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

My preferred method is to mock out and inject the actual threading mechanism so that under test it is not asynchronous. Some times that is not possible (if the threading of the method is part of the framework, or otherwise not under your control).

If you can't control thread creation, then waiting for the thread to finish in some way, either a while loop or just a timed wait for however long the thread is supposed to take and failing the test if the state is not there since it took too long anyway.

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);
}

Answer 3

check out my article on unit testing Silverlight applications

http://www.codeproject.com/KB/silverlight/Ag3DemoLOB.aspx

There is an example of unit testing a method that calls a WCF service asynchronously...