unfifed timeout handling and fixed memory usage due to Task.Delay tasks for each send package are present for the duration of the timeout resulting in memory usage. new approach uses cancellationtoken that will be cleaned up directly if operation completes before timeout

MQTTnet.Core/Adapter/MqttChannelCommunicationAdapter.cs

@@ -1,10 +1,12 @@
 using System;
 using System.IO;
+using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Core.Channel;
 using MQTTnet.Core.Client;
 using MQTTnet.Core.Diagnostics;
 using MQTTnet.Core.Exceptions;
+using MQTTnet.Core.Internal;
 using MQTTnet.Core.Packets;
 using MQTTnet.Core.Serializer;
 
@@ -24,7 +26,7 @@ namespace MQTTnet.Core.Adapter
 
         public Task ConnectAsync(MqttClientOptions options, TimeSpan timeout)
         {
-            return ExecuteWithTimeoutAsync(_channel.ConnectAsync(options), timeout);
+            return _channel.ConnectAsync(options).TimeoutAfter(timeout);
         }
 
         public Task DisconnectAsync()
@@ -38,7 +40,7 @@ namespace MQTTnet.Core.Adapter
 
             var writeBuffer = PacketSerializer.Serialize(packet);
             _sendTask = SendAsync( writeBuffer );
-            return ExecuteWithTimeoutAsync(_sendTask, timeout);
+            return _sendTask.TimeoutAfter(timeout);
         }
 
         private Task _sendTask = Task.FromResult(0); // this task is used to prevent overlapping write
@@ -54,7 +56,7 @@ namespace MQTTnet.Core.Adapter
             Tuple<MqttPacketHeader, MemoryStream> tuple;
             if (timeout > TimeSpan.Zero)
             {
-                tuple = await ExecuteWithTimeoutAsync(ReceiveAsync(), timeout).ConfigureAwait(false);
+                tuple = await ReceiveAsync().TimeoutAfter(timeout).ConfigureAwait(false);
             }
             else
             {
@@ -96,35 +98,5 @@ namespace MQTTnet.Core.Adapter
 
             return Tuple.Create(header, body);
         }
-
-        private static async Task<TResult> ExecuteWithTimeoutAsync<TResult>(Task<TResult> task, TimeSpan timeout)
-        {
-            var timeoutTask = Task.Delay(timeout);
-            if (await Task.WhenAny(timeoutTask, task).ConfigureAwait(false) == timeoutTask)
-            {
-                throw new MqttCommunicationTimedOutException();
-            }
-
-            if (task.IsFaulted)
-            {
-                throw new MqttCommunicationException(task.Exception);
-            }
-
-            return task.Result;
-        }
-
-        private static async Task ExecuteWithTimeoutAsync(Task task, TimeSpan timeout)
-        {
-            var timeoutTask = Task.Delay(timeout);
-            if (await Task.WhenAny(timeoutTask, task).ConfigureAwait(false) == timeoutTask)
-            {
-                throw new MqttCommunicationTimedOutException();
-            }
-
-            if (task.IsFaulted)
-            {
-                throw new MqttCommunicationException(task.Exception);
-            }
-        }
     }
 }

MQTTnet.Core/Client/MqttPacketDispatcher.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Threading.Tasks;
 using MQTTnet.Core.Diagnostics;
 using MQTTnet.Core.Exceptions;
+using MQTTnet.Core.Internal;
 using MQTTnet.Core.Packets;
 using System.Collections.Concurrent;
 
@@ -22,16 +23,19 @@ namespace MQTTnet.Core.Client
             var packetAwaiter = AddPacketAwaiter(request, responseType);
             DispatchPendingPackets();
 
-            var hasTimeout = await Task.WhenAny(Task.Delay(timeout), packetAwaiter.Task).ConfigureAwait(false) != packetAwaiter.Task;
-            RemovePacketAwaiter(request, responseType);
-
-            if (hasTimeout)
+            try
             {
-                MqttTrace.Warning(nameof(MqttPacketDispatcher), "Timeout while waiting for packet.");
-                throw new MqttCommunicationTimedOutException();
+                return await packetAwaiter.Task.TimeoutAfter( timeout );
+            }
+            catch ( MqttCommunicationTimedOutException )
+            {
+                MqttTrace.Warning( nameof( MqttPacketDispatcher ), "Timeout while waiting for packet." );
+                throw;
+            }
+            finally
+            {
+                RemovePacketAwaiter(request, responseType);
             }
-
-            return packetAwaiter.Task.Result;
         }
 
         public void Dispatch(MqttBasePacket packet)

MQTTnet.Core/Internal/TaskExtensions.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using MQTTnet.Core.Exceptions;
+
+namespace MQTTnet.Core.Internal
+{
+    public static class TaskExtensions
+    {
+        public static Task TimeoutAfter( this Task task, TimeSpan timeout )
+        {
+            return TimeoutAfter( task.ContinueWith( t => 0 ), timeout );
+        }
+
+        public static async Task<TResult> TimeoutAfter<TResult>(this Task<TResult> task, TimeSpan timeout)
+        {
+            using (var cancellationTokenSource = new CancellationTokenSource())
+            {
+                var tcs = new TaskCompletionSource<TResult>();
+
+                cancellationTokenSource.Token.Register(() =>
+                {
+                    tcs.TrySetCanceled();
+                } );
+                
+                try
+                {
+                    cancellationTokenSource.CancelAfter(timeout);
+                    task.ContinueWith( t =>
+                    {
+                        if (t.IsFaulted)
+                        {
+                            tcs.TrySetException(t.Exception);
+                        }
+
+                        if (t.IsCompleted)
+                        {
+                            tcs.TrySetResult(t.Result);
+                        }
+                    }, cancellationTokenSource.Token );
+
+                    return await tcs.Task;
+                }
+                catch (TaskCanceledException)
+                {
+                    throw new MqttCommunicationTimedOutException();
+                }
+                catch (Exception e)
+                {
+                    throw new MqttCommunicationException(e);
+                }
+            }
+        }
+    }
+}

Tests/MQTTnet.Core.Tests/ExtensionTests.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Core.Exceptions;
+using MQTTnet.Core.Internal;
+
+namespace MQTTnet.Core.Tests
+{
+    [TestClass]
+    public class ExtensionTests
+    {
+        [ExpectedException(typeof( MqttCommunicationTimedOutException ) )]
+        [TestMethod]
+        public async Task TestTimeoutAfter()
+        {
+            await Task.Delay(TimeSpan.FromMilliseconds(500)).TimeoutAfter(TimeSpan.FromMilliseconds(100));
+        }
+
+        [ExpectedException(typeof( MqttCommunicationTimedOutException))]
+        [TestMethod]
+        public async Task TestTimeoutAfterWithResult()
+        {
+            await Task.Delay(TimeSpan.FromMilliseconds(500)).ContinueWith(t =>  5).TimeoutAfter(TimeSpan.FromMilliseconds(100));
+        }
+        
+        [TestMethod]
+        public async Task TestTimeoutAfterCompleteInTime()
+        {
+            var result = await Task.Delay( TimeSpan.FromMilliseconds( 100 ) ).ContinueWith( t => 5 ).TimeoutAfter( TimeSpan.FromMilliseconds( 500 ) );
+            Assert.AreEqual( 5, result );
+        }
+    }
+}

Tests/MQTTnet.Core.Tests/MQTTnet.Core.Tests.csproj

@@ -86,6 +86,7 @@
   <ItemGroup>
     <Compile Include="ByteReaderTests.cs" />
     <Compile Include="ByteWriterTests.cs" />
+    <Compile Include="ExtensionTests.cs" />
     <Compile Include="MqttPacketSerializerTests.cs" />
     <Compile Include="MqttServerTests.cs" />
     <Compile Include="MqttSubscriptionsManagerTests.cs" />