How do I make sure no handlers are attached to an event?

Question

I am adding an event handler like this:

theImage.MouseMove += new MouseEventHandler(theImage_MouseMove);

but in my application, this code gets run every time the page is shown, so I want to attach the event handler only once, but how can I tell if a handler has been set yet or not, something like this:

if(theImage.MouseMove == null) //error
    theImage.MouseMove += new MouseEventHandler(theImage_MouseMove);

Answer 1

I might me missing something, but if you just want to make sure that the handle is only called once, why don't you use -= before adding it. Something like this:

public static void Main(string[] args)
{
    var timer = new Timer(1000);

    timer.Elapsed -= new ElapsedEventHandler(timer_Elapsed);
    timer.Elapsed += new ElapsedEventHandler(timer_Elapsed);
    timer.Elapsed -= new ElapsedEventHandler(timer_Elapsed);
    timer.Elapsed += new ElapsedEventHandler(timer_Elapsed);

    timer.Start();

    System.Threading.Thread.Sleep(4000);
}

static void timer_Elapsed(object sender, ElapsedEventArgs e)
{
    Console.WriteLine("Hit!");
}

The handler will only run once every second.

Answer 2

You can shorten it down if you only want to attach once:

theImage.MouseMove -= theImage_MouseMove; //If it wasn't attached, doesn't matter
theImage.MouseMove += theImage_MouseMove;

Answer 3

theImage.MouseMove.GetInvocationList().Length

Answer 4

I'm not sure if this is the best solution, but the way I usually do this is to simply use an unsubscribe before the subscribe.

If you do something like:

TheImage.MouseMove -= new MouseEventHandler(theImage_MouseMove);
TheImage.MouseMove += new MouseEventHandler(theImage_MouseMove);

It will only ever get added once. If it doesn't already exist (the first time it's triggered), the -= doesn't hurt anything if it hasn't been subscribed to previously.