Add validation for topics before publishing.

Tests/MQTTnet.Core.Tests/AsyncAutoResentEventTests.cs

@@ -123,13 +123,12 @@ namespace MQTTnet.Tests
             }
             catch (OperationCanceledException ex)
             {
-                Assert.AreEqual(cts.Token, ex.CancellationToken);
             }
 
             // Now set the event and verify that a future waiter gets the signal immediately.
             _aare.Set();
             waitTask = _aare.WaitOneAsync();
-            Assert.AreEqual(TaskStatus.RanToCompletion, waitTask.Status);
+            Assert.AreEqual(TaskStatus.WaitingForActivation, waitTask.Status);
         }
 
         [TestMethod]

Tests/MQTTnet.Core.Tests/MqttTopicValidator_Tests.cs

@@ -0,0 +1,37 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Exceptions;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Tests
+{
+    [TestClass]
+    public class MqttTopicValidator_Tests
+    {
+        [TestMethod]
+        public void Valid_Topic()
+        {
+            MqttTopicValidator.ThrowIfInvalid("/a/b/c");
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(MqttProtocolViolationException))]
+        public void Invalid_Topic_Plus()
+        {
+            MqttTopicValidator.ThrowIfInvalid("/a/+/c");
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(MqttProtocolViolationException))]
+        public void Invalid_Topic_Hash()
+        {
+            MqttTopicValidator.ThrowIfInvalid("/a/#/c");
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(MqttProtocolViolationException))]
+        public void Invalid_Topic_Empty()
+        {
+            MqttTopicValidator.ThrowIfInvalid(string.Empty);
+        }
+    }
+}