one less copy and allocation

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketSerializer.cs

@@ -24,19 +24,18 @@ namespace MQTTnet.Serializer
             {
                 // Leave enough head space for max header size (fixed + 4 variable remaining length)
                 stream.Position = 5;
-
                 var fixedHeader = SerializePacket(packet, writer);
 
-                var remainingLength = MqttPacketWriter.EncodeRemainingLength((int)stream.Length - 5);
+                stream.Position = 1;
+                var remainingLength = MqttPacketWriter.EncodeRemainingLength((int)stream.Length - 5, stream);
 
-                var headerSize = remainingLength.Length + 1;
+                var headerSize = remainingLength + 1;
                 var headerOffset = 5 - headerSize;
 
                 // Position cursor on correct offset on beginining of array (has leading 0x0)
                 stream.Position = headerOffset;
 
                 writer.Write(fixedHeader);
-                writer.Write(remainingLength, 0, remainingLength.Length);
 
 #if NET461 || NET452 || NETSTANDARD2_0
                 var buffer = stream.GetBuffer();

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketWriter.cs

@@ -56,14 +56,18 @@ namespace MQTTnet.Serializer
             Write(value);
         }
 
-        public static byte[] EncodeRemainingLength(int length)
+        public static int EncodeRemainingLength(int length,MemoryStream stream)
         {
+            //write the encoded remaining length right aligned on the 4 byte buffer
+
             if (length <= 0)
             {
-                return new[] { (byte)0 };
+                stream.Seek(3, SeekOrigin.Current);
+                stream.WriteByte(0);
+                return 1;
             }
 
-            var bytes = new byte[4];
+            byte[] buffer = new byte[4];
             var offset = 0;
 
             // Alorithm taken from http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html.
@@ -77,13 +81,15 @@ namespace MQTTnet.Serializer
                     encodedByte = encodedByte | 128;
                 }
 
-                bytes[offset] = (byte)encodedByte;
+                buffer[offset] = (byte)encodedByte;
 
                 offset++;
             } while (x > 0);
 
-            Array.Resize(ref bytes, offset);
-            return bytes;
+            stream.Seek(4 - offset, SeekOrigin.Current);
+            stream.Write(buffer, 0, offset);
+
+            return offset;
         }
     }
 }