C# – Benefits of Weak References

One useful example is the guys who run DB4O object oriented database. There, WeakReferences are used as a kind of light cache: it will keep your objects in memory only as long as your application does, allowing you to put a real cache on top.

Another use would be in the implementation of weak event handlers. Currently, one big source of memory leaks in .NET applications is forgetting to remove event handlers. E.g.

public MyForm()
{
    MyApplication.Foo += someHandler;
}

See the problem? In the above snippet, MyForm will be kept alive in memory forever as long as MyApplication is alive in memory. Create 10 MyForms, close them all, your 10 MyForms will still be in memory, kept alive by the event handler.

Enter WeakReference. You can build a weak event handler using WeakReferences so that someHandler is a weak event handler to MyApplication.Foo, thus fixing your memory leaks!

This isn't just theory. Dustin Campbell from the DidItWith.NET blog posted an implementation of weak event handlers using System.WeakReference.

Weak references are all about garbage collection. A standard object will not "disappear" until all references to it are severed, this means all the references your various objects have to it have to be removed before garbage collection will consider it garbage.

With a weak reference just because your object is referenced by other objects doesn't necessarily mean it's not garbage. It can still get picked up by GC and get removed from memory.

An example: If I have a bunch of Foo objects in my application I might want to use a Set to keep a central record of all the Foo's I have around. But, when other parts of my application remove a Foo object by deleting all references to it, I don't want the remaining reference my Set holds to that object to keep it from being garbage collected! Really I just want it to disappear from my set. This is where you'd use something like a Weak Set (Java has a WeakHashMap) instead, which uses weak references to its members instead of "strong" references.

If your objects aren't being garbage collected when you want them to then you've made an error in your book keeping, something's still holding a reference that you forgot to remove. Using weak references can ease the pain of such book keeping, since you don't have to worry about them keeping an object "alive" and un-garbage-collected, but you don't have to use them.