Writing unit tests for multi-threaded is not simple and could even be impossible for some scenarios – how could you test that an asynchronous method was not called?

Since most unit testing examples tend to be rather trivial I’ve decided to try and explain other more complex scenarios – preferably without using any calculator examples.

The “Timer” problem

Consider the Timer class (or rather classes), .NET has several classes called  Timer, In this post I’m referring to  System.Threading.Timer and  System.Timers.Timer which is built on top of the former. 
Both perform an action in another thread (using the Thread Pool) in intervals (not accurately – but I won’t go there in this post).

Consider the following class:

public class ClassUnderTest
{
    private readonly Timer _timer;
 
    public ClassUnderTest()
    {
        _timer = new Timer(1000);
        _timer.Elapsed += PerformPeriodicAction;
        _timer.Start();
    }
 
    public ClassUnderTest(Timer timer)
    {
        _timer = timer;
        _timer.Elapsed += PerformPeriodicAction;
        _timer.Start();
    }
 
    private void PerformPeriodicAction(object sender, ElapsedEventArgs e)
    {
        // Perform very important task here!
    }
}

Testing code that employs timers can be tricky. Unfortunately there’s a tendency to write a test that looks like this when timers are involved:

[Test]
public void TestJustWrong()
{
    var cut = new ClassUnderTest();
 
    // Make sure the timer executed at least once
    Thread.Sleep(5000);
 
    // Check that something happens
}

So what is the problem?

Although it might seem like a valid unit test – in fact it’s a test that would fail from time to time - whenever the timer would happen to “tick” for more than the sleep period.

[Test]
public void TestByMethod()
{
    var cut = new ClassUnderTestForTestability();
 
    // Let's call the method
    cut.PerformPeriodicAction(this, null);
 
    // Check that something happens
}

Solution 2 – invoke the timer at your leisure

Most modern Isolation (a.k.a Mocking) frameworks have the ability to invoke events as well as create fake objects.

Test]
public void TestWithFakeTimer()
{
    var fakeTimer = Isolate.Fake.Instance<Timer>();
 
    var cut = new ClassUnderTest(fakeTimer);
 
    // Let's invoke the event
    Isolate.Invoke.Event(() => fakeTimer.Elapsed += null, this, null);
 
    // Check that something happens
}

I’m using Typemock Isolator but you can use the isolation framework of your choice The syntax is strange but once you’ll get used to it, it’s simple.

The added value is that we’re causing the real flow of our program to execute – in a synchronized and timely manner.

Conclusion

Although there are many ways to test code that uses timers I find myself employing the two above most of the time.

I plan to write a few more posts on the other pitfall of testing multi-threaded code (and how to avoid them) but until then – happy coding.