Update docs.

Build/MQTTnet.nuspec

@@ -27,6 +27,7 @@
 * [Server] Fixed bug in PubRel packet generation (MQTTv5 only).
 * [Server] Improved message processing performance (+ ~5%).
 * [Server] Fix wrong usage of client session items for undelivered messages.
+* [Server] Allow to respond with a reason code in PUBACK/PUBREC (thanks to @muneebmajeed).
 
 Git commit: $gitCommit
     </releaseNotes>

Source/MQTTnet/Client/MqttClient.cs

@@ -637,25 +637,27 @@ namespace MQTTnet.Client
                     }
                     else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtLeastOnce)
                     {
-                        var reasonCode = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
+                        var eventArgs = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
-                        if (reasonCode != null && Enum.IsDefined(typeof(MqttPubAckReasonCode), reasonCode.Value))
+
+                        if (!eventArgs.ProcessingFailed)
                         {
                             await SendAsync(new MqttPubAckPacket
                             {
                                 PacketIdentifier = publishPacket.PacketIdentifier,
-                                ReasonCode = (MqttPubAckReasonCode)reasonCode.Value
+                                ReasonCode = (MqttPubAckReasonCode)eventArgs.ReasonCode
                             }, cancellationToken).ConfigureAwait(false);
                         }
                     }
                     else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.ExactlyOnce)
                     {
-                        var reasonCode = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
+                        var eventArgs = await HandleReceivedApplicationMessageAsync(publishPacket).ConfigureAwait(false);
-                        if (reasonCode != null && Enum.IsDefined(typeof(MqttPubRecReasonCode), reasonCode.Value))
+
+                        if (!eventArgs.ProcessingFailed)
                         {
                             await SendAsync(new MqttPubRecPacket
                             {
                                 PacketIdentifier = publishPacket.PacketIdentifier,
-                                ReasonCode = (MqttPubRecReasonCode)reasonCode.Value
+                                ReasonCode = (MqttPubRecReasonCode)eventArgs.ReasonCode
                             }, cancellationToken).ConfigureAwait(false);
                         }
                     }
@@ -756,22 +758,18 @@ namespace MQTTnet.Client
             return _adapter.PacketFormatterAdapter.DataConverter.CreatePublishResult(pubRecPacket, pubCompPacket);
         }
 
-        async Task<int?> HandleReceivedApplicationMessageAsync(MqttPublishPacket publishPacket)
+        async Task<MqttApplicationMessageReceivedEventArgs> HandleReceivedApplicationMessageAsync(MqttPublishPacket publishPacket)
         {
-            int? reasonCode = 0;
             var applicationMessage = _adapter.PacketFormatterAdapter.DataConverter.CreateApplicationMessage(publishPacket);
+            var eventArgs = new MqttApplicationMessageReceivedEventArgs(Options.ClientId, applicationMessage);
 
             var handler = ApplicationMessageReceivedHandler;
             if (handler != null)
             {
-                var eventArgs = new MqttApplicationMessageReceivedEventArgs(Options.ClientId, applicationMessage);
                 await handler.HandleApplicationMessageReceivedAsync(eventArgs).ConfigureAwait(false);
-                if (eventArgs.ProcessingFailed)
-                {
-                    reasonCode = eventArgs.ReasonCode;
-                }
             }
-            return reasonCode;
+
+            return eventArgs;
         }
 
         async Task WaitForTaskAsync(Task task, Task sender)

Source/MQTTnet/MqttApplicationMessageReceivedEventArgs.cs

@@ -1,5 +1,4 @@
-using MQTTnet.Protocol;
+using System;
-using System;
 
 namespace MQTTnet
 {
@@ -17,6 +16,6 @@ namespace MQTTnet
 
         public bool ProcessingFailed { get; set; }
 
-        public int? ReasonCode { get; set; }
+        public MqttApplicationMessageReceivedReasonCode ReasonCode { get; set; } = MqttApplicationMessageReceivedReasonCode.Success;
     }
 }

Source/MQTTnet/MqttApplicationMessageReceivedReasonCode.cs

@@ -0,0 +1,15 @@
+namespace MQTTnet
+{
+    public enum MqttApplicationMessageReceivedReasonCode
+    {
+        Success = 0,
+        NoMatchingSubscribers = 16,
+        UnspecifiedError = 128,
+        ImplementationSpecificError = 131,
+        NotAuthorized = 135,
+        TopicNameInvalid = 144,
+        PacketIdentifierInUse = 145,
+        QuotaExceeded = 151,
+        PayloadFormatInvalid = 153
+    }
+}