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Creating a weak event

, 8 Apr 2009
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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.

Introduction

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.

Background

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
{
    add
    {
        fMyWeakEvent.Add(value);
    }
    remove
    {
        fMyWeakEvent.Remove(value);
    }
}

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.

License

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

About the Author

Paulo Zemek
Architect
Canada Canada
I started to program computers when I was 11 years old, as a hobbist, 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 they work easier, faster and with less errors.
 
Want more info or simply want to contact me?
Take a look at: http://paulozemek.azurewebsites.net/
Or e-mail me at: paulozemek@outlook.com
 
Codeproject MVP 2012
Microsoft MVP 2013

Comments and Discussions

 
GeneralMy vote of 1 PinmemberYahia Alhami26-Apr-10 9:30 
GeneralAlternate solution Pinmemberchimeric6931-Aug-09 20:44 
GeneralRe: Alternate solution [modified] PinmemberPaulo Zemek2-Sep-09 15:04 
Generalgood work PinmemberDonsw12-May-09 4:35 
GeneralThanks. PinmemberPaulo Zemek13-May-09 1:43 
GeneralWeakDelegate PinmemberPaulo Zemek8-Apr-09 12:29 
GeneralRe: WeakDelegate Pinmembersupercat99-Apr-09 6:31 
GeneralRe: WeakDelegate PinmemberPaulo Zemek9-Apr-09 6:48 
GeneralRe: WeakDelegate Pinmembersupercat99-Apr-09 7:17 
GeneralRe: WeakDelegate PinmemberPaulo Zemek9-Apr-09 7:23 

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