How to know how many event handlers for an event?

Question

Hi All,

How to know how many event handlers for an event?

I want a way to execute the following code:

// if (control.CheckedChanged.Handlers.Length == 0)
{
    control.CheckedChanged += (s, e) =>
    {
      // code;
    }
}

Note: this code is out side the control class.

Thanks in advance.

Answer 1

You can't, because only the type that exposes the event has access to the actual delegate. From within the control, you could do something like that:

if (MyEvent!= null)
{
    EventHandler[] handlers = (EventHandler[])MyEvent.GetInvocationList();
    foreach(EventHandler handler in handlers)
    {
        ...
    }
}

Or, for what you're trying to do:

if (CheckedChanged == null)
{
    CheckedChanged += (s, e) =>
    {
      // code;
    }
}

Answer 2

My answer is more of a comment for Thomas Levesque, but I can't comment yet, so here goes nothing. I find this area of C# a little ugly, since there's a possibility to introduce race conditions - i.e. different threads may race and you may enter the if statement with CheckedChanged != null

if (CheckedChanged == null)
{
    CheckedChanged += (s, e) =>
    {
      // code;
    }
}

You should either lock this code, but in many cases you will find yourself writing code like this

//Invoke SomeEvent if there are any handlers  attached to it.
if(SomeEvent != null) SomeEvent(); 

But SomeEvent may be nulled in the process, so it would be safer to write something like this

SomeEVentHandler handler = SomeEvent;
if (handler != null) handler();

...just to be extra safe.