Unit testing a multithreaded application?

Question

Does anyone have any advice for a consistent way to unit test a multithreaded application? I have done one application where our mock \"worker threads\" had a thread.sleep

Answer 1

If you have to test that a background thread does something, a simple technique I find handy is to to have a WaitUntilTrue method, which looks something like this:

bool WaitUntilTrue(Func func,
              int timeoutInMillis,
              int timeBetweenChecksMillis)
{
    Stopwatch stopwatch = Stopwatch.StartNew();

    while(stopwatch.ElapsedMilliseconds < timeoutInMillis)
    {
        if (func())
            return true;
        Thread.Sleep(timeBetweenChecksMillis);
    }   
    return false;
}

Used like this:

volatile bool backgroundThreadHasFinished = false;
//run your multithreaded test and make sure the thread sets the above variable.

Assert.IsTrue(WaitUntilTrue(x => backgroundThreadHasFinished, 1000, 10));

This way you don't have to sleep your main testing thread for a long time to give the background thread time to finish. If the background doesn't finish in a reasonable amount of time, the test fails.