Properly disposing a System.Threading.Timer

The following test fixture shows how a timer can be properly disposed in order to ensure that the thread executing the callback has completed (or we've timed out waiting for it):

[TestClass]
public class TimerFixture
{
    [TestMethod]
    public void Check_timer_dispose_behaviour()
    {
        System.Threading.Timer timer = new System.Threading.Timer(new System.Threading.TimerCallback(OnTick), null, 100, 0);

        while (!isProcessing) Thread.Sleep(50);

        WaitHandle waitHandle = new AutoResetEvent(false);

        timer.Dispose(waitHandle);

        isStopping = true;

        WaitHandle.WaitAll(new[] { waitHandle }, 1000);

        Assert.IsFalse(isProcessing);
    }
        
    private void OnTick(object state)
    {
        isProcessing = true;

        while (!isStopping) Thread.Sleep(100);

        Thread.Sleep(200);

        isProcessing = false;
    }

    private bool isStopping = false;
    private bool isProcessing = false;
}