Covnert the body buffer to a memory stream which will grow automatically.

Frameworks/MQTTnet.NetStandard/Serializer/IMqttPacketSerializer.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
 using MQTTnet.Packets;
 
 namespace MQTTnet.Serializer
@@ -10,6 +11,6 @@ namespace MQTTnet.Serializer
 
         ICollection<ArraySegment<byte>> Serialize(MqttBasePacket mqttPacket);
 
-        MqttBasePacket Deserialize(MqttPacketHeader header, byte[] body);
+        MqttBasePacket Deserialize(MqttPacketHeader header, MemoryStream body);
     }
 }

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketReader.cs

@@ -22,7 +22,7 @@ namespace MQTTnet.Serializer
 
         public bool EndOfRemainingData => BaseStream.Position == _header.BodyLength;
 
-        public static async Task<MqttPacketHeader> ReadHeaderFromSourceAsync(Stream stream, CancellationToken cancellationToken)
+        public static async Task<MqttPacketHeader> ReadHeaderAsync(Stream stream, CancellationToken cancellationToken)
         {
             // Wait for the next package which starts with the header. At this point there will probably
             // some large delay and thus the thread should be put back to the pool (await). So ReadByte()
@@ -36,7 +36,7 @@ namespace MQTTnet.Serializer
 
             var fixedHeader = buffer[0];
             var controlPacketType = (MqttControlPacketType)(fixedHeader >> 4);
-            var bodyLength = await ReadBodyLengthFromSourceAsync(stream, cancellationToken).ConfigureAwait(false);
+            var bodyLength = await ReadBodyLengthAsync(stream, cancellationToken).ConfigureAwait(false);
 
             return new MqttPacketHeader
             {
@@ -84,7 +84,7 @@ namespace MQTTnet.Serializer
             return ReadBytes(_header.BodyLength - (int)BaseStream.Position);
         }
 
-        private static async Task<int> ReadBodyLengthFromSourceAsync(Stream stream, CancellationToken cancellationToken)
+        private static async Task<int> ReadBodyLengthAsync(Stream stream, CancellationToken cancellationToken)
         {
             // Alorithm taken from https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/errata01/os/mqtt-v3.1.1-errata01-os-complete.html.
             var multiplier = 1;

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketSerializer.cs

@@ -42,13 +42,12 @@ namespace MQTTnet.Serializer
             }
         }
 
-        public MqttBasePacket Deserialize(MqttPacketHeader header, byte[] body)
+        public MqttBasePacket Deserialize(MqttPacketHeader header, MemoryStream body)
         {
             if (header == null) throw new ArgumentNullException(nameof(header));
             if (body == null) throw new ArgumentNullException(nameof(body));
 
-            using (var bodyStream = new MemoryStream(body))
-            using (var reader = new MqttPacketReader(header, bodyStream))
+            using (var reader = new MqttPacketReader(header, body))
             {
                 return Deserialize(header, reader);
             }