How to unregister an event

t4ure4n

I have a procject in which i am using a custom webclient. I have following event _WebClient.DocumentComplete which i can register and it works. Now if I want to unregister the first event handler and assign a new event handler I gives me error because I can't use _WebClient.DocumentComplete = null; Can some 1 tell me how to unregister the event

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kubben

When you add an event handler you use += to remove one you use -= _WebClient.DocumentComplete -= new EventHandler(yourEventHandlerNameHere); Hope that helps. Ben

Luc Pattyn

Hi, you can undo someEvent+=new SomeEventHandler(myHandler); with the following magical line: someEvent-=new SomeEventHandler(myHandler); The magic is you create a new object to remove an old (but very similar) object ! :)

Luc Pattyn

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b_umika

Have you tried using _WebClient.DocumentComplete -= ; Is that what you are talking about?

Dan Neely

Luc Pattyn wrote:

The magic is you create a new object to remove an old (but very similar) object !

I'd always thought you needed to save the old one to unregister it. Learn something new everyday. :) How does this work if buggy code results in the same handler being registered twice?

-- You have to explain to them [VB coders] what you mean by "typed". their first response is likely to be something like, "Of course my code is typed. Do you think i magically project it onto the screen with the power of my mind?" --- John Simmons / outlaw programmer

Martin 0

Hello Luc,

Luc Pattyn wrote:

The magic is you create a new object to remove an old (but very similar) object !

I remember that Scott Dorman once pointed out that it is even possible without creating an new object (I think only in .Net>1.1) Here is the thread![^] P.S.: This was my Nr.:1000 post.

All the best, Martin

Luc Pattyn

Dont know, havent seen it spec'd, so I can only suggest you try it. I can confirm if you register twice, you execute twice (that is a common bug, that harms performance and is not always apparent). :)

Luc Pattyn

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Luc Pattyn

Hi Martin, I did remember that thread; I havent tried it yet, I still tend to write code that runs also on 1.1

Martin# wrote:

This was my Nr.:1000 post

Aha, that explains a few things. Now take that holiday ! :)

Luc Pattyn

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Martin 0

Luc Pattyn wrote:

Aha, that explains a few things. Now take that holiday !

:) :rose:

All the best, Martin

Bijesh

dan neely wrote:

How does this work if buggy code results in the same handler being registered twice?

Its ugly. You basically get the same handler invoked twice on that event.. I've had to track down so many of these 'event leaks'.. :sigh:

Luc Pattyn

Hi Dan, I ran a little test:

	private int clicks=0;
	private int handlers=0;
	Button btn=new Button();

	public override void Run(int arg) {
		doClick();
		addHandler();
		doClick();
		addHandler();
		doClick();
		removeHandler();
		doClick();
		removeHandler();
		doClick();
		removeHandler();
		doClick();
	}

	void doClick() {
		log("CLICK");
		btn.PerformClick();
	}

	void addHandler() {
		handlers++;
		log("ADD HANDLER "+handlers);
		btn.Click+=new EventHandler(btn\_Click);
	}

	void removeHandler() {
		log("REMOVE HANDLER "+handlers);
		handlers--;
		try {btn.Click-=new EventHandler(btn\_Click);
		} catch { log("failed to remove handler"); }
	}

	void btn\_Click(object sender, EventArgs e) {
		clicks++;
		log("  got click "+clicks);
	}

and it resulted in the following log:

15.109 CLICK
15.125 ADD HANDLER 1
15.140 CLICK
15.156 got click 1
15.203 ADD HANDLER 2
15.218 CLICK
15.296 got click 2
15.312 got click 3
15.328 REMOVE HANDLER 2
15.343 CLICK
15.343 got click 4
15.437 REMOVE HANDLER 1
15.468 CLICK
15.484 REMOVE HANDLER 0
15.593 CLICK

So each -= effectively removes exactly one matching delegate if it finds one, and does not throw otherwise. :)

Luc Pattyn

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Luc Pattyn

Hi Dan, FYI: I just finised turning this subject into a short article. You may want to skim it. Regards

Luc Pattyn

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