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Creating a weak event

, 8 Apr 2009 CPOL 19.1K 208 14
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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)

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About the Author

Paulo Zemek
Engineer Microsoft Corporation
United States United States
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.

Now I just started working as a Senior Software Engineer at Microsoft.

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, 2015
Microsoft MVP 2013-2014

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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 
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 
Why do you use reflection if you are using the interface?
But I can see the point. Here I am trying to present two simple solutions. One of them (and this is the one I use myself) is to create the event as an weakevent. So, the objects that register to it don't have the option to be weak.
The other is a wrapper for custom events to work in a weak manner. This one, that I presented in my last message, actually has the problem that it allows the target to be collected, but the wrapper itself lives while the object in which it was registered lives. My next approach for this is to make a "RegisterAsWeakEvent" method that receives the event (still thinking if I use EventInfo) and the original delegate, and registers a wrapper class that also knows how to de-register when the target is collected. The problem is that to support any delegate, as I can't use generics for the base type, I would need to use reflection and codedom to create the wrapper class. But, my main problem with this is not the performance, but the fact that I will by loading a new dll into memory, that can, in many cases, use more memory than the "lost" delegate itself.
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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