C#: Notification before WeakReference is collected?

Question

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

Answer 1

.Net 4.0 has the solution you need: ConditionalWeakTable. Here is a short program that demonstrates the idea. (discussed here as well)

using System;
using System.Runtime.CompilerServices;

namespace GCCollectNotification
{
    class ObjectToWatch { }

    class Notifier
    {
        public object ObjectToWatch { get; set; }
        ~Notifier() { Console.WriteLine("object is collected"); }
    }

    class Program
    {
        private ConditionalWeakTable<object, Notifier> map
            = new ConditionalWeakTable<object, Notifier>();

        public void Test()
        {
            var obj = new ObjectToWatch();
            var notifier = map.GetOrCreateValue(obj);
            notifier.ObjectToWatch = obj;
        }

        static void Main(string[] args)
        {
            new Program().Test();

            GC.Collect();
            GC.WaitForPendingFinalizers();

            // "object is collected" should have been printed by now

            Console.WriteLine("end of program");
        }
    }
}