Event subscription to a static instance. How to ensure GC.

Question

Hi.

I'm fixing a User control component using the .NET 1.1 framework.

There are many instances of this user control referencing a singleton wrapping a COM resource. They subscribe to events from this resource.

I suspect that the reason why we are having a degrading performance is because the singleton is maintaining a reference to the user control classes even after their respective windows are gone. Thus preventing GC.

They unsubscribe the event in their finalize method. Hence I suspect a chicken and egg problem. The finalize wont execute because they are being referenced through their event subscription in the longer lived Singleton, preventing GC.

Where in a User Control should I perform the event unsubscribe to make it eligible for GC?

I do not own the application hosting the user control.

Answer 1

You should unsubscribe when the control is disposed, really. If you can't easily modify the Dispose method yourself, you could add an event handler to the Disposed event:

ComSingleton.Foo += FooHandler;
this.Disposed += delegate { ComSingleton.Foo -= FooHandler; };

If you want to subscribe to events using anonymous methods or lambda expressions, you'll need to use a separate local variable so that you can refer to it again:

EventHandler tmp = delegate { Console.WriteLine("Something happened"); };
ComSingleton.Foo += tmp;
this.Disposed += delegate { ComSingleton.Foo -= tmp; };

An alternative is to use "weak event handlers" - there are many articles about this on the web; here's one which goes into a fair amount of detail.