NotifyingWeakReference

WeakReferences in C# and Java are nice but sometimes it'd be nice if there was an easy way to get notified when the referenced object is collected.

For Platform.NET I implemented a WeakReference type that has a ReferenceCollected event.

The type actually inherits from a generic class named WeakReference which isn't listed below but the point of this post is to demonstrate the relatively simple technique used.

Caveats:

• Don't use the type if you plan on having hundreds of NotifyingWeakReference instances. Each NotifyingWeakReferences will create an object with a finalizer which slows down the garbage collection process.
• The event will be raised sometime after the NotifyingWeakReference is invalidated but not immediately.

public class NotifyingWeakReference<T>
     : WeakReference<T>
     where T : class
 {
     public virtual event EventHandler ReferenceCollected;

     protected virtual void OnReferenceCollected(EventArgs eventArgs)
     {
         if (ReferenceCollected != null)
         {
             ReferenceCollected(this, eventArgs);
         }
     }

     private class GarbageCollectionListener
     {
         private NotifyingWeakReference<T> m_NotifyingReference;

         public GarbageCollectionListener(NotifyingWeakReference<T>
              notifyingReference)
         {
             m_NotifyingReference = notifyingReference;
         }

         ~GarbageCollectionListener()
         {
             if (m_NotifyingReference.Target == null)
             {
                 m_NotifyingReference.OnReferenceCollected(EventArgs.Empty);
             }
             else
             {
                 new GarbageCollectionListener(m_NotifyingReference);
             }
         }
     }

     public NotifyingWeakReference(T value)
         : base(value)
     {
         new GarbageCollectionListener(this);
     }
 }

PS. Does anyone else wish C# had runtime-level support for Java-like SoftReferences? I emulate them by using TimedReferences (WeakReferences that hold strong references for a specified amount of time after the reference target was last accessed).