Fix for #1568 when RaiseAsync with a void Delegate returns null results in System.NullReferenceException

src/Moq.Tests/EventHandlersFixture.cs

@@ -179,11 +179,29 @@ public async Task Can_raise_parameterized_async_event_using_RaiseAsync()
             Assert.Equal(42, received);
         }
 
-        public class HasAsyncEvent
+        [Fact]
+        public async Task RaiseAsync_WithVoidEventDelegate_ThrowsArgumentOutOfRangeException()
         {
-#pragma warning disable CS0067 // Event never used
-            public virtual event Func<Task> Event;
-            public virtual event Func<int, Task> ParameterizedEvent;
+            //Arrange
+            var mock = new Mock<IWithEvent>();
+
+            await Assert.ThrowsAnyAsync<ArgumentOutOfRangeException>(async () => await mock.RaiseAsync(e => e.CustomEvent += null, "foo", 5));
         }
+
     }
 }
+
+public delegate void CustomEvent(string message, int value);
+
+public interface IWithEvent
+{
+    event CustomEvent CustomEvent;
+
+}
+
+public class HasAsyncEvent
+{
+#pragma warning disable CS0067 // Event never used
+    public virtual event Func<Task> Event;
+    public virtual event Func<int, Task> ParameterizedEvent;
+}

src/Moq.Tests/Moq.Tests.csproj

@@ -28,7 +28,7 @@
     <PackageReference Include="System.Net.Http" Version="4.3.4" />
     <PackageReference Include="System.Text.RegularExpressions" Version="4.3.1" />
     <PackageReference Include="xunit" Version="2.9.3" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.0" PrivateAssets="all"/>
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.0" PrivateAssets="all" />
     <PackageReference Include="Castle.Core" Version="5.2.1" />
     <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="8.0.1" />
     <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.3" PrivateAssets="All" />

src/Moq/Mock`1.cs

@@ -1423,6 +1423,7 @@ public void Raise(Action<T> eventExpression, params object[] args)
         /// </example>
         public Task RaiseAsync(Action<T> eventExpression, params object[] args)
         {
+            if (eventExpression.Method.ReturnType == typeof(void)) throw new ArgumentOutOfRangeException(nameof(eventExpression), "The event expression must return Task or ValueTask.");
             return Mock.RaiseEventAsync(this, eventExpression, args);
         }