Fix removal of event handlers from async event.

Source/MQTTnet.Tests/Internal/AsyncEvent_Tests.cs

@@ -0,0 +1,66 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Implementations;
+using MQTTnet.Internal;
+
+namespace MQTTnet.Tests.Internal
+{
+    [TestClass]
+    public sealed class AsyncEvent_Tests
+    {
+        int _testEventAsyncCount;
+
+        [TestMethod]
+        public async Task Attach_EventHandler()
+        {
+            var testClass = new TestClass();
+
+            testClass.TestEventAsync += OnTestEventAsync;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+
+            Assert.AreEqual(1, _testEventAsyncCount);
+        }
+
+        [TestMethod]
+        public async Task Detach_EventHandler()
+        {
+            var testClass = new TestClass();
+
+            testClass.TestEventAsync += OnTestEventAsync;
+            testClass.TestEventAsync -= OnTestEventAsync;
+
+            await testClass.FireEventAsync(EventArgs.Empty);
+
+            Assert.AreEqual(0, _testEventAsyncCount);
+        }
+
+        Task OnTestEventAsync(EventArgs arg)
+        {
+            Interlocked.Increment(ref _testEventAsyncCount);
+            return PlatformAbstractionLayer.CompletedTask;
+        }
+
+        sealed class TestClass
+        {
+            readonly AsyncEvent<EventArgs> _testEvent = new AsyncEvent<EventArgs>();
+
+            public event Func<EventArgs, Task> TestEventAsync
+            {
+                add => _testEvent.AddHandler(value);
+                remove => _testEvent.RemoveHandler(value);
+            }
+
+            public Task FireEventAsync(EventArgs eventArgs)
+            {
+                return _testEvent.InvokeAsync(eventArgs);
+            }
+        }
+    }
+}

Source/MQTTnet/Internal/AsyncEventInvocator.cs

@@ -21,12 +21,14 @@ namespace MQTTnet.Internal
 
         public bool WrapsHandler(Action<TEventArgs> handler)
         {
-            return ReferenceEquals(_handler, handler);
+            // Do not use ReferenceEquals! It will not work with delegates.
+            return handler == _handler;
         }
         
         public bool WrapsHandler(Func<TEventArgs, Task> handler)
         {
-            return ReferenceEquals(_asyncHandler, handler);
+            // Do not use ReferenceEquals! It will not work with delegates.
+            return handler == _asyncHandler;
         }
         
         public Task InvokeAsync(TEventArgs eventArgs)