Fix cancellation issues when connecting.

Build/MQTTnet.nuspec

@@ -26,7 +26,7 @@
 * [Server] Improved message processing performance (+ ~5%).
 * [Server] Fix wrong usage of client session items for undelivered messages.
 
-      Git commit: $gitCommit
+Git commit: $gitCommit
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2020</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin Blazor</tags> 

Source/MQTTnet/Exceptions/MqttCommunicationException.cs

@@ -13,8 +13,8 @@ namespace MQTTnet.Exceptions
         {
         }
 
-        public MqttCommunicationException(string message)
+        public MqttCommunicationException(string message, Exception innerException = null)
-            : base(message)
+            : base(message, innerException)
         {
         }
     }

Source/MQTTnet/Implementations/CrossPlatformSocket.cs

@@ -4,6 +4,7 @@ using System.Net;
 using System.Net.Sockets;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Exceptions;
 
 namespace MQTTnet.Implementations
 {
@@ -123,9 +124,19 @@ namespace MQTTnet.Implementations
                     _networkStream = new NetworkStream(_socket, true);
                 }
             }
+            catch (SocketException socketException)
+            {
+                if (socketException.SocketErrorCode == SocketError.OperationAborted)
+                {
+                    throw new OperationCanceledException();
+                }
+
+                throw new MqttCommunicationException($"Error while connecting with host '{host}:{port}'.", socketException);
+            }
             catch (ObjectDisposedException)
             {
                 // This will happen when _socket.EndConnect gets called by Task library but the socket is already disposed.
+                cancellationToken.ThrowIfCancellationRequested();
             }
         }
 

Tests/MQTTnet.Core.Tests/CrossPlatformSocket_Tests.cs

@@ -29,14 +29,15 @@ namespace MQTTnet.Tests
         }
 
         [TestMethod]
+        [ExpectedException(typeof(OperationCanceledException))]
         public async Task Try_Connect_Invalid_Host()
         {
             var crossPlatformSocket = new CrossPlatformSocket();
 
-            var cancellationToken = new CancellationTokenSource(TimeSpan.FromSeconds(3));
+            var cancellationToken = new CancellationTokenSource(TimeSpan.FromSeconds(5));
             cancellationToken.Token.Register(() => crossPlatformSocket.Dispose());
 
-            await crossPlatformSocket.ConnectAsync("www.google.de", 1234, CancellationToken.None);
+            await crossPlatformSocket.ConnectAsync("www.google.de", 54321, cancellationToken.Token);
         }
 
         //[TestMethod]