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

Frameworks/MQTTnet.NetStandard/Adapter/MqttChannelAdapter.cs

@@ -17,8 +17,7 @@ namespace MQTTnet.Adapter
     public class MqttChannelAdapter : IMqttChannelAdapter
     {
         private const uint ErrorOperationAborted = 0x800703E3;
-
-        private static readonly byte[] EmptyBody = new byte[0];
+        private const int ReadBufferSize = 4096;  // TODO: Move buffer size to config
 
         private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
         private readonly IMqttNetLogger _logger;
@@ -91,28 +90,35 @@ namespace MQTTnet.Adapter
             MqttBasePacket packet = null;
             await ExecuteAndWrapExceptionAsync(async () =>
             {
-                ReceivedMqttPacket receivedMqttPacket;
-                if (timeout > TimeSpan.Zero)
-                {
-                    receivedMqttPacket = await ReceiveAsync(_channel.ReceiveStream, cancellationToken).TimeoutAfter(timeout).ConfigureAwait(false);
-                }
-                else
+                ReceivedMqttPacket receivedMqttPacket = null;
+                try
                 {
-                    receivedMqttPacket = await ReceiveAsync(_channel.ReceiveStream, cancellationToken).ConfigureAwait(false);
-                }
+                    if (timeout > TimeSpan.Zero)
+                    {
+                        receivedMqttPacket = await ReceiveAsync(_channel.ReceiveStream, cancellationToken).TimeoutAfter(timeout).ConfigureAwait(false);
+                    }
+                    else
+                    {
+                        receivedMqttPacket = await ReceiveAsync(_channel.ReceiveStream, cancellationToken).ConfigureAwait(false);
+                    }
 
-                if (receivedMqttPacket == null || cancellationToken.IsCancellationRequested)
-                {
-                    throw new TaskCanceledException();
-                }
+                    if (receivedMqttPacket == null || cancellationToken.IsCancellationRequested)
+                    {
+                        throw new TaskCanceledException();
+                    }
+
+                    packet = PacketSerializer.Deserialize(receivedMqttPacket.Header, receivedMqttPacket.Body);
+                    if (packet == null)
+                    {
+                        throw new MqttProtocolViolationException("Received malformed packet.");
+                    }
 
-                packet = PacketSerializer.Deserialize(receivedMqttPacket.Header, receivedMqttPacket.Body);
-                if (packet == null)
+                    _logger.Trace<MqttChannelAdapter>("RX <<< {0}", packet);
+                }
+                finally
                 {
-                    throw new MqttProtocolViolationException("Received malformed packet.");
+                    receivedMqttPacket?.Dispose();
                 }
-
-                _logger.Trace<MqttChannelAdapter>("RX <<< {0}", packet);
             }).ConfigureAwait(false);
 
             return packet;
@@ -120,7 +126,7 @@ namespace MQTTnet.Adapter
 
         private static async Task<ReceivedMqttPacket> ReceiveAsync(Stream stream, CancellationToken cancellationToken)
         {
-            var header = await MqttPacketReader.ReadHeaderFromSourceAsync(stream, cancellationToken).ConfigureAwait(false);
+            var header = await MqttPacketReader.ReadHeaderAsync(stream, cancellationToken).ConfigureAwait(false);
             if (header == null)
             {
                 return null;
@@ -128,23 +134,26 @@ namespace MQTTnet.Adapter
 
             if (header.BodyLength == 0)
             {
-                return new ReceivedMqttPacket(header, EmptyBody);
+                return new ReceivedMqttPacket(header, new MemoryStream(new byte[0], false));
             }
 
-            var body = new byte[header.BodyLength];
-
-            var offset = 0;
-            do
+            var body = header.BodyLength <= ReadBufferSize ? new MemoryStream(new byte[header.BodyLength]) : new MemoryStream();
+            
+            var buffer = new byte[ReadBufferSize];
+            while (body.Length < header.BodyLength)
             {
-                var readBytesCount = await stream.ReadAsync(body, offset, body.Length - offset, cancellationToken).ConfigureAwait(false);
+                var readBytesCount = await stream.ReadAsync(buffer, 0, buffer.Length, cancellationToken).ConfigureAwait(false);
+                
                 // Check if the client closed the connection before sending the full body.
                 if (readBytesCount == 0)
                 {
                     throw new MqttCommunicationException("Connection closed while reading remaining packet body.");
                 }
 
-                offset += readBytesCount;
-            } while (offset < header.BodyLength);
+                body.Write(buffer, 0, readBytesCount);
+            }
+
+            body.Seek(0L, SeekOrigin.Begin);
             
             return new ReceivedMqttPacket(header, body);
         }

Frameworks/MQTTnet.NetStandard/Adapter/ReceivedMqttPacket.cs

@@ -1,11 +1,12 @@
 using System;
+using System.IO;
 using MQTTnet.Packets;
 
 namespace MQTTnet.Adapter
 {
-    public class ReceivedMqttPacket
+    public sealed class ReceivedMqttPacket : IDisposable
     {
-        public ReceivedMqttPacket(MqttPacketHeader header, byte[] body)
+        public ReceivedMqttPacket(MqttPacketHeader header, MemoryStream body)
         {
             Header = header ?? throw new ArgumentNullException(nameof(header));
             Body = body ?? throw new ArgumentNullException(nameof(body));
@@ -13,6 +14,11 @@ namespace MQTTnet.Adapter
 
         public MqttPacketHeader Header { get; }
 
-        public byte[] Body { get; }
+        public MemoryStream Body { get; }
+
+        public void Dispose()
+        {
+            Body?.Dispose();
+        }
     }
 }

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);
             }

Tests/MQTTnet.Core.Tests/MqttPacketReaderTests.cs

@@ -12,7 +12,7 @@ namespace MQTTnet.Core.Tests
         public void MqttPacketReader_EmptyStream()
         {
             var memStream = new MemoryStream();
-            var header = MqttPacketReader.ReadHeaderFromSourceAsync(memStream, CancellationToken.None).GetAwaiter().GetResult();
+            var header = MqttPacketReader.ReadHeaderAsync(memStream, CancellationToken.None).GetAwaiter().GetResult();
 
             Assert.IsNull(header);
         }

Tests/MQTTnet.Core.Tests/MqttPacketSerializerTests.cs

@@ -405,11 +405,11 @@ namespace MQTTnet.Core.Tests
 
             using (var headerStream = new MemoryStream(Join(buffer1)))
             {
-                var header = MqttPacketReader.ReadHeaderFromSourceAsync(headerStream, CancellationToken.None).GetAwaiter().GetResult();
+                var header = MqttPacketReader.ReadHeaderAsync(headerStream, CancellationToken.None).GetAwaiter().GetResult();
 
                 using (var bodyStream = new MemoryStream(Join(buffer1), (int)headerStream.Position, header.BodyLength))
                 {
-                    var deserializedPacket = serializer.Deserialize(header, bodyStream.ToArray());
+                    var deserializedPacket = serializer.Deserialize(header, bodyStream);
                     var buffer2 = serializer.Serialize(deserializedPacket);
 
                     Assert.AreEqual(expectedBase64Value, Convert.ToBase64String(Join(buffer2)));