Question

I’m making a Calendar application for myself to use, and to learn.

I’ve had no trouble until now with mutliple forms, and opening new ones on top of each other, etc.

Here’s an example:

private void button1_Click(object sender, EventArgs e)
{
    if (ceForm != null) ceForm.Close();
    ceForm = new CalendarEventForm();
    ceForm.Show();
}

Anyway, I now started to add timers to pop up a ‘reminder’ form before important events on my calendar will occur (i.e. 1 hour before etc.).

The code sets up the timers when the program is loaded, and then when each timer elapses, this is called:

static void lazyTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
{
    mainForm.ShowReminder((sender as LazyTimer).ReferredEvent);
}

LazyTimer is exactly the same as a System.Timers.Timer except the added propery ‘ReferredEvent’, which refers to the calendar event that is to be reminded of.

MainForm.ShowReminder() is as follows:

public void ShowReminder(LazyEvent lazyEvent)
{
    ReminderForm newReminder = new ReminderForm();
    newReminder.LoadEvent(lazyEvent);
    newReminder.Show();
}

The weird thing is that ReminderForm crashes. I’ve tried it with other forms (such as CalendarEventForm, which I know works normally) and they crash too. However, when I try to load the ReminderForm by pressing a button on my main form, it works fine.

Why do my forms crash when loaded (indirectly) by a timer?

Answer 1

Short answer: Use a System.Windows.Forms.Timer, not a System.Timers.Timer.

The reason is that the System.Timer.Timers class will fire the timer event on another thread, and you are not able to directly do UI operations from another thread than the main UI thread.