Unit testing event-based asynchronous code

AsyncUnitTestManualResetEvent.cs

[Test]
public void TestCalculations()
{
    // A simple web service offering basic aritmetich operations
    Service service = new Service();

    // Test data...
    int a = 10;
    int b = 10;

    // Variables to track is a asyn call completed.
    bool addCompleted = false;
    bool substractCompleted = false;

    // Used to signal the waiting test thread that a async operation have completed.
    ManualResetEvent manualEvent = new ManualResetEvent(false);

    // Async callback events are anonomous and are in the same scope as the test code,
    // and therefore have access to the manualEvent variable.
    service.AddCompleted += delegate(object sender, AddCompletedEventArgs args)
    {
        // Some basic assertions on the code.
        Assert.AreEqual(a + b, args.Result);
        addCompleted = true;

        // Signal the waiting NUnit thread that we're ready to move on.
        manualEvent.Set();
    };
    
    service.SubstractCompleted += delegate(object sender, SubstractCompletedEventArgs args)
    {
        Assert.AreEqual(a - b, args.Result);
        substractCompleted = true;
        manualEvent.Set();
    };

    // Call the add method asyncronous.
    service.AddAsync(a, b);

    // Block the current thread untill the callback event signals
    // or the call times out (1500 ms).
    manualEvent.WaitOne(1500, false);

    // Check if we completed the async call, or fail the test if we timed out.
    Assert.IsTrue(addCompleted);

    // Set the event to non-signaled before making next async call.
    manualEvent.Reset();

    service.SubstractAsync(a, b);
    manualEvent.WaitOne(1500, false);
    Assert.IsTrue(substractCompleted);
}