public void RetvalAsync() { var handler = CreateClass("MyService", "RetvalAsync")(null); service.RetvalAsync().Returns(Task.FromResult(123456789)); var expectedData = new byte[4]; fixed(byte *pData = expectedData) { *(int *)pData = 123456789; } var resultingTask = handler.Handle(service, null, 0); Assert.That(resultingTask.Result, Is.EqualTo(expectedData)); }