Question

I’m building a timer class but I can’t make a test pass. I want to test that a method is called when the time on the timer is elapsed.

I have the following class:

public class TimeOutTimer
{
    private readonly ISubscriber _subscriber;
    private Timer _timer;

    public TimeOutTimer(ISubscriber subscriber)
    {
        _subscriber = subscriber;
    }

    public void Start()
    {
        _timer = new Timer(1000);
        _timer.Start();
        _timer.Elapsed += TimerOnElapsed;
    }

    private void TimerOnElapsed(object sender, ElapsedEventArgs elapsedEventArgs)
    {
        _subscriber.TimeReached();   
    }
}

And the test, using Moq:

[Test]
public void Start_WithValidParameters_TriggersTimeReached()
{
    var subscriberMock = new Mock<ISubscriber>();
    var timer = new TimeOutTimer(subscriberMock.Object);

    timer.Start();

    subscriberMock.Verify(subscriber => subscriber.TimeReached());
}

If I take the Timer out and make a direct call to _subscriber.TimeReached(), it works.

Am I doing something wrong?

Answer 1

In your example the event should be invoked after 1000ms, whereas you verify straight away. Clearly the event cannot be invoked at that point.

The easy way out is to put a thread sleep in your test.

[Test]
public void Start_WithValidParameters_TriggersTimeReached()
{
    var subscriberMock = new Mock<ISubscriber>();
    var timer = new TimeOutTimer(subscriberMock.Object);        

    timer.Start();
    Thread.Sleep(1000);

    subscriberMock.Verify(subscriber => subscriber.TimeReached());
}

However this is not really the correct way to generate a unit test for a time based class.

The correct way would be to create an ITimer interface which is then implemented by an adapter based on Timer.

The the ITimer then becomes a dependency of TimeoutTimer, and is passed to it in the constructor or as a property.

In your test you can then Mock the timer so it synchronises with your test without waiting (e.g. by being able to manually trigger the event by controlling the class).