Problem …
Long title … simple problem: I am writing automated unit test cases for an asynchronous method call which returns back via an event handler (a callback).

If I had architected the method that I am calling I would have used the standard Begin/End methods to handle the asynchronous calls but I am using a pre-written source code so I need to adhere to its methodology.

Solution …
Nonetheless, I needed a way to have a TestMethod make the asynchronous method call and then somehow get the results back from the event handler. I was not (am still not) aware of anything built-in to NUnit or Visual Studio Unit Testing to handle this
so I figured I was on my own.

I figured that all I really needed was a way to sleep the test method until the event handler is called and then signal it to continue and analyze the data returned from the event. Signal. I had heard that before when talking about Thread Synchronization … threading … that is what the event callback is … duh. I need a thread signal!

Enter AutoResetEvent …
Very fortunately for me, .NET has built-in just such a signal using a class called AutoResetEvent (System.Threading). Once I realized this, the code was frighteningly simple. One thread creates the signal object (initializing its state to un-signaled) and waits on it. The second thread sets the signal and the initial thread continues.

Here’s some code:

1. using System.Threading;
2. using Microsoft.VisualStudio.TestTools.UnitTesting;
4. namespace MyTests
5. {
6. [TestClass]
7. public class MyTestClass
8. {
9. AutoResetEvent _TestTrigger;
10. DataType _MyData;
12. [TestMethod]
13. public void MyTestMethod()
14. {
15. this._TestTrigger = new AutoResetEvent(false);
16. MyAsyncCall();
17. this._TestTrigger.WaitOne();
18. Assert.AreEqual(…)
20. }
22. private void My_EventHandler(…)
23. {
24. this._MyData = result from event handler;
25. this._TestTrigger.Set();
27. }
29. }
31. }

You can make modifications to this so that the calling method does not wait indefinitely as well as other handy mods. If you come up with anything cool please drop a comment here so everyone can share in your find!