How to check a eventhandler is registered

Question

I have a class X and this class have a event: EventX.

And I have a class B, I register event EventX of objectX to a eventhandler HandlerB of class B.

X objectX = X.GetStaticObject();
objectX.EventX += HandlerB;

How to check EventX of objectX contained eventhandler HandlerB. Thanks.

Notes: objectX is a static global which event can be registered from anywhere with any objects which are not object B.

msergey · Accepted Answer · 2011-07-13T10:04:54.630

To check that specifically your HandlerB was registered, you can use Delegate.GetInvocationList() method to get appropriate info.

EDIT:

After V4Vendetta comment I tried to compile code and it failed. I slightly changed it.

To be able to do that check, method Test should be added to X (I assume that it's static, otherwise use this instead of X):

public static void Test(Delegate delegateToTest)
{
   if (X.EventX != null)
   {
       foreach (Delegate existingHandler in X.EventX.GetInvocationList())
       {
           if (existingHandler == delegateToTest)
           {
               // registered
           }
       }
    }
}

And then test from somewhere where HandlerB is accessible:

X.Test(new EventHandler(HandlerB));

@V4Vendetta: you are right. I did not check it in VS and completely forget that it may cause error. However I edited code to make it work. — msergey, Jul 13 '11 at 09:53

score 3 · Answer 2 · answered Jul 13 '11 at 08:42

3

If it is not null, it is registered (though it might be with an empty handler).

if(objectX.EventX != null)
{
  // registered!
}

As far as I know, you can't tell what is registered with it. Being able to would defeat the point of having an event in the first place (decoupling code and double dispatch).

answered Jul 13 '11 at 08:42

Oded

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However, it can registered from anywhere, because objectX is a global object (static). Thanks. – Leo Vo Jul 13 '11 at 08:44
I add more info. Please help me. THanks. – Leo Vo Jul 13 '11 at 08:56

score 1 · Answer 3 · edited May 23 '17 at 12:23

If you want to prevent an event handler to be registered twice for the same event, you could remove it before you register it - something like this:

objectX.EventX -= HandlerB; 
objectX.EventX += HandlerB;

This can be done in the registering code, or even in the event itself. Have a look at this question for details.

Note: removing HandlerB should not fail, even if it was not registered before! (Unless somebody changed the remove part in an explicit event definition to do so.)

How to check a eventhandler is registered

3 Answers3