Hi,
In C#/.NET, is there any way to get a notification before the object pointed to by a weak reference is destructed? Basically, I want to allow an object to be collected, but do something right before the object is destroyed, without modifying code to add destructors (since I won't know exactly what types of objects will be sued with my code).
Thanks, Robert
No there is no way to achieve this functionality.
After a bit of speculation, I don't believe it's possible to implement a feature in the manner you are describing.
Consider that at the point the object held by a WeakReference is collected, there are no more references (hence it's collectable). For an event to have any use to you it would need to provide the object as part of the event. This means that the reference has gone from collectable to not-collectable. There is nothing stopping the handling code from retaking a reference on that object. Hence the object can no longer be considered collectable. The CLR would need to make a second pass on the object to re-ensure it was collectable.
You can see how the second time around the event could not be raised because it would lead to uncollectable objects.
It would be a misuse of naming to claim this event was raised just before an object was collected. Simply because any handler could prevent this from being collected by establishing a new reference to the object. Instead it would have to be "ObjectMaybeAboutToBeCollected". This probably won't give you the behavior you're looking for.
Your question doesn't make sense to me. Where is the code that's going to be called supposed to reside? Given that the weak references will be nulled before the referenced object is destroyed, it doesn't make sense for it to be part of the class that referenced the about-to-be destroyed object. And there's already code in the referenced object that's called before the object is destroyed - that's the destructor.
What's the actual design problem you want to solve? There may be a better way.
You can't do that. However what you can do is to watch when a GC is approaching ( there are new GC APIs in CLR v3.5Sp1 that allows you to do so, GCNotifications )