Add interfaces, optimize performance, fix QoS level 2

MQTTnet.Core/Packets/IMqttPacketWithIdentifier.cs

@@ -1,6 +1,6 @@
 namespace MQTTnet.Core.Packets
 {
-    public interface IPacketWithIdentifier
+    public interface IMqttPacketWithIdentifier
     {
         ushort PacketIdentifier { get; set; }
     }

MQTTnet.Core/Packets/MqttBasePacket.cs

@@ -1,25 +1,6 @@
-using System;
-
-namespace MQTTnet.Core.Packets
+namespace MQTTnet.Core.Packets
 {
     public abstract class MqttBasePacket
     {
-        public TResponsePacket CreateResponse<TResponsePacket>()
-        {
-            var responsePacket = Activator.CreateInstance<TResponsePacket>();
-            var responsePacketWithIdentifier = responsePacket as IPacketWithIdentifier;
-            if (responsePacketWithIdentifier != null)
-            {
-                var requestPacketWithIdentifier = this as IPacketWithIdentifier;
-                if (requestPacketWithIdentifier == null)
-                {
-                    throw new InvalidOperationException("Response packet has PacketIdentifier but request packet does not.");
-                }
-
-                responsePacketWithIdentifier.PacketIdentifier = requestPacketWithIdentifier.PacketIdentifier;
-            }
-
-            return responsePacket;
-        }
     }
 }

MQTTnet.Core/Packets/MqttBasePublishPacket.cs

@@ -1,6 +1,6 @@
 namespace MQTTnet.Core.Packets
 {
-    public class MqttBasePublishPacket : MqttBasePacket, IPacketWithIdentifier
+    public class MqttBasePublishPacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
         public ushort PacketIdentifier { get; set; }
     }

MQTTnet.Core/Packets/MqttPacketExtensions.cs

@@ -0,0 +1,27 @@
+using System;
+
+namespace MQTTnet.Core.Packets
+{
+    public static class MqttPacketExtensions
+    {
+        public static TResponsePacket CreateResponse<TResponsePacket>(this MqttBasePacket packet)
+        {
+            if (packet == null) throw new ArgumentNullException(nameof(packet));
+
+            var responsePacket = Activator.CreateInstance<TResponsePacket>();
+
+            if (responsePacket is IMqttPacketWithIdentifier responsePacketWithIdentifier)
+            {
+                var requestPacketWithIdentifier = packet as IMqttPacketWithIdentifier;
+                if (requestPacketWithIdentifier == null)
+                {
+                    throw new InvalidOperationException("Response packet has PacketIdentifier but request packet does not.");
+                }
+
+                responsePacketWithIdentifier.PacketIdentifier = requestPacketWithIdentifier.PacketIdentifier;
+            }
+
+            return responsePacket;
+        }
+    }
+}

MQTTnet.Core/Packets/MqttSubAckPacket.cs

@@ -4,7 +4,7 @@ using MQTTnet.Core.Protocol;
 
 namespace MQTTnet.Core.Packets
 {
-    public sealed class MqttSubAckPacket : MqttBasePacket, IPacketWithIdentifier
+    public sealed class MqttSubAckPacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
         public ushort PacketIdentifier { get; set; }
 

MQTTnet.Core/Packets/MqttSubscribePacket.cs

@@ -3,7 +3,7 @@ using System.Linq;
 
 namespace MQTTnet.Core.Packets
 {
-    public sealed class MqttSubscribePacket : MqttBasePacket, IPacketWithIdentifier
+    public sealed class MqttSubscribePacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
         public ushort PacketIdentifier { get; set; }
         

MQTTnet.Core/Packets/MqttUnsubAckPacket.cs

@@ -1,6 +1,6 @@
 namespace MQTTnet.Core.Packets
 {
-    public sealed class MqttUnsubAckPacket : MqttBasePacket, IPacketWithIdentifier
+    public sealed class MqttUnsubAckPacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
         public ushort PacketIdentifier { get; set; }
     }

MQTTnet.Core/Packets/MqttUnsubscribe.cs

@@ -2,7 +2,7 @@
 
 namespace MQTTnet.Core.Packets
 {
-    public sealed class MqttUnsubscribePacket : MqttBasePacket, IPacketWithIdentifier
+    public sealed class MqttUnsubscribePacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
         public ushort PacketIdentifier { get; set; }