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Posted 8 Apr 2009

Creating a weak event

, 8 Apr 2009 CPOL
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This article shows how to use WeakDelegateSet class to create weak events. With weak events, objects can register themselves in events, but are allowed to be collected if the only references for them are such events.


This simple article shows that creating a weak event (an event that allows objects who respond to it to be collected) is really simple with the help of the WeakDelegateSet class.


I created this class when I discovered a stupid bug (but, one that can turn to be a serious problem) in my other article. Classes that registered themselves in GCUtils.Collected where not allowed to be collected themselves if they didn't unregister from the event. It must be a weak event, so I did it, and created a class that helps this process.

Using the code

To create a weak event, you must create a WeakDelegateSet, and must redirect the add and remove handlers of the event to that WeakDelegateSet. For example:

private WeakDelegateSet fMyWeakEvent = new WeakDelegateSet();
public event EventHandler MyWeakEvent

And then, you invoke the event using the Invoke method in the WeakDelegateSet, passing all the required parameters to it. So, for my example event:

fMyWeakEvent.Invoke(this, EventArgs.Empty);

This is all that is needed to create a weak event. Maybe this is not a very common requirement, but I needed the weak event to be able to collect objects that were registered for the Collected event in case Dispose was not invoked. And, I am sure many events may use this as many times we need to be informed of something when the object is really alive.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Paulo Zemek
Engineer YouTube
United States United States
I started to program computers when I was 11 years old, as a hobbyist, programming in AMOS Basic and Blitz Basic for Amiga.
At 12 I had my first try with assembler, but it was too difficult at the time. Then, in the same year, I learned C and, after learning C, I was finally able to learn assembler (for Motorola 680x0).
Not sure, but probably between 12 and 13, I started to learn C++. I always programmed "in an object oriented way", but using function pointers instead of virtual methods.

At 15 I started to learn Pascal at school and to use Delphi. At 16 I started my first internship (using Delphi). At 18 I started to work professionally using C++ and since then I've developed my programming skills as a professional developer in C++ and C#, generally creating libraries that help other developers do their work easier, faster and with less errors.

Want more info or simply want to contact me?
Take a look at:
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Codeproject MVP 2012, 2015 & 2016
Microsoft MVP 2013-2014 (now I work at Microsoft so I can't be a Microsoft MVP anymore)

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Comments and Discussions

GeneralMy vote of 2 Pin
Peter Huber SG26-Jul-15 21:01
memberPeter Huber SG26-Jul-15 21:01 
GeneralRe: My vote of 2 Pin
Paulo Zemek26-Jul-15 21:13
mvpPaulo Zemek26-Jul-15 21:13 
GeneralMy vote of 1 Pin
Yahia Alhami26-Apr-10 9:30
memberYahia Alhami26-Apr-10 9:30 
GeneralAlternate solution Pin
chimeric6931-Aug-09 20:44
memberchimeric6931-Aug-09 20:44 
GeneralRe: Alternate solution Pin
Paulo Zemek2-Sep-09 15:04
memberPaulo Zemek2-Sep-09 15:04 
To be honest I didn't know this solution.
But, looking at it, I must say it remembers me the Observer/Listener pattern in Java. It's not really an event.

Also, look at this:
"Then, your handling logic for that event is specified within one of the cases of the ReceiveWeakEvent implementation on your class, not as a conventional delegate-based handler."

With my WeakEvent solution, the Event is weak, so anyone registers to it normally (via += ) and everything will work fine.
Of course, there are differences as, in the weakevent I suggest, the purpose is to have an event that treats ANY of it's observers as Weak. I use this in the Collected event handler, for example. Why keep an object alive and allow it to try to free some memory, when the object itself can be collected?

modified 28-Aug-12 19:03pm.

Generalgood work Pin
Donsw12-May-09 4:35
memberDonsw12-May-09 4:35 
GeneralThanks. Pin
Paulo Zemek13-May-09 1:43
memberPaulo Zemek13-May-09 1:43 
GeneralWeakDelegate Pin
Paulo Zemek8-Apr-09 12:29
memberPaulo Zemek8-Apr-09 12:29 
GeneralRe: WeakDelegate Pin
supercat99-Apr-09 6:31
membersupercat99-Apr-09 6:31 
GeneralRe: WeakDelegate Pin
Paulo Zemek9-Apr-09 6:48
memberPaulo Zemek9-Apr-09 6:48 
GeneralRe: WeakDelegate Pin
supercat99-Apr-09 7:17
membersupercat99-Apr-09 7:17 
GeneralRe: WeakDelegate Pin
Paulo Zemek9-Apr-09 7:23
memberPaulo Zemek9-Apr-09 7:23 

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