Fix exception handling when using invalid host names upon connect.

Build/MQTTnet.nuspec

@@ -17,6 +17,7 @@
 * [Client] Added method to trigger PING/PONG manually (connection check etc.).
 * [Client] Added support for certificate validation callback when using Web Sockets (requires netstandard2.1+).
 * [Client] Fixed a memory leak when web socket based connections trying to reconnect with an offline server.
+* [Client] Fixed an issue when connecting to an invalid host (format).
 * [ManagedClient] Added method to trigger PING/PONG manually (connection check etc.).
 * [MQTTnet.AspNetCore] improved compatibility with AspNetCore 3.1.
 * [MQTTnet.AspNetCore] Fixed a packaging issue with the Nuget package.

Source/MQTTnet/Client/MqttClient.cs

@@ -333,7 +333,7 @@ namespace MQTTnet.Client
 
                 await Task.WhenAll(receiverTask, publishPacketReceiverTask, keepAliveTask).ConfigureAwait(false);
 
-                _publishPacketReceiverQueue.Dispose();
+                _publishPacketReceiverQueue?.Dispose();
             }
             catch (Exception e)
             {

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -71,6 +71,8 @@ namespace MQTTnet.Implementations
 
             await socket.ConnectAsync(_options.Server, _options.GetPort(), cancellationToken).ConfigureAwait(false);
 
+            cancellationToken.ThrowIfCancellationRequested();
+
             var networkStream = socket.GetStream();
 
             if (_options.TlsOptions?.UseTls == true)

Tests/MQTTnet.Core.Tests/MqttClient_Tests.cs

@@ -23,6 +23,30 @@ namespace MQTTnet.Tests
     {
         public TestContext TestContext { get; set; }
 
+        [TestMethod]
+        [ExpectedException(typeof(MqttCommunicationTimedOutException))]
+        public async Task Connect_To_Invalid_Server_Wrong_IP()
+        {
+            var client = new MqttFactory().CreateMqttClient();
+            await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("1.2.3.4").WithCommunicationTimeout(TimeSpan.FromSeconds(2)).Build());
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(MqttCommunicationException))]
+        public async Task Connect_To_Invalid_Server_Port_Not_Opened()
+        {
+            var client = new MqttFactory().CreateMqttClient();
+            await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("127.0.0.1", 12345).WithCommunicationTimeout(TimeSpan.FromSeconds(2)).Build());
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(MqttCommunicationException))]
+        public async Task Connect_To_Invalid_Server_Wrong_Protocol()
+        {
+            var client = new MqttFactory().CreateMqttClient();
+            await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("http://127.0.0.1", 12345).WithCommunicationTimeout(TimeSpan.FromSeconds(2)).Build());
+        }
+
         [TestMethod]
         public async Task Send_Manual_Ping()
         {