Add support for topic alias.

Tests/MQTTnet.Core.Tests/Mockups/TestEnvironment.cs

@@ -124,6 +124,20 @@ namespace MQTTnet.Tests.Mockups
             return client;
         }
 
+        public async Task<IMqttClient> ConnectClientAsync(Action<MqttClientOptionsBuilder> optionsBuilder)
+        {
+            if (optionsBuilder == null) throw new ArgumentNullException(nameof(optionsBuilder));
+
+            var options = new MqttClientOptionsBuilder();
+            options = options.WithTcpServer("localhost", ServerPort);
+            optionsBuilder.Invoke(options);
+
+            var client = CreateClient();
+            await client.ConnectAsync(options.Build()).ConfigureAwait(false);
+
+            return client;
+        }
+
         public async Task<IMqttClient> ConnectClientAsync(MqttClientOptionsBuilder options)
         {
             if (options == null) throw new ArgumentNullException(nameof(options));

Tests/MQTTnet.Core.Tests/Server_TopicAlias_Tests.cs

@@ -0,0 +1,57 @@
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Formatter;
+using MQTTnet.Tests.Mockups;
+
+namespace MQTTnet.Tests
+{
+    [TestClass]
+    public class Server_TopicAlias_Tests
+    {
+        [TestMethod]
+        public async Task Publish_After_Client_Connects()
+        {
+            using (var testEnvironment = new TestEnvironment())
+            {
+                await testEnvironment.StartServerAsync();
+
+                var receivedTopics = new List<string>();
+
+                var c1 = await testEnvironment.ConnectClientAsync(options => options.WithProtocolVersion(MqttProtocolVersion.V500));
+                c1.UseApplicationMessageReceivedHandler(e =>
+                {
+                    lock (receivedTopics)
+                    {
+                        receivedTopics.Add(e.ApplicationMessage.Topic);
+                    }
+                });
+
+                await c1.SubscribeAsync("#");
+
+                var c2 = await testEnvironment.ConnectClientAsync(options => options.WithProtocolVersion(MqttProtocolVersion.V500));
+
+                var message = new MqttApplicationMessage
+                {
+                    Topic = "this_is_the_topic",
+                    TopicAlias = 22
+                };
+
+                await c2.PublishAsync(message);
+
+                message.Topic = null;
+
+                await c2.PublishAsync(message);
+                await c2.PublishAsync(message);
+
+                await Task.Delay(500);
+
+                Assert.AreEqual(3, receivedTopics.Count);
+                CollectionAssert.AllItemsAreNotNull(receivedTopics);
+                Assert.IsTrue(receivedTopics.All(t => t.Equals("this_is_the_topic")));
+            }
+        }
+    }
+}