NullReferenceException occuring in System.Windows.Forms.Timer tick event.

Question

I have a simple class with two variables and a Close function which is called from OnTimerTick. On very rare occasions a NullReferenceException is occurring in Close() function, but I fail to understand what those occasions can be. Can somebody explain?

System.Windows.Forms.Timer timer = new Timer();
//timer.Tick is wired up in Constructor to OnTimerTick

private void OnTimerTick(object sender, EventArgs e)
{
    timer.Tick -= OnTimerTick;
    Close();
}

private void Close()
{
    if (varOne != null)
    {
        varOne.SomeEvent -= onSomeEvent;
        varOne.Dispose();
        varOne = null;
    }

    if (varTwo != null)
    {
        varTwo.AnotherEvent -= onAnotherEvent;
        varTwo.Dispose();
        varTwo = null;
    }
}

Answer 1

Assuming that no other threads are mutating your variables, and assuming that onSomeEvent and onAnotherEvent are on the current instance (i.e. no chance of a null reference there), then perhaps the most likely thing is that Dispose() is throwing?

This is possible - typically when in an error-state (indeed, it plagues WCF); try wrapping the dispose.

Oh; I'm also assuming here that varTwo has simple event handlers; it is also entirely possible for an event unsubscribe to fail; for example, if it is using an EventHandlerList and has already thrown it away...

So putting those together, something like:

// very paranoid cleanup
try {varOne.SomeEvent -= onSomeEvent; }
catch (Exception ex) { Trace.WriteLine(ex); } // best endeavours...
try { varOne.Dispose(); }
catch (Exception ex) { Trace.WriteLine(ex); } // best endeavours...

Normally this type of paranoia isn't necessary; but sometimes it is.