Add Unit Tests.

Source/MQTTnet.Tests/Mockups/TestEnvironment.cs

@@ -96,28 +96,32 @@ namespace MQTTnet.Tests.Mockups
             return ConnectClient(Factory.CreateClientOptionsBuilder().WithProtocolVersion(_protocolVersion));
         }
 
-        public async Task<MqttClient> ConnectClient(Action<MqttClientOptionsBuilder> optionsBuilder, TimeSpan timeout = default)
+        public async Task<MqttClient> ConnectClient(Action<MqttClientOptionsBuilder> configureOptions, TimeSpan timeout = default)
         {
-            if (optionsBuilder == null)
+            if (configureOptions == null)
             {
-                throw new ArgumentNullException(nameof(optionsBuilder));
+                throw new ArgumentNullException(nameof(configureOptions));
             }
 
-            var options = Factory.CreateClientOptionsBuilder().WithProtocolVersion(_protocolVersion).WithTcpServer("127.0.0.1", ServerPort);
+            // Start with initial default values.
+            var optionsBuilder = Factory.CreateClientOptionsBuilder().WithProtocolVersion(_protocolVersion).WithTcpServer("127.0.0.1", ServerPort);
 
-            optionsBuilder.Invoke(options);
+            // Let the caller override settings. Do not touch the options after this.
+            configureOptions.Invoke(optionsBuilder);
 
+            var options = optionsBuilder.Build();
+            
             var client = CreateClient();
 
             if (timeout == TimeSpan.Zero)
             {
-                await client.ConnectAsync(options.Build()).ConfigureAwait(false);
+                await client.ConnectAsync(options).ConfigureAwait(false);
             }
             else
             {
                 using (var timeoutToken = new CancellationTokenSource(timeout))
                 {
-                    await client.ConnectAsync(options.Build(), timeoutToken.Token).ConfigureAwait(false);
+                    await client.ConnectAsync(options, timeoutToken.Token).ConfigureAwait(false);
                 }
             }
 

Source/MQTTnet.Tests/Server/Cross_Version_Tests.cs

@@ -0,0 +1,67 @@
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Formatter;
+
+namespace MQTTnet.Tests.Server
+{
+    [TestClass]
+    public sealed class Cross_Version_Tests : BaseTestClass
+    {
+        [TestMethod]
+        public async Task Send_V311_Receive_V500()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                await testEnvironment.StartServer();
+
+                var receiver = await testEnvironment.ConnectClient(o => o.WithProtocolVersion(MqttProtocolVersion.V500));
+                var receivedApplicationMessages = testEnvironment.CreateApplicationMessageHandler(receiver);
+                await receiver.SubscribeAsync("#");
+
+                var sender = await testEnvironment.ConnectClient();
+
+                var applicationMessage = new MqttApplicationMessageBuilder().WithTopic("My/Message").WithPayload("My_Payload").Build();
+                await sender.PublishAsync(applicationMessage);
+
+                await LongTestDelay();
+
+                Assert.AreEqual(1, receivedApplicationMessages.ReceivedEventArgs.Count);
+                Assert.AreEqual("My/Message", receivedApplicationMessages.ReceivedEventArgs.First().ApplicationMessage.Topic);
+                Assert.AreEqual("My_Payload", receivedApplicationMessages.ReceivedEventArgs.First().ApplicationMessage.ConvertPayloadToString());
+            }
+        }
+
+        [TestMethod]
+        public async Task Send_V500_Receive_V311()
+        {
+            using (var testEnvironment = CreateTestEnvironment(MqttProtocolVersion.V500))
+            {
+                await testEnvironment.StartServer();
+
+                var receiver = await testEnvironment.ConnectClient(o => o.WithProtocolVersion(MqttProtocolVersion.V311));
+                var receivedApplicationMessages = testEnvironment.CreateApplicationMessageHandler(receiver);
+                await receiver.SubscribeAsync("#");
+                
+                var sender = await testEnvironment.ConnectClient();
+
+                var applicationMessage = new MqttApplicationMessageBuilder().WithTopic("My/Message")
+                    .WithPayload("My_Payload")
+                    .WithUserProperty("A", "B")
+                    .WithResponseTopic("Response")
+                    .WithCorrelationData(Encoding.UTF8.GetBytes("Correlation"))
+                    .Build();
+                
+                await sender.PublishAsync(applicationMessage);
+
+                await LongTestDelay();
+
+                Assert.AreEqual(1, receivedApplicationMessages.ReceivedEventArgs.Count);
+                Assert.AreEqual("My/Message", receivedApplicationMessages.ReceivedEventArgs.First().ApplicationMessage.Topic);
+                Assert.AreEqual("My_Payload", receivedApplicationMessages.ReceivedEventArgs.First().ApplicationMessage.ConvertPayloadToString());
+            }
+        }
+    }
+}