoutToken.Token.Register(() => _packet.TrySetCanceled());
-
-                var packet = await _packet.Task.ConfigureAwait(false);
-                return (TPacket)packet;
-            }
-        }
-
-        public void Complete(MqttBasePacket packet)
-        {
-            if (packet == null) throw new ArgumentNullException(nameof(packet));
-
-            _packet.TrySetResult(packet);
-        }
-
-        public void Fail(Exception exception)
-        {
-            if (exception == null) throw new ArgumentNullException(nameof(exception));
-
-            _packet.TrySetException(exception);
-        }
-    }
-}

Source/MQTTnet/Client/Receiving/IMqttApplicationMessageHandler.cs

@@ -0,0 +1,9 @@
+using System.Threading.Tasks;
+
+namespace MQTTnet.Client.Receiving
+{
+    public interface IMqttApplicationMessageHandler
+    {
+        Task HandleApplicationMessageAsync(MqttApplicationMessageHandlerContext context);
+    }
+}

Source/MQTTnet/Client/Receiving/MqttApplicationMessageHandlerContext.cs

@@ -0,0 +1,15 @@
+namespace MQTTnet.Client.Receiving
+{
+    public class MqttApplicationMessageHandlerContext
+    {
+        public MqttApplicationMessageHandlerContext(string senderClientId, MqttApplicationMessage applicationMessage)
+        {
+            SenderClientId = senderClientId;
+            ApplicationMessage = applicationMessage;
+        }
+
+        public string SenderClientId { get; }
+
+        public MqttApplicationMessage ApplicationMessage { get; }
+    }
+}

Source/MQTTnet/Client/Receiving/MqttApplicationMessageHandlerDelegate.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Client.Receiving
+{
+    public class MqttApplicationMessageHandlerDelegate : IMqttApplicationMessageHandler
+    {
+        private readonly Func<MqttApplicationMessageHandlerContext, Task> _handler;
+
+        public MqttApplicationMessageHandlerDelegate(Action<MqttApplicationMessageHandlerContext> handler)
+        {
+            if (handler == null) throw new ArgumentNullException(nameof(handler));
+
+            _handler = context =>
+            {
+                handler(context);
+                return Task.FromResult(0);
+            };
+        }
+
+        public MqttApplicationMessageHandlerDelegate(Func<MqttApplicationMessageHandlerContext, Task> handler)
+        {
+            _handler = handler ?? throw new ArgumentNullException(nameof(handler));
+        }
+
+        public Task HandleApplicationMessageAsync(MqttApplicationMessageHandlerContext context)
+        {
+            return _handler(context);
+        }
+    }
+}

Source/MQTTnet/Client/Subscribing/MqttClientSubscribeOptions.cs

@@ -0,0 +1,12 @@
+using System.Collections.Generic;
+using MQTTnet.Packets;
+
+namespace MQTTnet.Client.Subscribing
+{
+    public class MqttClientSubscribeOptions
+    {
+        public List<TopicFilter> TopicFilters { get; set; } = new List<TopicFilter>();
+
+        public List<MqttUserProperty> UserProperties { get; set; } = new List<MqttUserProperty>();
+    }
+}

Source/MQTTnet/Client/Unsubscribing/MqttClientUnsubscribeOptions.cs

@@ -0,0 +1,12 @@
+using System.Collections.Generic;
+using MQTTnet.Packets;
+
+namespace MQTTnet.Client.Unsubscribing
+{
+    public class MqttClientUnsubscribeOptions
+    {
+        public List<string> TopicFilters { get; set; } = new List<string>();
+
+        public List<MqttUserProperty> UserProperties { get; set; } = new List<MqttUserProperty>();
+    }
+}

Source/MQTTnet/Exceptions/MqttCommunicationClosedGracefullyException.cs

@@ -1,6 +0,0 @@
-namespace MQTTnet.Exceptions
-{
-    public class MqttCommunicationClosedGracefullyException : MqttCommunicationException
-    {
-    }
-}

Source/MQTTnet/Formatter/IMqttDataConverter.cs

@@ -19,5 +19,9 @@ namespace MQTTnet.Formatter
         MqttClientSubscribeResult CreateClientSubscribeResult(MqttSubscribePacket subscribePacket, MqttSubAckPacket subAckPacket);
 
         MqttClientUnsubscribeResult CreateClientUnsubscribeResult(MqttUnsubscribePacket unsubscribePacket, MqttUnsubAckPacket unsubAckPacket);
+
+        MqttSubscribePacket CreateSubscribePacket(MqttClientSubscribeOptions options);
+
+        MqttUnsubscribePacket CreateUnsubscribePacket(MqttClientUnsubscribeOptions options);
     }
 }

Source/MQTTnet/Formatter/MqttPacketReader.cs

@@ -18,7 +18,7 @@ namespace MQTTnet.Formatter
             _channel = channel ?? throw new ArgumentNullException(nameof(channel));
         }
 
-        public async Task<MqttFixedHeader> ReadFixedHeaderAsync(byte[] fixedHeaderBuffer, CancellationToken cancellationToken)
+        public async Task<ReadFixedHeaderResult> ReadFixedHeaderAsync(byte[] fixedHeaderBuffer, CancellationToken cancellationToken)
         {
             // The MQTT fixed header contains 1 byte of flags and at least 1 byte for the remaining data length.
             // So in all cases at least 2 bytes must be read for a complete MQTT packet.
@@ -32,15 +32,25 @@ namespace MQTTnet.Formatter
                 var bytesRead = await _channel.ReadAsync(buffer, totalBytesRead, buffer.Length - totalBytesRead, cancellationToken).ConfigureAwait(false);
 
                 cancellationToken.ThrowIfCancellationRequested();
-                ExceptionHelper.ThrowIfGracefulSocketClose(bytesRead);
 
+                if (bytesRead == 0)
+                {
+                    return new ReadFixedHeaderResult
+                    {
+                        ConnectionClosed = true
+                    };
+                }
+                
                 totalBytesRead += bytesRead;
             }
 
             var hasRemainingLength = buffer[1] != 0;
             if (!hasRemainingLength)
             {
-                return new MqttFixedHeader(buffer[0], 0, totalBytesRead);
+                return new ReadFixedHeaderResult
+                {
+                    FixedHeader = new MqttFixedHeader(buffer[0], 0, totalBytesRead)
+                };
             }
 
 #if WINDOWS_UWP
@@ -54,12 +64,23 @@ namespace MQTTnet.Formatter
             var bodyLength = ReadBodyLength(buffer[1], cancellationToken);
 #endif
 
-            totalBytesRead += bodyLength;
-            return new MqttFixedHeader(buffer[0], bodyLength, totalBytesRead);
+            if (!bodyLength.HasValue)
+            {
+                return new ReadFixedHeaderResult
+                {
+                    ConnectionClosed = true
+                };
+            }
+
+            totalBytesRead += bodyLength.Value;
+            return new ReadFixedHeaderResult
+            {
+                FixedHeader = new MqttFixedHeader(buffer[0], bodyLength.Value, totalBytesRead)
+            };
         }
 
 #if !WINDOWS_UWP
-        private int ReadBodyLength(byte initialEncodedByte, CancellationToken cancellationToken)
+        private int? ReadBodyLength(byte initialEncodedByte, CancellationToken cancellationToken)
         {
             var offset = 0;
             var multiplier = 128;
@@ -74,9 +95,18 @@ namespace MQTTnet.Formatter
                     throw new MqttProtocolViolationException("Remaining length is invalid.");
                 }
 
-                cancellationToken.ThrowIfCancellationRequested();
+                if (cancellationToken.IsCancellationRequested)
+                {
+                    return null;
+                }
+
+                var buffer = ReadByte(cancellationToken);
+                if (!buffer.HasValue)
+                {
+                    return null;
+                }
 
-                encodedByte = ReadByte(cancellationToken);
+                encodedByte = buffer.Value;
 
                 value += (encodedByte & 127) * multiplier;
                 multiplier *= 128;
@@ -85,15 +115,18 @@ namespace MQTTnet.Formatter
             return value;
         }
 
-        private byte ReadByte(CancellationToken cancellationToken)
+        private byte? ReadByte(CancellationToken cancellationToken)
         {
             var readCount = _channel.ReadAsync(_singleByteBuffer, 0, 1, cancellationToken).ConfigureAwait(false).GetAwaiter().GetResult();
 
-            cancellationToken.ThrowIfCancellationRequested();
+            if (cancellationToken.IsCancellationRequested)
+            {
+                return null;
+            }
 
-            if (readCount <= 0)
+            if (readCount == 0)
             {
-                ExceptionHelper.ThrowGracefulSocketClose();
+                return null;
             }
 
             return _singleByteBuffer[0];
@@ -101,7 +134,7 @@ namespace MQTTnet.Formatter
 
 #else
         
-        private async Task<int> ReadBodyLengthAsync(byte initialEncodedByte, CancellationToken cancellationToken)
+        private async Task<int?> ReadBodyLengthAsync(byte initialEncodedByte, CancellationToken cancellationToken)
         {
             var offset = 0;
             var multiplier = 128;
@@ -116,9 +149,18 @@ namespace MQTTnet.Formatter
                     throw new MqttProtocolViolationException("Remaining length is invalid.");
                 }
 
-                cancellationToken.ThrowIfCancellationRequested();
+                if (cancellationToken.IsCancellationRequested)
+                {
+                    return null;
+                }
+
+                var buffer = await ReadByteAsync(cancellationToken).ConfigureAwait(false);
+                if (!buffer.HasValue)
+                {
+                    return null;
+                }
 
-                encodedByte = await ReadByteAsync(cancellationToken).ConfigureAwait(false);
+                encodedByte = buffer.Value;
 
                 value += (encodedByte & 127) * multiplier;
                 multiplier *= 128;
@@ -127,15 +169,18 @@ namespace MQTTnet.Formatter
             return value;
         }
 
-        private async Task<byte> ReadByteAsync(CancellationToken cancellationToken)
+        private async Task<byte?> ReadByteAsync(CancellationToken cancellationToken)
         {
             var readCount = await _channel.ReadAsync(_singleByteBuffer, 0, 1, cancellationToken).ConfigureAwait(false);
 
-            cancellationToken.ThrowIfCancellationRequested();
+            if (cancellationToken.IsCancellationRequested)
+            {
+                return null;
+            }
 
-            if (readCount <= 0)
+            if (readCount == 0)
             {
-                ExceptionHelper.ThrowGracefulSocketClose();
+                return null;
             }
 
             return _singleByteBuffer[0];

Source/MQTTnet/Formatter/ReadFixedHeaderResult.cs

@@ -0,0 +1,9 @@
+namespace MQTTnet.Formatter
+{
+    public class ReadFixedHeaderResult
+    {
+        public bool ConnectionClosed { get; set; }
+
+        public MqttFixedHeader FixedHeader { get; set; }
+    }
+}

Source/MQTTnet/Formatter/V3/MqttV310DataConverter.cs

@@ -143,5 +143,35 @@ namespace MQTTnet.Formatter.V3
 
             return result;
         }
+
+        public MqttSubscribePacket CreateSubscribePacket(MqttClientSubscribeOptions options)
+        {
+            if (options == null) throw new ArgumentNullException(nameof(options));
+
+            if (options.UserProperties?.Any() == true)
+            {
+                throw new MqttProtocolViolationException("User properties are not supported in MQTT version 3.");
+            }
+
+            var subscribePacket = new MqttSubscribePacket();
+            subscribePacket.TopicFilters.AddRange(options.TopicFilters);
+            
+            return subscribePacket;
+        }
+
+        public MqttUnsubscribePacket CreateUnsubscribePacket(MqttClientUnsubscribeOptions options)
+        {
+            if (options == null) throw new ArgumentNullException(nameof(options));
+
+            if (options.UserProperties?.Any() == true)
+            {
+                throw new MqttProtocolViolationException("User properties are not supported in MQTT version 3.");
+            }
+
+            var unsubscribePacket = new MqttUnsubscribePacket();
+            unsubscribePacket.TopicFilters.AddRange(options.TopicFilters);
+
+            return unsubscribePacket;
+        }
     }
 }

Source/MQTTnet/Formatter/V5/MqttV500DataConverter.cs

@@ -128,5 +128,35 @@ namespace MQTTnet.Formatter.V5
 
             return result;
         }
+
+        public MqttSubscribePacket CreateSubscribePacket(MqttClientSubscribeOptions options)
+        {
+            if (options == null) throw new ArgumentNullException(nameof(options));
+
+            var packet = new MqttSubscribePacket
+            {
+                Properties = new MqttSubscribePacketProperties()
+            };
+
+            packet.TopicFilters.AddRange(options.TopicFilters);
+            packet.Properties.UserProperties.AddRange(options.UserProperties);
+
+            return packet;
+        }
+
+        public MqttUnsubscribePacket CreateUnsubscribePacket(MqttClientUnsubscribeOptions options)
+        {
+            if (options == null) throw new ArgumentNullException(nameof(options));
+
+            var packet = new MqttUnsubscribePacket
+            {
+                Properties = new MqttUnsubscribePacketProperties()
+            };
+
+            packet.TopicFilters.AddRange(options.TopicFilters);
+            packet.Properties.UserProperties.AddRange(options.UserProperties);
+
+            return packet;
+        }
     }
 }

Source/MQTTnet/IApplicationMessageReceiver.cs

@@ -1,9 +1,13 @@
 using System;
+using MQTTnet.Client.Receiving;
 
 namespace MQTTnet
 {
     public interface IApplicationMessageReceiver
     {
+        IMqttApplicationMessageHandler ReceivedApplicationMessageHandler { get; set; }
+
+        [Obsolete("Use _ReceivedApplicationMessageHandler_ instead.")]
         event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived;
     }
 }

Source/MQTTnet/Internal/ExceptionHelper.cs

@@ -1,20 +0,0 @@
-using MQTTnet.Exceptions;
-
-namespace MQTTnet.Internal
-{
-    public static class ExceptionHelper
-    {
-        public static void ThrowGracefulSocketClose()
-        {
-            throw new MqttCommunicationClosedGracefullyException();
-        }
-
-        public static void ThrowIfGracefulSocketClose(int readBytesCount)
-        {
-            if (readBytesCount <= 0)
-            {
-                throw new MqttCommunicationClosedGracefullyException();
-            }
-        }
-    }
-}

Source/MQTTnet/MQTTnet.csproj

@@ -63,20 +63,8 @@
   <ItemGroup Condition="'$(TargetFramework)'=='net461'">
   </ItemGroup>
 
-  <ItemGroup>
-    <Compile Remove="NewFolder\**" />
-    <EmbeddedResource Remove="NewFolder\**" />
-    <None Remove="NewFolder\**" />
-  </ItemGroup>
-
   <ItemGroup>
     <PackageReference Include="Microsoft.NETCore.UniversalWindowsPlatform" Version="6.2.2" />
   </ItemGroup>
 
-  <ItemGroup>
-    <Reference Include="System.Net.Requests">
-      <HintPath>C:\Program Files\dotnet\sdk\NuGetFallbackFolder\microsoft.netcore.app\2.1.0\ref\netcoreapp2.1\System.Net.Requests.dll</HintPath>
-    </Reference>
-  </ItemGroup>
-
 </Project>

Source/MQTTnet/MessageStream/MqttMessageStream.cs

@@ -0,0 +1,79 @@
+using System;
+using System.Collections.Generic;
+
+using System.Runtime.CompilerServices;
+using System.Threading;
+using System.Threading.Tasks;
+using MQTTnet.Internal;
+using MQTTnet.Packets;
+
+namespace MQTTnet.MessageStream
+{
+    public class MqttMessageStream
+    {
+        private readonly LinkedList<MqttBasePacket> _queue = new LinkedList<MqttBasePacket>();
+        private readonly LinkedList<TaskCompletionSource<MqttBasePacket>> _waitHandles = new LinkedList<TaskCompletionSource<MqttBasePacket>>();
+
+        private readonly SemaphoreSlim _lock = new SemaphoreSlim(1, 1);
+
+        private readonly BlockingQueue<MqttBasePacket> _packets = new BlockingQueue<MqttBasePacket>();
+
+        public void Enqueue(MqttBasePacket packet)
+        {
+            if (packet == null) throw new ArgumentNullException(nameof(packet));
+
+            
+
+            lock (_queue)
+            {
+                //_queue.AddLast(packet);
+
+                _packets.Enqueue(packet);
+            }
+
+            //lock (_queue)
+            //{
+            //    _queue.AddLast(packet);
+
+            //    foreach (var waitHandle in _waitHandles)
+            //    {
+            //        waitHandle.TrySetResult(true);
+            //    }
+
+            //    _waitHandles.Clear();
+            //}
+        }
+        
+        public Task<MqttBasePacket> TakeAsync(CancellationToken cancellationToken)
+        {
+            lock (_packets)
+            {
+                var packet = _packets.Dequeue();
+                return Task.FromResult(packet);
+            }
+            
+
+            //while (!cancellationToken.IsCancellationRequested)
+            //{
+            //    TaskCompletionSource<bool> waitHandle;
+            //    lock (_queue)
+            //    {
+            //        if (_queue.Count > 0)
+            //        {
+            //            var node = _queue.First;
+            //            _queue.RemoveFirst();
+
+            //            return node.Value;
+            //        }
+
+            //        waitHandle = new TaskCompletionSource<bool>();
+            //        _waitHandles.Add(waitHandle);
+            //    }
+
+            //    await waitHandle.Task;
+            //}
+
+            //return null;
+        }
+    }
+}

Source/MQTTnet/Client/PacketDispatcher/IMqttPacketAwaiter.cs → Source/MQTTnet/PacketDispatcher/IMqttPacketAwaiter.cs

@@ -1,12 +1,14 @@
 using System;
 using MQTTnet.Packets;
 
-namespace MQTTnet.Client.PacketDispatcher
+namespace MQTTnet.PacketDispatcher
 {
-    public interface IMqttPacketAwaiter
+    public interface IMqttPacketAwaiter : IDisposable
     {
         void Complete(MqttBasePacket packet);
 
         void Fail(Exception exception);
+
+        void Cancel();
     }
 }

Source/MQTTnet/PacketDispatcher/MqttPacketAwaiter.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using MQTTnet.Packets;
+
+namespace MQTTnet.PacketDispatcher
+{
+    public sealed class MqttPacketAwaiter<TPacket> : IMqttPacketAwaiter where TPacket : MqttBasePacket
+    {
+        private readonly TaskCompletionSource<MqttBasePacket> _taskCompletionSource = new TaskCompletionSource<MqttBasePacket>();
+        private readonly ushort? _packetIdentifier;
+        private readonly MqttPacketDispatcher _owningPacketDispatcher;
+        
+        public MqttPacketAwaiter(ushort? packetIdentifier, MqttPacketDispatcher owningPacketDispatcher)
+        {
+            _packetIdentifier = packetIdentifier;
+            _owningPacketDispatcher = owningPacketDispatcher ?? throw new ArgumentNullException(nameof(owningPacketDispatcher));
+        }
+
+        public async Task<TPacket> WaitOneAsync(TimeSpan timeout)
+        {
+            using (var timeoutToken = new CancellationTokenSource(timeout))
+            {
+                timeoutToken.Token.Register(() => _taskCompletionSource.TrySetCanceled());
+
+                var packet = await _taskCompletionSource.Task.ConfigureAwait(false);
+                return (TPacket)packet;
+            }
+        }
+
+        public void Complete(MqttBasePacket packet)
+        {
+            if (packet == null) throw new ArgumentNullException(nameof(packet));
+            
+            // To prevent deadlocks it is required to call the _TrySetResult_ method
+            // from a new thread because the awaiting code will not(!) be executed in
+            // a new thread automatically (due to await). Furthermore _this_ thread will
+            // do it. But _this_ thread is also reading incoming packets -> deadlock.
+            Task.Run(() => _taskCompletionSource.TrySetResult(packet));
+        }
+
+        public void Fail(Exception exception)
+        {
+            if (exception == null) throw new ArgumentNullException(nameof(exception));
+
+            Task.Run(() => _taskCompletionSource.TrySetException(exception));
+        }
+
+        public void Cancel()
+        {
+            Task.Run(() => _taskCompletionSource.TrySetCanceled());
+        }
+
+        public void Dispose()
+        {
+            _owningPacketDispatcher.RemovePacketAwaiter<TPacket>(_packetIdentifier);
+        }
+    }
+}

Source/MQTTnet/Client/PacketDispatcher/MqttPacketDispatcher.cs → Source/MQTTnet/PacketDispatcher/MqttPacketDispatcher.cs

@@ -1,22 +1,24 @@
 using System;
 using System.Collections.Concurrent;
-using System.Threading.Tasks;
+using System.Threading;
 using MQTTnet.Packets;
 
-namespace MQTTnet.Client.PacketDispatcher
+namespace MQTTnet.PacketDispatcher
 {
     public class MqttPacketDispatcher
     {
-        private readonly ConcurrentDictionary<Tuple<ushort, Type>, IMqttPacketAwaiter> _awaiters = new ConcurrentDictionary<Tuple<ushort, Type>, IMqttPacketAwaiter>();
+        private readonly ConcurrentDictionary<Tuple<ushort, Type>, IMqttPacketAwaiter> _packetAwaiters = new ConcurrentDictionary<Tuple<ushort, Type>, IMqttPacketAwaiter>();
+
+        private BlockingCollection<MqttBasePacket> _inboundPackagesQueue = new BlockingCollection<MqttBasePacket>();
 
         public void Dispatch(Exception exception)
         {
-            foreach (var awaiter in _awaiters)
+            foreach (var awaiter in _packetAwaiters)
             {
-                Task.Run(() => awaiter.Value.Fail(exception)); // Task.Run fixes a dead lock. Without this the client only receives one message.
+                awaiter.Value.Fail(exception);
             }
 
-            _awaiters.Clear();
+            _packetAwaiters.Clear();
         }
 
         public void Dispatch(MqttBasePacket packet)
@@ -32,31 +34,56 @@ namespace MQTTnet.Client.PacketDispatcher
             var type = packet.GetType();
             var key = new Tuple<ushort, Type>(identifier, type);
 
-            if (_awaiters.TryRemove(key, out var awaiter))
+            if (_packetAwaiters.TryRemove(key, out var awaiter))
             {
-                Task.Run(() => awaiter.Complete(packet)); // Task.Run fixes a dead lock. Without this the client only receives one message.
+                awaiter.Complete(packet);
                 return;
             }
 
-            throw new InvalidOperationException($"Packet of type '{type.Name}' not handled or dispatched.");
+            lock (_inboundPackagesQueue)
+            {
+                _inboundPackagesQueue.Add(packet);
+            }
+        }
+
+        public MqttBasePacket Take(CancellationToken cancellationToken)
+        {
+            BlockingCollection<MqttBasePacket> inboundPackagesQueue;
+            lock (_inboundPackagesQueue)
+            {
+                inboundPackagesQueue = _inboundPackagesQueue;
+            }
+
+            return inboundPackagesQueue.Take(cancellationToken);
         }
 
         public void Reset()
         {
-            _awaiters.Clear();
+            foreach (var awaiter in _packetAwaiters)
+            {
+                awaiter.Value.Cancel();
+            }
+
+            lock (_inboundPackagesQueue)
+            {
+                _inboundPackagesQueue?.Dispose();
+                _inboundPackagesQueue = new BlockingCollection<MqttBasePacket>();
+            }
+
+            _packetAwaiters.Clear();
         }
 
         public MqttPacketAwaiter<TResponsePacket> AddPacketAwaiter<TResponsePacket>(ushort? identifier) where TResponsePacket : MqttBasePacket
         {
-            var awaiter = new MqttPacketAwaiter<TResponsePacket>();
-
             if (!identifier.HasValue)
             {
                 identifier = 0;
             }
 
+            var awaiter = new MqttPacketAwaiter<TResponsePacket>(identifier, this);
+
             var key = new Tuple<ushort, Type>(identifier.Value, typeof(TResponsePacket));
-            if (!_awaiters.TryAdd(key, awaiter))
+            if (!_packetAwaiters.TryAdd(key, awaiter))
             {
                 throw new InvalidOperationException($"The packet dispatcher already has an awaiter for packet of type '{key.Item2.Name}' with identifier {key.Item1}.");
             }
@@ -71,8 +98,8 @@ namespace MQTTnet.Client.PacketDispatcher
                 identifier = 0;
             }
 
-            var key = new Tuple<ushort, Type>(identifier ?? 0, typeof(TResponsePacket));
-            _awaiters.TryRemove(key, out _);
+            var key = new Tuple<ushort, Type>(identifier.Value, typeof(TResponsePacket));
+            _packetAwaiters.TryRemove(key, out _);
         }
     }
 }

Source/MQTTnet/Server/MqttClientSession.cs

@@ -7,6 +7,8 @@ using MQTTnet.Adapter;
 using MQTTnet.Client;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
+using MQTTnet.MessageStream;
+using MQTTnet.PacketDispatcher;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
@@ -15,6 +17,7 @@ namespace MQTTnet.Server
     public class MqttClientSession : IMqttClientSession
     {
         private readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new MqttPacketIdentifierProvider();
+        private readonly MqttPacketDispatcher _packetDispatcher = new MqttPacketDispatcher();
 
         private readonly MqttRetainedMessagesManager _retainedMessagesManager;
         private readonly MqttServerEventDispatcher _eventDispatcher;
@@ -56,7 +59,7 @@ namespace MQTTnet.Server
 
             _keepAliveMonitor = new MqttClientKeepAliveMonitor(this, _logger);
             _subscriptionsManager = new MqttClientSubscriptionsManager(clientId, _options, eventDispatcher);
-            _pendingMessagesQueue = new MqttClientSessionPendingMessagesQueue(_options, this, _logger);
+            _pendingMessagesQueue = new MqttClientSessionPendingMessagesQueue(_options, this, _packetDispatcher, _logger);
         }
 
         public string ClientId { get; }
@@ -144,13 +147,14 @@ namespace MQTTnet.Server
             }
 
             var publishPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreatePublishPacket(applicationMessage);
+            publishPacket.QualityOfServiceLevel = checkSubscriptionsResult.QualityOfServiceLevel;
 
             // Set the retain flag to true according to [MQTT-3.3.1-8] and [MQTT-3.3.1-9].
             publishPacket.Retain = isRetainedApplicationMessage;
 
             if (publishPacket.QualityOfServiceLevel > 0)
             {
-                publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNewPacketIdentifier();
+                publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
             }
 
             if (_options.ClientMessageQueueInterceptor != null)
@@ -178,6 +182,8 @@ namespace MQTTnet.Server
             _pendingMessagesQueue.Enqueue(publishPacket);
         }
 
+
+
         private async Task RunInternalAsync(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter)
         {
             if (channelAdapter == null) throw new ArgumentNullException(nameof(channelAdapter));
@@ -212,13 +218,41 @@ namespace MQTTnet.Server
 
                 _isCleanSession = false;
 
+                Task.Run(async () =>
+                {
+                    while (!cancellationTokenSource.IsCancellationRequested)
+                    {
+                        var packet = _packetDispatcher.Take(cancellationTokenSource.Token);
+                        await ProcessReceivedPacketAsync(packet, cancellationTokenSource.Token).ConfigureAwait(false);
+                    }
+                }, cancellationTokenSource.Token);
+
+                Task.Run(async () =>
+                {
+                    while (!cancellationTokenSource.IsCancellationRequested)
+                    {
+                        try
+                        {
+                            var packet = await _outboundMessageStream.TakeAsync(cancellationTokenSource.Token);
+                            await channelAdapter.SendPacketAsync(packet, cancellationTokenSource.Token);
+                        }
+                        catch (Exception e)
+                        {
+                            _logger.Error(e, "sdfsdf");
+                            await StopAsync(MqttClientDisconnectType.NotClean);
+
+                        }
+                        
+                    }
+                },cancellationTokenSource.Token);
+
                 while (!cancellationTokenSource.IsCancellationRequested)
                 {
                     var packet = await channelAdapter.ReceivePacketAsync(TimeSpan.Zero, cancellationTokenSource.Token).ConfigureAwait(false);
                     if (packet != null)
                     {
                         _keepAliveMonitor.PacketReceived(packet);
-                        await ProcessReceivedPacketAsync(channelAdapter, packet, cancellationTokenSource.Token).ConfigureAwait(false);
+                        _packetDispatcher.Dispatch(packet);
                     }
                 }
             }
@@ -229,17 +263,7 @@ namespace MQTTnet.Server
             {
                 if (exception is MqttCommunicationException)
                 {
-                    if (exception is MqttCommunicationClosedGracefullyException)
-                    {
-                        _logger.Verbose("Client '{0}': Connection closed gracefully.", ClientId);
-
-                        StopInternal(MqttClientDisconnectType.Clean);
-                        return;
-                    }
-                    else
-                    {
-                        _logger.Warning(exception, "Client '{0}': Communication exception while receiving client packets.", ClientId);
-                    }
+                    _logger.Warning(exception, "Client '{0}': Communication exception while receiving client packets.", ClientId);
                 }
                 else
                 {
@@ -256,18 +280,18 @@ namespace MQTTnet.Server
                 }
 
                 _willMessage = null;
-                
+
                 _channelAdapter.ReadingPacketStarted -= OnAdapterReadingPacketStarted;
                 _channelAdapter.ReadingPacketCompleted -= OnAdapterReadingPacketCompleted;
                 _channelAdapter = null;
-                
+
                 _logger.Info("Client '{0}': Session stopped.", ClientId);
                 _eventDispatcher.OnClientDisconnected(ClientId, _wasCleanDisconnect);
 
                 _workerTask = null;
             }
         }
-        
+
         private void StopInternal(MqttClientDisconnectType type)
         {
             var cts = _cancellationTokenSource;
@@ -278,55 +302,33 @@ namespace MQTTnet.Server
 
             _wasCleanDisconnect = type == MqttClientDisconnectType.Clean;
             _cancellationTokenSource?.Cancel(false);
+            _packetDispatcher.Reset();
         }
 
-        private Task ProcessReceivedPacketAsync(IMqttChannelAdapter channelAdapter, MqttBasePacket packet, CancellationToken cancellationToken)
+        private readonly MqttMessageStream _outboundMessageStream = new MqttMessageStream();
+
+        private Task ProcessReceivedPacketAsync(MqttBasePacket packet, CancellationToken cancellationToken)
         {
             if (packet is MqttPublishPacket publishPacket)
             {
-                return HandleIncomingPublishPacketAsync(channelAdapter, publishPacket, cancellationToken);
+                return HandleIncomingPublishPacketAsync(publishPacket, cancellationToken);
             }
 
             if (packet is MqttPingReqPacket)
             {
-                return channelAdapter.SendPacketAsync(new MqttPingRespPacket(), cancellationToken);
-            }
-
-            if (packet is MqttPubRelPacket pubRelPacket)
-            {
-                var responsePacket = new MqttPubCompPacket
-                {
-                    PacketIdentifier = pubRelPacket.PacketIdentifier,
-                    ReasonCode = MqttPubCompReasonCode.Success
-                };
-
-                return channelAdapter.SendPacketAsync(responsePacket, cancellationToken);
-            }
-
-            if (packet is MqttPubRecPacket pubRecPacket)
-            {
-                var responsePacket = new MqttPubRelPacket
-                {
-                    PacketIdentifier = pubRecPacket.PacketIdentifier,
-                    ReasonCode = MqttPubRelReasonCode.Success
-                };
-
-                return channelAdapter.SendPacketAsync(responsePacket, cancellationToken);
-            }
-
-            if (packet is MqttPubAckPacket || packet is MqttPubCompPacket)
-            {
+                //return channelAdapter.SendPacketAsync(new MqttPingRespPacket(), cancellationToken);
+                _outboundMessageStream.Enqueue(new MqttPingRespPacket());
                 return Task.FromResult(0);
             }
 
             if (packet is MqttSubscribePacket subscribePacket)
             {
-                return HandleIncomingSubscribePacketAsync(channelAdapter, subscribePacket, cancellationToken);
+                return HandleIncomingSubscribePacketAsync(subscribePacket, cancellationToken);
             }
 
             if (packet is MqttUnsubscribePacket unsubscribePacket)
             {
-                return HandleIncomingUnsubscribePacketAsync(channelAdapter, unsubscribePacket, cancellationToken);
+                return HandleIncomingUnsubscribePacketAsync(unsubscribePacket, cancellationToken);
             }
 
             if (packet is MqttDisconnectPacket)
@@ -335,6 +337,18 @@ namespace MQTTnet.Server
                 return Task.FromResult(0);
             }
 
+            //if (packet is MqttAuthPacket ||
+            //    packet is MqttSubAckPacket ||
+            //    packet is MqttUnsubAckPacket ||
+            //    packet is MqttPubAckPacket ||
+            //    packet is MqttPubCompPacket ||
+            //    packet is MqttPubRecPacket ||
+            //    packet is MqttPubRelPacket)
+            //{
+            //    _packetDispatcher.TryDispatch(packet);
+            //    return Task.FromResult(0);
+            //}
+
             _logger.Warning(null, "Client '{0}': Received invalid packet ({1}). Closing connection.", ClientId, packet);
 
             StopInternal(MqttClientDisconnectType.NotClean);
@@ -350,10 +364,14 @@ namespace MQTTnet.Server
             }
         }
 
-        private async Task HandleIncomingSubscribePacketAsync(IMqttChannelAdapter adapter, MqttSubscribePacket subscribePacket, CancellationToken cancellationToken)
+        private async Task HandleIncomingSubscribePacketAsync(MqttSubscribePacket subscribePacket, CancellationToken cancellationToken)
         {
             var subscribeResult = await _subscriptionsManager.SubscribeAsync(subscribePacket).ConfigureAwait(false);
-            await adapter.SendPacketAsync(subscribeResult.ResponsePacket, cancellationToken).ConfigureAwait(false);
+
+            _outboundMessageStream.Enqueue(subscribeResult.ResponsePacket);
+            //await adapter.SendPacketAsync(subscribeResult.ResponsePacket, cancellationToken).ConfigureAwait(false);
+
+            // TODO: Add "WaitForDelivery".
 
             if (subscribeResult.CloseConnection)
             {
@@ -364,13 +382,17 @@ namespace MQTTnet.Server
             await EnqueueSubscribedRetainedMessagesAsync(subscribePacket.TopicFilters).ConfigureAwait(false);
         }
 
-        private Task HandleIncomingUnsubscribePacketAsync(IMqttChannelAdapter adapter, MqttUnsubscribePacket unsubscribePacket, CancellationToken cancellationToken)
+        private Task HandleIncomingUnsubscribePacketAsync(MqttUnsubscribePacket unsubscribePacket, CancellationToken cancellationToken)
         {
             var unsubscribeResult = _subscriptionsManager.Unsubscribe(unsubscribePacket);
-            return adapter.SendPacketAsync(unsubscribeResult, cancellationToken);
+
+            _outboundMessageStream.Enqueue(unsubscribeResult);
+
+            //return adapter.SendPacketAsync(unsubscribeResult, cancellationToken);
+            return Task.FromResult(0);
         }
 
-        private Task HandleIncomingPublishPacketAsync(IMqttChannelAdapter adapter, MqttPublishPacket publishPacket, CancellationToken cancellationToken)
+        private Task HandleIncomingPublishPacketAsync(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
         {
             Interlocked.Increment(ref _receivedMessagesCount);
 
@@ -382,11 +404,11 @@ namespace MQTTnet.Server
                     }
                 case MqttQualityOfServiceLevel.AtLeastOnce:
                     {
-                        return HandleIncomingPublishPacketWithQoS1Async(adapter, publishPacket, cancellationToken);
+                        return HandleIncomingPublishPacketWithQoS1Async(publishPacket, cancellationToken);
                     }
                 case MqttQualityOfServiceLevel.ExactlyOnce:
                     {
-                        return HandleIncomingPublishPacketWithQoS2Async(adapter, publishPacket, cancellationToken);
+                        return HandleIncomingPublishPacketWithQoS2Async(publishPacket, cancellationToken);
                     }
                 default:
                     {
@@ -405,7 +427,6 @@ namespace MQTTnet.Server
         }
 
         private Task HandleIncomingPublishPacketWithQoS1Async(
-            IMqttChannelAdapter adapter,
             MqttPublishPacket publishPacket,
             CancellationToken cancellationToken)
         {
@@ -419,24 +440,32 @@ namespace MQTTnet.Server
                 ReasonCode = MqttPubAckReasonCode.Success
             };
 
-            return adapter.SendPacketAsync(response, cancellationToken);
+            _outboundMessageStream.Enqueue(response);
+
+            //return adapter.SendPacketAsync(response, cancellationToken);
+            return Task.FromResult(0);
         }
 
-        private Task HandleIncomingPublishPacketWithQoS2Async(
-            IMqttChannelAdapter adapter,
+        private async Task HandleIncomingPublishPacketWithQoS2Async(
             MqttPublishPacket publishPacket,
             CancellationToken cancellationToken)
         {
             // QoS 2 is implement as method "B" (4.3.3 QoS 2: Exactly once delivery)
             _sessionsManager.EnqueueApplicationMessage(this, _channelAdapter.PacketFormatterAdapter.DataConverter.CreateApplicationMessage(publishPacket));
 
-            var response = new MqttPubRecPacket
+            using (var pubRelPacketAwaiter = _packetDispatcher.AddPacketAwaiter<MqttPubRelPacket>(publishPacket.PacketIdentifier))
             {
-                PacketIdentifier = publishPacket.PacketIdentifier,
-                ReasonCode = MqttPubRecReasonCode.Success
-            };
+                var pubRecPacket = new MqttPubRecPacket
+                {
+                    PacketIdentifier = publishPacket.PacketIdentifier,
+                    ReasonCode = MqttPubRecReasonCode.Success
+                };
+
+                //await adapter.SendPacketAsync(pubRecPacket, cancellationToken).ConfigureAwait(false);
+                _outboundMessageStream.Enqueue(pubRecPacket);
 
-            return adapter.SendPacketAsync(response, cancellationToken);
+                await pubRelPacketAwaiter.WaitOneAsync(_options.DefaultCommunicationTimeout).ConfigureAwait(false);
+            }
         }
 
         private void OnAdapterReadingPacketCompleted(object sender, EventArgs e)

Source/MQTTnet/Server/MqttClientSessionPendingMessagesQueue.cs

@@ -6,6 +6,7 @@ using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
 using MQTTnet.Internal;
+using MQTTnet.PacketDispatcher;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
@@ -13,20 +14,26 @@ namespace MQTTnet.Server
 {
     public class MqttClientSessionPendingMessagesQueue : IDisposable
     {
-        private readonly Queue<MqttBasePacket> _queue = new Queue<MqttBasePacket>();
+        private readonly Queue<MqttPublishPacket> _queue = new Queue<MqttPublishPacket>();
         private readonly AsyncAutoResetEvent _queueLock = new AsyncAutoResetEvent();
 
         private readonly IMqttServerOptions _options;
         private readonly MqttClientSession _clientSession;
+        private readonly MqttPacketDispatcher _packetDispatcher;
         private readonly IMqttNetChildLogger _logger;
 
         private long _sentPacketsCount;
 
-        public MqttClientSessionPendingMessagesQueue(IMqttServerOptions options, MqttClientSession clientSession, IMqttNetChildLogger logger)
+        public MqttClientSessionPendingMessagesQueue(
+            IMqttServerOptions options,
+            MqttClientSession clientSession, 
+            MqttPacketDispatcher packetDispatcher,
+            IMqttNetChildLogger logger)
         {
             if (logger == null) throw new ArgumentNullException(nameof(logger));
             _options = options ?? throw new ArgumentNullException(nameof(options));
             _clientSession = clientSession ?? throw new ArgumentNullException(nameof(clientSession));
+            _packetDispatcher = packetDispatcher ?? throw new ArgumentNullException(nameof(packetDispatcher));
 
             _logger = logger.CreateChildLogger(nameof(MqttClientSessionPendingMessagesQueue));
         }
@@ -115,7 +122,7 @@ namespace MQTTnet.Server
 
         private async Task TrySendNextQueuedPacketAsync(IMqttChannelAdapter adapter, CancellationToken cancellationToken)
         {
-            MqttBasePacket packet = null;
+            MqttPublishPacket packet = null;
             try
             {
                 if (cancellationToken.IsCancellationRequested)
@@ -137,7 +144,34 @@ namespace MQTTnet.Server
                     return;
                 }
 
-                await adapter.SendPacketAsync(packet, cancellationToken).ConfigureAwait(false);
+                if (packet.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtMostOnce)
+                {
+                    await adapter.SendPacketAsync(packet, cancellationToken).ConfigureAwait(false);
+                }
+                else if (packet.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtLeastOnce)
+                {
+                    var awaiter = _packetDispatcher.AddPacketAwaiter<MqttPubAckPacket>(packet.PacketIdentifier);
+                    await adapter.SendPacketAsync(packet, cancellationToken).ConfigureAwait(false);
+                    await awaiter.WaitOneAsync(_options.DefaultCommunicationTimeout).ConfigureAwait(false);
+                }
+                else if (packet.QualityOfServiceLevel == MqttQualityOfServiceLevel.ExactlyOnce)
+                {
+                    var awaiter1 = _packetDispatcher.AddPacketAwaiter<MqttPubRecPacket>(packet.PacketIdentifier);
+                    var awaiter2 = _packetDispatcher.AddPacketAwaiter<MqttPubCompPacket>(packet.PacketIdentifier);
+                    try
+                    {
+                        await adapter.SendPacketAsync(packet, cancellationToken).ConfigureAwait(false);
+                        await awaiter1.WaitOneAsync(_options.DefaultCommunicationTimeout).ConfigureAwait(false);
+                        
+                        await adapter.SendPacketAsync(new MqttPubRelPacket { PacketIdentifier = packet.PacketIdentifier }, cancellationToken).ConfigureAwait(false);
+                        await awaiter2.WaitOneAsync(_options.DefaultCommunicationTimeout).ConfigureAwait(false);
+                    }
+                    finally
+                    {
+                        _packetDispatcher.RemovePacketAwaiter<MqttPubRecPacket>(packet.PacketIdentifier);
+                        _packetDispatcher.RemovePacketAwaiter<MqttPubCompPacket>(packet.PacketIdentifier);
+                    }
+                }
 
                 _logger.Verbose("Enqueued packet sent (ClientId: {0}).", _clientSession.ClientId);
 
@@ -153,7 +187,7 @@ namespace MQTTnet.Server
                 {
                     _logger.Warning(exception, "Sending publish packet failed: Communication exception (ClientId: {0}).", _clientSession.ClientId);
                 }
-                else if (exception is OperationCanceledException)
+                else if (exception is OperationCanceledException && cancellationToken.IsCancellationRequested)
                 {
                 }
                 else
@@ -161,14 +195,11 @@ namespace MQTTnet.Server
                     _logger.Error(exception, "Sending publish packet failed (ClientId: {0}).", _clientSession.ClientId);
                 }
 
-                if (packet is MqttPublishPacket publishPacket)
+                if (packet?.QualityOfServiceLevel > MqttQualityOfServiceLevel.AtMostOnce)
                 {
-                    if (publishPacket.QualityOfServiceLevel > MqttQualityOfServiceLevel.AtMostOnce)
-                    {
-                        publishPacket.Dup = true;
+                    packet.Dup = true;
 
-                        Enqueue(publishPacket);
-                    }
+                    Enqueue(packet);
                 }
 
                 if (!cancellationToken.IsCancellationRequested)

Source/MQTTnet/Server/MqttServer.cs

@@ -5,6 +5,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
 using MQTTnet.Client.Publishing;
+using MQTTnet.Client.Receiving;
 using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Server
@@ -31,7 +32,18 @@ namespace MQTTnet.Server
             _eventDispatcher.ClientDisconnected += (s, e) => ClientDisconnected?.Invoke(s, e);
             _eventDispatcher.ClientSubscribedTopic += (s, e) => ClientSubscribedTopic?.Invoke(s, e);
             _eventDispatcher.ClientUnsubscribedTopic += (s, e) => ClientUnsubscribedTopic?.Invoke(s, e);
-            _eventDispatcher.ApplicationMessageReceived += (s, e) => ApplicationMessageReceived?.Invoke(s, e);
+            _eventDispatcher.ApplicationMessageReceived += async (s, e) =>
+            {
+                // TODO: Migrate EventDispatcher to proper handlers and no events anymore.
+                ApplicationMessageReceived?.Invoke(s, e);
+
+                var handler = ReceivedApplicationMessageHandler;
+                if (handler != null)
+                {
+                    await handler.HandleApplicationMessageAsync(
+                        new MqttApplicationMessageHandlerContext(e.ClientId, e.ApplicationMessage)).ConfigureAwait(false);
+                }
+            };
         }
 
         public event EventHandler Started;
@@ -42,6 +54,7 @@ namespace MQTTnet.Server
         public event EventHandler<MqttClientSubscribedTopicEventArgs> ClientSubscribedTopic;
         public event EventHandler<MqttClientUnsubscribedTopicEventArgs> ClientUnsubscribedTopic;
 
+        public IMqttApplicationMessageHandler ReceivedApplicationMessageHandler { get; set; }
         public event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived;
 
         public IMqttServerOptions Options { get; private set; }

Tests/MQTTnet.Benchmarks/SerializerBenchmark.cs

@@ -52,7 +52,7 @@ namespace MQTTnet.Benchmarks
             {
                 channel.Reset();
 
-                var header = reader.ReadFixedHeaderAsync(fixedHeader, CancellationToken.None).GetAwaiter().GetResult();
+                var header = reader.ReadFixedHeaderAsync(fixedHeader, CancellationToken.None).GetAwaiter().GetResult().FixedHeader;
 
                 var receivedPacket = new ReceivedMqttPacket(
                     header.Flags,

Tests/MQTTnet.Core.Tests/ManagedMqttClientTests.cs

@@ -13,26 +13,32 @@ namespace MQTTnet.Tests
         {
             var factory = new MqttFactory();
             var managedClient = factory.CreateManagedMqttClient();
+            try
+            {
+                var clientOptions = new ManagedMqttClientOptionsBuilder()
+                    .WithMaxPendingMessages(5)
+                    .WithPendingMessagesOverflowStrategy(MqttPendingMessagesOverflowStrategy.DropNewMessage);
 
-            var clientOptions = new ManagedMqttClientOptionsBuilder()
-                .WithMaxPendingMessages(5)
-                .WithPendingMessagesOverflowStrategy(MqttPendingMessagesOverflowStrategy.DropNewMessage);
+                clientOptions.WithClientOptions(o => o.WithTcpServer("localhost"));
 
-            clientOptions.WithClientOptions(o => o.WithTcpServer("localhost"));
+                await managedClient.StartAsync(clientOptions.Build());
 
-            await managedClient.StartAsync(clientOptions.Build());
+                await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "1" });
+                await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "2" });
+                await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "3" });
+                await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "4" });
+                await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "5" });
 
-            await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "1" });
-            await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "2" });
-            await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "3" });
-            await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "4" });
-            await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "5" });
+                await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "6" });
+                await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "7" });
+                await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "8" });
 
-            await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "6" });
-            await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "7" });
-            await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "8" });
-
-            Assert.AreEqual(5, managedClient.PendingApplicationMessagesCount);
+                Assert.AreEqual(5, managedClient.PendingApplicationMessagesCount);
+            }
+            finally
+            {
+                await managedClient.StopAsync();
+            }
         }
     }
 }

Tests/MQTTnet.Core.Tests/MqttClientTests.cs

@@ -2,11 +2,14 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Net.Sockets;
+using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Client.Options;
+using MQTTnet.Client.Receiving;
 using MQTTnet.Exceptions;
+using MQTTnet.Protocol;
 using MQTTnet.Server;
 
 namespace MQTTnet.Tests
@@ -39,11 +42,100 @@ namespace MQTTnet.Tests
             Assert.IsInstanceOfType(ex.InnerException, typeof(SocketException));
         }
 
+        [TestMethod]
+        public async Task Client_Preserve_Message_Order()
+        {
+            // The messages are sent in reverse or to ensure that the delay in the handler
+            // needs longer for the first messages and later messages may be processed earlier (if there
+            // is an issue).
+            const int MessagesCount = 50;
+
+            using (var testSetup = new TestSetup())
+            {
+                await testSetup.StartServerAsync();
+
+                var client1 = await testSetup.ConnectClientAsync();
+                await client1.SubscribeAsync("x");
+
+                var receivedValues = new List<int>();
+     
+                async Task Handler1(MqttApplicationMessageHandlerContext context)
+                {
+                    var value = int.Parse(context.ApplicationMessage.ConvertPayloadToString());
+                    await Task.Delay(value);
+
+                    lock (receivedValues)
+                    {
+                        receivedValues.Add(value);
+                    }
+                }
+
+                client1.UseReceivedApplicationMessageHandler(Handler1);
+
+                var client2 = await testSetup.ConnectClientAsync();
+                for (var i = MessagesCount; i > 0; i--)
+                {
+                    await client2.PublishAsync("x", i.ToString());
+                }
+
+                await Task.Delay(5000);
+
+                for (var i = MessagesCount; i > 0; i--)
+                {
+                    Assert.AreEqual(i, receivedValues[MessagesCount - i]);
+                }
+            }
+        }
+
+        [TestMethod]
+        public async Task Client_Send_Reply_For_Any_Received_Message()
+        {
+            using (var testSetup = new TestSetup())
+            {
+                await testSetup.StartServerAsync();
+
+                var client1 = await testSetup.ConnectClientAsync();
+                await client1.SubscribeAsync("request/+");
+
+                async Task Handler1(MqttApplicationMessageHandlerContext context)
+                {
+                    await client1.PublishAsync($"reply/{context.ApplicationMessage.Topic}");
+                }
+
+                client1.UseReceivedApplicationMessageHandler(Handler1);
+
+                var client2 = await testSetup.ConnectClientAsync();
+                await client2.SubscribeAsync("reply/#");
+
+                var replies = new List<string>();
+
+                void Handler2(MqttApplicationMessageHandlerContext context)
+                {
+                    lock (replies)
+                    {
+                        replies.Add(context.ApplicationMessage.Topic);
+                    }
+                }
+                
+                client2.UseReceivedApplicationMessageHandler((Action<MqttApplicationMessageHandlerContext>) Handler2);
+
+                await Task.Delay(500);
+
+                await client2.PublishAsync("request/a");
+                await client2.PublishAsync("request/b");
+                await client2.PublishAsync("request/c");
+
+                await Task.Delay(500);
+
+                Assert.AreEqual("reply/request/a,reply/request/b,reply/request/c", string.Join("," , replies));
+            }
+        }
+
         [TestMethod]
         public async Task Client_Publish()
         {
             var server = new MqttFactory().CreateMqttServer();
-            
+
             try
             {
                 var receivedMessages = new List<MqttApplicationMessage>();
@@ -125,39 +217,85 @@ namespace MQTTnet.Tests
             }
         }
 
-//#if DEBUG
-//        [TestMethod]
-//        public async Task Client_Cleanup_On_Authentification_Fails()
-//        {
-//            var channel = new TestMqttCommunicationAdapter();
-//            var channel2 = new TestMqttCommunicationAdapter();
-//            channel.Partner = channel2;
-//            channel2.Partner = channel;
-
-//            Task.Run(async () => {
-//                var connect = await channel2.ReceivePacketAsync(TimeSpan.Zero, CancellationToken.None);
-//                await channel2.SendPacketAsync(new MqttConnAckPacket
-//                {
-//                    ConnectReturnCode = Protocol.MqttConnectReturnCode.ConnectionRefusedNotAuthorized
-//                }, CancellationToken.None);
-//            });
-            
-//            var fake = new TestMqttCommunicationAdapterFactory(channel);
-
-//            var client = new MqttClient(fake, new MqttNetLogger());
-
-//            try
-//            {
-//                await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("any-server").Build());
-//            }
-//            catch (Exception ex)
-//            {
-//                Assert.IsInstanceOfType(ex, typeof(MqttConnectingFailedException));
-//            }
-
-//            Assert.IsTrue(client._packetReceiverTask == null || client._packetReceiverTask.IsCompleted, "receive loop not completed");
-//            Assert.IsTrue(client._keepAliveMessageSenderTask == null || client._keepAliveMessageSenderTask.IsCompleted, "keepalive loop not completed");
-//        }
-//#endif
+        [TestMethod]
+        public async Task Client_Exception_In_Application_Message_Handler()
+        {
+            using (var testSetup = new TestSetup())
+            {
+                testSetup.IgnoreClientLogErrors = true;
+                testSetup.IgnoreServerLogErrors = true;
+
+                await testSetup.StartServerAsync(
+                    new MqttServerOptionsBuilder()
+                        .WithPersistentSessions()
+                        .WithDefaultCommunicationTimeout(TimeSpan.FromMilliseconds(50)));
+
+                var client1 = await testSetup.ConnectClientAsync(new MqttClientOptionsBuilder()
+                    .WithCleanSession(false));
+
+                await client1.SubscribeAsync("x", MqttQualityOfServiceLevel.AtLeastOnce);
+
+                var retries = 0;
+
+                async Task Handler1(MqttApplicationMessageHandlerContext context)
+                {
+                    retries++;
+
+                    await Task.Delay(50);
+                    throw new Exception("Broken!");
+                }
+
+                client1.UseReceivedApplicationMessageHandler(Handler1);
+
+                var client2 = await testSetup.ConnectClientAsync();
+                await client2.PublishAsync("x");
+
+                await Task.Delay(1000);
+
+                // The server should disconnect clients which are not responding.
+                Assert.IsFalse(client1.IsConnected);
+
+                await client1.ReconnectAsync().ConfigureAwait(false);
+                
+                await Task.Delay(1000);
+
+                Assert.AreEqual(2, retries);
+            }
+        }
+
+        //#if DEBUG
+        //        [TestMethod]
+        //        public async Task Client_Cleanup_On_Authentification_Fails()
+        //        {
+        //            var channel = new TestMqttCommunicationAdapter();
+        //            var channel2 = new TestMqttCommunicationAdapter();
+        //            channel.Partner = channel2;
+        //            channel2.Partner = channel;
+
+        //            Task.Run(async () => {
+        //                var connect = await channel2.ReceivePacketAsync(TimeSpan.Zero, CancellationToken.None);
+        //                await channel2.SendPacketAsync(new MqttConnAckPacket
+        //                {
+        //                    ConnectReturnCode = Protocol.MqttConnectReturnCode.ConnectionRefusedNotAuthorized
+        //                }, CancellationToken.None);
+        //            });
+
+        //            var fake = new TestMqttCommunicationAdapterFactory(channel);
+
+        //            var client = new MqttClient(fake, new MqttNetLogger());
+
+        //            try
+        //            {
+        //                await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("any-server").Build());
+        //            }
+        //            catch (Exception ex)
+        //            {
+        //                Assert.IsInstanceOfType(ex, typeof(MqttConnectingFailedException));
+        //            }
+
+        //            Assert.IsTrue(client._packetReceiverTask == null || client._packetReceiverTask.IsCompleted, "receive loop not completed");
+        //            Assert.IsTrue(client._keepAliveMessageSenderTask == null || client._keepAliveMessageSenderTask.IsCompleted, "keepalive loop not completed");
+        //        }
+        //#endif
     }
 }

Tests/MQTTnet.Core.Tests/MqttPacketIdentifierProviderTests.cs

@@ -10,10 +10,10 @@ namespace MQTTnet.Tests
         public void Reset()
         {
             var p = new MqttPacketIdentifierProvider();
-            Assert.AreEqual(1, p.GetNewPacketIdentifier());
-            Assert.AreEqual(2, p.GetNewPacketIdentifier());
+            Assert.AreEqual(1, p.GetNextPacketIdentifier());
+            Assert.AreEqual(2, p.GetNextPacketIdentifier());
             p.Reset();
-            Assert.AreEqual(1, p.GetNewPacketIdentifier());
+            Assert.AreEqual(1, p.GetNextPacketIdentifier());
         }
 
         [TestMethod]
@@ -23,10 +23,10 @@ namespace MQTTnet.Tests
 
             for (ushort i = 0; i < ushort.MaxValue; i++)
             {
-                Assert.AreEqual(i + 1, p.GetNewPacketIdentifier());
+                Assert.AreEqual(i + 1, p.GetNextPacketIdentifier());
             }
 
-            Assert.AreEqual(1, p.GetNewPacketIdentifier());
+            Assert.AreEqual(1, p.GetNextPacketIdentifier());
         }
     }
 }

Tests/MQTTnet.Core.Tests/MqttPacketReaderTests.cs

@@ -1,7 +1,7 @@
 using System.IO;
 using System.Threading;
+using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
-using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
 using MQTTnet.Internal;
 
@@ -11,12 +11,13 @@ namespace MQTTnet.Tests
     public class MqttPacketReaderTests
     {
         [TestMethod]
-        [ExpectedException(typeof(MqttCommunicationClosedGracefullyException))]
-        public void MqttPacketReader_EmptyStream()
+        public async Task MqttPacketReader_EmptyStream()
         {
             var fixedHeader = new byte[2];
             var reader = new MqttPacketReader(new TestMqttChannel(new MemoryStream()));
-            reader.ReadFixedHeaderAsync(fixedHeader, CancellationToken.None).GetAwaiter().GetResult();
+            var readResult = await reader.ReadFixedHeaderAsync(fixedHeader, CancellationToken.None);
+
+            Assert.IsTrue(readResult.ConnectionClosed);
         }
     }
 }

Tests/MQTTnet.Core.Tests/MqttPacketSerializerTests.cs

@@ -182,7 +182,7 @@ namespace MQTTnet.Tests
 
             var header = new MqttPacketReader(testChannel).ReadFixedHeaderAsync(
                 new byte[2],
-                CancellationToken.None).GetAwaiter().GetResult();
+                CancellationToken.None).GetAwaiter().GetResult().FixedHeader;
 
             var eof = buffer.Offset + buffer.Count;
 
@@ -550,7 +550,7 @@ namespace MQTTnet.Tests
             {
                 var channel = new TestMqttChannel(headerStream);
                 var fixedHeader = new byte[2];
-                var header = new MqttPacketReader(channel).ReadFixedHeaderAsync(fixedHeader, CancellationToken.None).GetAwaiter().GetResult();
+                var header = new MqttPacketReader(channel).ReadFixedHeaderAsync(fixedHeader, CancellationToken.None).GetAwaiter().GetResult().FixedHeader;
 
                 using (var bodyStream = new MemoryStream(Join(buffer1), (int)headerStream.Position, header.RemainingLength))
                 {
@@ -586,7 +586,7 @@ namespace MQTTnet.Tests
                 var channel = new TestMqttChannel(headerStream);
                 var fixedHeader = new byte[2];
 
-                var header = new MqttPacketReader(channel).ReadFixedHeaderAsync(fixedHeader, CancellationToken.None).GetAwaiter().GetResult();
+                var header = new MqttPacketReader(channel).ReadFixedHeaderAsync(fixedHeader, CancellationToken.None).GetAwaiter().GetResult().FixedHeader;
 
                 using (var bodyStream = new MemoryStream(Join(buffer1), (int)headerStream.Position, (int)header.RemainingLength))
                 {

Tests/MQTTnet.Core.Tests/MqttServerTests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 using System.Net.Sockets;
@@ -19,36 +20,36 @@ namespace MQTTnet.Tests
     public class MqttServerTests
     {
         [TestMethod]
-        public void MqttServer_PublishSimple_AtMostOnce()
+        public async Task MqttServer_PublishSimple_AtMostOnce()
         {
-            TestPublishAsync(
+            await TestPublishAsync(
                 "A/B/C",
                 MqttQualityOfServiceLevel.AtMostOnce,
                 "A/B/C",
                 MqttQualityOfServiceLevel.AtMostOnce,
-                1).Wait();
+                1);
         }
 
         [TestMethod]
-        public void MqttServer_PublishSimple_AtLeastOnce()
+        public async Task MqttServer_PublishSimple_AtLeastOnce()
         {
-            TestPublishAsync(
+            await TestPublishAsync(
                 "A/B/C",
                 MqttQualityOfServiceLevel.AtLeastOnce,
                 "A/B/C",
                 MqttQualityOfServiceLevel.AtLeastOnce,
-                1).Wait();
+                1);
         }
 
         [TestMethod]
-        public void MqttServer_PublishSimple_ExactlyOnce()
+        public async Task MqttServer_PublishSimple_ExactlyOnce()
         {
-            TestPublishAsync(
+            await TestPublishAsync(
                 "A/B/C",
                 MqttQualityOfServiceLevel.ExactlyOnce,
                 "A/B/C",
                 MqttQualityOfServiceLevel.ExactlyOnce,
-                1).Wait();
+                1);
         }
 
         [TestMethod]
@@ -143,32 +144,27 @@ namespace MQTTnet.Tests
         }
 
         [TestMethod]
-        public async Task MqttServer_Publish()
+        public async Task MqttServer_Publish_From_Server()
         {
-            var serverAdapter = new TestMqttServerAdapter();
-            var s = new MqttFactory().CreateMqttServer(new[] { serverAdapter }, new MqttNetLogger());
-            var receivedMessagesCount = 0;
-
-            try
+            using (var testSetup = new TestSetup())
             {
-                await s.StartAsync(new MqttServerOptions());
+                var server = await testSetup.StartServerAsync();
 
-                var c1 = await serverAdapter.ConnectTestClient("c1");
+                var receivedMessagesCount = 0;
 
-                c1.ApplicationMessageReceived += (_, __) => receivedMessagesCount++;
+                var client = await testSetup.ConnectClientAsync();
+                client.UseReceivedApplicationMessageHandler(c => Interlocked.Increment(ref receivedMessagesCount));
 
                 var message = new MqttApplicationMessageBuilder().WithTopic("a").WithAtLeastOnceQoS().Build();
-                await c1.SubscribeAsync(new TopicFilter { Topic = "a", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce });
+                await client.SubscribeAsync(new TopicFilter { Topic = "a", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce });
 
-                await s.PublishAsync(message);
-                await Task.Delay(500);
-            }
-            finally
-            {
-                await s.StopAsync();
-            }
+                await server.PublishAsync(message);
 
-            Assert.AreEqual(1, receivedMessagesCount);
+                await Task.Delay(1000);
+                await server.StopAsync();
+
+                Assert.AreEqual(1, receivedMessagesCount);
+            }
         }
 
         [TestMethod]
@@ -438,40 +434,43 @@ namespace MQTTnet.Tests
         [TestMethod]
         public async Task MqttServer_Lots_Of_Retained_Messages()
         {
-            const int ClientCount = 100;
+            const int ClientCount = 25;
 
-            var server = new MqttFactory().CreateMqttServer();
-            try
+            using (var testSetup = new TestSetup())
             {
-                await server.StartAsync(new MqttServerOptionsBuilder().Build());
-
-                Parallel.For(
-                    0,
-                    ClientCount,
-                    new ParallelOptions { MaxDegreeOfParallelism = 10 },
-                    i =>
+                var server = await testSetup.StartServerAsync();
+                
+                var tasks = new ConcurrentBag<Task>();
+                for (var i = 0; i < ClientCount; i++)
                 {
-                    using (var client = new MqttFactory().CreateMqttClient())
+                    var clientId = i;
+                    tasks.Add(Task.Run(async () =>
                     {
-                        client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("localhost").Build())
-                            .GetAwaiter().GetResult();
-
-                        for (var j = 0; j < 10; j++)
+                        try
                         {
-                            // Clear retained message.
-                            client.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("r" + i)
-                                .WithPayload(new byte[0]).WithRetainFlag().Build()).GetAwaiter().GetResult();
+                            using (var client = await testSetup.ConnectClientAsync())
+                            {
+                                // Clear retained message.
+                                await client.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("r" + clientId)
+                                    .WithPayload(new byte[0]).WithRetainFlag().Build());
 
-                            // Set retained message.
-                            client.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("r" + i)
-                                .WithPayload("value" + j).WithRetainFlag().Build()).GetAwaiter().GetResult();
-                        }
+                                // Set retained message.
+                                await client.PublishAsync(new MqttApplicationMessageBuilder().WithTopic("r" + clientId)
+                                    .WithPayload("value").WithRetainFlag().Build());
 
-                        Thread.Sleep(100);
+                                await Task.Delay(10);
 
-                        client.DisconnectAsync().GetAwaiter().GetResult();
-                    }
-                });
+                                await client.DisconnectAsync();
+                            }
+                        }
+                        catch (Exception exception)
+                        {
+                            testSetup.TrackException(exception);
+                        }
+                    }));
+                }
+
+                await Task.WhenAll(tasks.ToArray());
 
                 await Task.Delay(1000);
 
@@ -484,10 +483,6 @@ namespace MQTTnet.Tests
                     Assert.IsTrue(retainedMessages.Any(m => m.Topic == "r" + i));
                 }
             }
-            finally
-            {
-                await server.StopAsync();
-            }
         }
 
         [TestMethod]
@@ -986,6 +981,23 @@ namespace MQTTnet.Tests
             MqttQualityOfServiceLevel filterQualityOfServiceLevel,
             int expectedReceivedMessagesCount)
         {
+            //using (var testSetup = new TestSetup())
+            //{
+            //    var server = await testSetup.StartServerAsync(new MqttServerOptionsBuilder());
+
+            //    var clientOptions = new MqttClientOptionsBuilder();
+            //    var c1 = await testSetup.ConnectClientAsync(clientOptions);
+            //    await Task.Delay(500);
+            //    Assert.AreEqual(1, (await server.GetClientSessionsStatusAsync()).Count);
+
+            //    await c1.DisconnectAsync();
+            //    await Task.Delay(500);
+
+            //    Assert.AreEqual(0, (await server.GetClientSessionsStatusAsync()).Count);
+            //}
+
+
+
             var s = new MqttFactory().CreateMqttServer();
 
             var receivedMessagesCount = 0;
@@ -994,13 +1006,15 @@ namespace MQTTnet.Tests
                 await s.StartAsync(new MqttServerOptions());
 
                 var c1 = new MqttFactory().CreateMqttClient();
-                c1.ApplicationMessageReceived += (_, __) => receivedMessagesCount++;
-                await c1.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("localhost").Build());
+                c1.UseReceivedApplicationMessageHandler(c => receivedMessagesCount++);
+
+                await c1.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build());
                 await c1.SubscribeAsync(new TopicFilterBuilder().WithTopic(topicFilter).WithQualityOfServiceLevel(filterQualityOfServiceLevel).Build());
 
                 var c2 = new MqttFactory().CreateMqttClient();
-                await c2.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("localhost").Build());
+                await c2.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("broker.hivemq.com").Build());
                 await c2.PublishAsync(builder => builder.WithTopic(topic).WithPayload(new byte[0]).WithQualityOfServiceLevel(qualityOfServiceLevel));
+                await c2.DisconnectAsync().ConfigureAwait(false);
 
                 await Task.Delay(500);
                 await c1.UnsubscribeAsync(topicFilter);

Tests/MQTTnet.Core.Tests/TestSetup.cs

@@ -1,6 +1,6 @@
 using System;
 using System.Collections.Generic;
-using System.Threading;
+using System.Linq;
 using System.Threading.Tasks;
 using MQTTnet.Client;
 using MQTTnet.Client.Options;
@@ -16,10 +16,15 @@ namespace MQTTnet.Tests
         private readonly IMqttNetLogger _serverLogger = new MqttNetLogger("server");
         private readonly IMqttNetLogger _clientLogger = new MqttNetLogger("client");
 
+        private readonly List<string> _serverErrors = new List<string>();
+        private readonly List<string> _clientErrors = new List<string>();
+
+        private readonly List<Exception> _exceptions = new List<Exception>();
+
         private IMqttServer _server;
 
-        private long _serverErrorsCount;
-        private long _clientErrorsCount;
+        public bool IgnoreClientLogErrors { get; set; }
+        public bool IgnoreServerLogErrors { get; set; }
 
         public TestSetup()
         {
@@ -27,19 +32,30 @@ namespace MQTTnet.Tests
             {
                 if (e.TraceMessage.Level == MqttNetLogLevel.Error)
                 {
-                    Interlocked.Increment(ref _serverErrorsCount);
+                    lock (_serverErrors)
+                    {
+                        _serverErrors.Add(e.TraceMessage.ToString());
+                    }
                 }
             };
 
             _clientLogger.LogMessagePublished += (s, e) =>
             {
-                if (e.TraceMessage.Level == MqttNetLogLevel.Error)
+                lock (_clientErrors)
                 {
-                    Interlocked.Increment(ref _clientErrorsCount);
+                    if (e.TraceMessage.Level == MqttNetLogLevel.Error)
+                    {
+                        _clientErrors.Add(e.TraceMessage.ToString());
+                    }
                 }
             };
         }
 
+        public Task<IMqttServer> StartServerAsync()
+        {
+            return StartServerAsync(new MqttServerOptionsBuilder());
+        }
+
         public async Task<IMqttServer> StartServerAsync(MqttServerOptionsBuilder options)
         {
             if (_server != null)
@@ -53,40 +69,63 @@ namespace MQTTnet.Tests
             return _server;
         }
 
+        public Task<IMqttClient> ConnectClientAsync()
+        {
+            return ConnectClientAsync(new MqttClientOptionsBuilder());
+        }
+
         public async Task<IMqttClient> ConnectClientAsync(MqttClientOptionsBuilder options)
         {
             var client = _mqttFactory.CreateMqttClient(_clientLogger);
-            _clients.Add(client);
-
             await client.ConnectAsync(options.WithTcpServer("localhost", 1888).Build());
 
+            _clients.Add(client);
             return client;
         }
 
         public void ThrowIfLogErrors()
         {
-            if (_serverErrorsCount > 0)
+            lock (_serverErrors)
             {
-                throw new Exception($"Server had {_serverErrorsCount} errors.");
+                if (!IgnoreServerLogErrors && _serverErrors.Count > 0)
+                {
+                    throw new Exception($"Server had {_serverErrors.Count} errors (${string.Join(Environment.NewLine, _serverErrors)}).");
+                }
             }
 
-            if (_clientErrorsCount > 0)
+            lock (_clientErrors)
             {
-                throw new Exception($"Client(s) had {_clientErrorsCount} errors.");
+                if (!IgnoreClientLogErrors && _clientErrors.Count > 0)
+                {
+                    throw new Exception($"Client(s) had {_clientErrors.Count} errors (${string.Join(Environment.NewLine, _clientErrors)}).");
+                }
             }
         }
 
         public void Dispose()
         {
-            ThrowIfLogErrors();
-
             foreach (var mqttClient in _clients)
             {
-                mqttClient.DisconnectAsync().GetAwaiter().GetResult();
-                mqttClient.Dispose();
+                mqttClient?.DisconnectAsync().GetAwaiter().GetResult();
+                mqttClient?.Dispose();
             }
 
             _server.StopAsync().GetAwaiter().GetResult();
+
+            ThrowIfLogErrors();
+
+            if (_exceptions.Any())
+            {
+                throw new Exception($"{_exceptions.Count} exceptions tracked.");
+            }
+        }
+
+        public void TrackException(Exception exception)
+        {
+            lock (_exceptions)
+            {
+                _exceptions.Add(exception);
+            }
         }
     }
 }