How does the Garbage Collector decide when to kill objects held by WeakReferences?

Answer 1

As per wikipedia "An object referenced only by weak references is considered unreachable (or "weakly reachable") and so may be collected at any time. Weak references are used to avoid keeping memory referenced by unneeded objects"

I am not sure if your case is about weak references...

Answer 2

Try calling GC.WaitForPendingFinalizers() right after GC.Collect().

Another possible option: don't ever use a WeakReference for any purpose. In the wild, I've only ever seen them used as a mechanism for lowering an application's memory footprint (i.e. a form of caching). As the mighty MSDN says:

Avoid using weak references as an automatic solution to memory management problems. Instead, develop an effective caching policy for handling your application's objects.