fixed tests and added testcase for user property serialization

Source/MQTTnet.AspnetCore/SpanBasedMqttPacketBodyReader.cs

@@ -51,6 +51,11 @@ namespace MQTTnet.AspNetCore
         public unsafe string ReadStringWithLengthPrefix()
         {
             var buffer = ReadSegmentWithLengthPrefix();
+            if (buffer.Length == 0)
+            {
+                return string.Empty;
+            }
+
             fixed (byte* bytes = &buffer.GetPinnableReference())
             {
                 var result = Encoding.UTF8.GetString(bytes, buffer.Length);

Source/MQTTnet.AspnetCore/SpanBasedMqttPacketWriter.cs

@@ -60,12 +60,11 @@ namespace MQTTnet.AspNetCore
 
         public void Write(IMqttPacketWriter propertyWriter)
         {
-            if (propertyWriter is SpanBasedMqttPacketWriter writer)
-            {
-                GrowIfNeeded(1);
-            }
+            if (propertyWriter == null) throw new ArgumentNullException(nameof(propertyWriter));
 
-            throw new InvalidOperationException($"{nameof(propertyWriter)} must be of type {typeof(SpanBasedMqttPacketWriter).Name}");
+            GrowIfNeeded(propertyWriter.Length);
+            Write(propertyWriter.GetBuffer(), 0, propertyWriter.Length);
+            Commit(propertyWriter.Length);
         }
 
         public void Write(byte[] payload, int start, int length)

Source/MQTTnet/Formatter/MqttPacketFormatterAdapter.cs

@@ -89,28 +89,30 @@ namespace MQTTnet.Formatter
             }
 
             ProtocolVersion = protocolVersion;
+            _formatter = GetMqttPacketFormatter(protocolVersion, Writer);
+        }
 
+        public static IMqttPacketFormatter GetMqttPacketFormatter(MqttProtocolVersion protocolVersion, IMqttPacketWriter writer)
+        {
+            if (protocolVersion == MqttProtocolVersion.Unknown)
+            {
+                throw new InvalidOperationException("MQTT protocol version is invalid.");
+            }
+            
             switch (protocolVersion)
             {
                 case MqttProtocolVersion.V500:
                     {
-                        _formatter = new MqttV500PacketFormatter(Writer);
-                        break;
+                        return new MqttV500PacketFormatter(writer);
                     }
-
                 case MqttProtocolVersion.V311:
                     {
-                        _formatter = new MqttV311PacketFormatter(Writer);
-                        break;
+                        return new MqttV311PacketFormatter(writer);
                     }
-
                 case MqttProtocolVersion.V310:
                     {
-
-                        _formatter = new MqttV310PacketFormatter(Writer);
-                        break;
+                        return new MqttV310PacketFormatter(writer);
                     }
-                    
                 default:
                     {
                         throw new NotSupportedException();

Source/MQTTnet/Formatter/MqttPacketWriter.cs

@@ -154,6 +154,7 @@ namespace MQTTnet.Formatter
                 }
 
                 Write(writer._buffer, 0, writer.Length);
+                return;
             }
 
             throw new InvalidOperationException($"{nameof(propertyWriter)} must be of type {typeof(MqttPacketWriter).Name}");

Tests/MQTTnet.Core.Tests/MqttPacketSerializer_Tests.cs

@@ -231,6 +231,32 @@ namespace MQTTnet.Tests
             SerializeAndCompare(p, "Ow4ABUEvQi9DAHtIRUxMTw==");
         }
 
+        [TestMethod]
+        public void SerializeV500_MqttPublishPacket()
+        {
+            var prop = new MqttPublishPacketProperties();
+
+            prop.ResponseTopic = "/Response";
+
+            prop.UserProperties.Add(new MqttUserProperty("Foo", "Bar"));
+
+            var p = new MqttPublishPacket
+            {
+                PacketIdentifier = 123,
+                Dup = true,
+                Retain = true,
+                Payload = Encoding.ASCII.GetBytes("HELLO"),
+                QualityOfServiceLevel = MqttQualityOfServiceLevel.AtLeastOnce,
+                Topic = "A/B/C",
+                Properties = prop
+            };
+
+            var deserialized = Roundtrip(p, MqttProtocolVersion.V500);
+
+            Assert.AreEqual(prop.ResponseTopic, deserialized.Properties.ResponseTopic);
+            Assert.IsTrue(deserialized.Properties.UserProperties.Any(x => x.Name == "Foo"));
+        }
+
         [TestMethod]
         public void DeserializeV311_MqttPublishPacket()
         {
@@ -542,19 +568,7 @@ namespace MQTTnet.Tests
         {
             var writer = WriterFactory();
 
-            IMqttPacketFormatter serializer;
-            if (protocolVersion == MqttProtocolVersion.V311)
-            {
-                serializer = new MqttV311PacketFormatter(writer);
-            }
-            else if (protocolVersion == MqttProtocolVersion.V310)
-            {
-                serializer = new MqttV310PacketFormatter(writer);
-            }
-            else
-            {
-                throw new NotSupportedException();
-            }
+            var serializer = MqttPacketFormatterAdapter.GetMqttPacketFormatter(protocolVersion, writer);
 
             var buffer1 = serializer.Encode(packet);
 
@@ -580,20 +594,8 @@ namespace MQTTnet.Tests
         {
             var writer = WriterFactory();
 
-            IMqttPacketFormatter serializer;
-            if (protocolVersion == MqttProtocolVersion.V311)
-            {
-                serializer = new MqttV311PacketFormatter(writer);
-            }
-            else if (protocolVersion == MqttProtocolVersion.V310)
-            {
-                serializer = new MqttV310PacketFormatter(writer);
-            }
-            else
-            {
-                throw new NotSupportedException();
-            }
-
+            var serializer = MqttPacketFormatterAdapter.GetMqttPacketFormatter(protocolVersion, writer);
+            
             var buffer1 = serializer.Encode(packet);
 
             using (var headerStream = new MemoryStream(Join(buffer1)))