Is there a way to unit test an async method?

Question

I am using Xunit and NMock on .NET platform. I am testing a presentation model where a method is asynchronous. The method creates an async task and executes it so the method returns immediately and the state I need to check aren't ready yet.

I can set a flag upon finish without modifying the SUT but that would mean I would have to keep checking the flag in a while loop for example, with perhaps timeout.

What are my options?

Answer 1

Just thought you might want an update on this since the #1 answer is actually recommending an older pattern to solve this problem.

In .net 4.5 + xUnit 1.9 or higher you can simply return a Task and optionally use the async keyword from your test to have xunit wait for the test to complete asynchronously.

See this article on xUnit.net 1.9

[Fact] public async Task MyAsyncUnitTest() {       // ... setup code here ...        var result = await CallMyAsyncApi(...);        // ... assertions here ... } 

Answer 2

Does your object feature any sort of signal that the asynchronous method is finished, such as an event? If that is the case, you can use the following approach:

[Test] public void CanTestAsync() {     MyObject instance = new MyObject()     AutoResetEvent waitHandle = new AutoResetEvent(false);      // create and attach event handler for the "Finished" event     EventHandler eventHandler = delegate(object sender, EventArgs e)      {         waitHandle.Set();  // signal that the finished event was raised     }      instance.AsyncMethodFinished += eventHandler;      // call the async method     instance.CallAsyncMethod();      // Wait until the event handler is invoked     if (!waitHandle.WaitOne(5000, false))       {           Assert.Fail("Test timed out.");       }       instance.AsyncMethodFinished -= eventHandler;         Assert.AreEqual("expected", instance.ValueToCheck); }