Question

I would like to call the event handler just one time, and then detach it. I tried to write:

EventHandler handler = (s, e) =>
{
    // Do something
    //
    // blabla

    // Detach the handler
    SizeChanged -= handler;
};
SizeChanged += handler;

However on the line SizeChanged -= handler I get this error

Use of unassigned local variable 'handler'

Do you have idead on how I should proceed ? I thought about using a boolean flag, but I will do that only if I can’t find a way to detach the handler.

Answer 1

This is because it really is unassigned yet. Try making a named method out of it, so the symbol is known prehand.

private void OnEvent(object sender, EventArgs e)
{
    // Do something

    AnEvent -= OnEvent;
}

private void RegisterOnce()
{
    AnEvent += OnEvent;
}

I would also recommend to run the DoSmething code only after detatch and implement some locking mechanism, in case you have multithrading, to prevent from multiple threads call the event at the exact same time, not having time to detatch and therefore, all run.