Question

The fundamental question is how do I create a unit test that needs to call a method, wait for an event to happen on the tested class and then call another method (the one that we actually want to test)?

Here’s the scenario if you have time to read further:

I’m developing an application that has to control a piece of hardware. In order to avoid dependency from hardware availability, when I create my object I specify that we are running in test mode. When that happens, the class that is being tested creates the appropriate driver hierarchy (in this case a thin mock layer of hardware drivers).

Imagine that the class in question is an Elevator and I want to test the method that gives me the floor number that the elevator is. Here is how my fictitious test looks like right now:

[TestMethod]
public void TestGetCurrentFloor()
{
    var elevator = new Elevator(Elevator.Environment.Offline);
    elevator.ElevatorArrivedOnFloor += TestElevatorArrived;

    elevator.GoToFloor(5);

    //Here's where I'm getting lost... I could block
    //until TestElevatorArrived gives me a signal, but
    //I'm not sure it's the best way

    int floor = elevator.GetCurrentFloor();

    Assert.AreEqual(floor, 5);
}

Edit:

Thanks for all the answers. This is how I ended up implementing it:

    [TestMethod]
    public void TestGetCurrentFloor()
    {
        var elevator = new Elevator(Elevator.Environment.Offline);
        elevator.ElevatorArrivedOnFloor += (s, e) => { Monitor.Pulse(this); };

        lock (this)
        {
            elevator.GoToFloor(5);

            if (!Monitor.Wait(this, Timeout))
                Assert.Fail("Elevator did not reach destination in time");

            int floor = elevator.GetCurrentFloor();

            Assert.AreEqual(floor, 5);
        }
    }

Answer 1

I think you are on the right lines already. The test needs to wait until either the event happens or you judge that it has taken too long to arrive and should give up waiting.

To do this, you could use Monitor.Wait with a timeout in your test and have it signalled with Monitor.Pulse when the event arrives.

[TestMethod]
public void TestGetCurrentFloor()
{
    var elevator = new Elevator(Elevator.Environment.Offline);
    elevator.ElevatorArrivedOnFloor += TestElevatorArrived;

    lock (this)
    {
        elevator.GoToFloor(5); // NOTE: this must hand off to a second thread, and the ElevatorArrivedOnFloor must be raised by this other thread otherwise the Monitor will be pulse before we've started waiting for it

        if (!Monitor.Wait(this, TIMEOUT)) Assert.Fail("Event did not arrive in time.");
    }

    int floor = elevator.GetCurrentFloor();

    Assert.AreEqual(floor, 5);
}

private void TestElevatorArrived(int floor)
{
    lock (this)
    {
        Monitor.Pulse(this);
    }
}

(The Assert.Fail() call here should be replaced with whatever mechanism your unit-testing tool uses for explicitly failing a test — or you could throw an exception.)