Question

Suppose I have a WeakReference of a target strong reference. I’d like to be informed when the target object itself is being collected by the GC. Is it possible?

EDIT: Adding code to the finalizer/destructor is not an option here. I need something that is not dependent on class code.

Answer 1

It’s possible under .NET 4.0 and following using ConditionalWeakTable<TKey, TValue>. Thanks this, and other sites. It follows proof of concept code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;

namespace Test
{
    public static class GCInterceptor
    {
        private static ConditionalWeakTable<object, CallbackRef> _table;

        static GCInterceptor()
        {
            _table = new ConditionalWeakTable<object, CallbackRef>();
        }

        public static void RegisterGCEvent(this object obj, Action<int> action)
        {
            CallbackRef callbackRef;
            bool found = _table.TryGetValue(obj, out callbackRef);
            if (found)
            {
                callbackRef.Collected += action;
                return;
            }

            int hashCode = RuntimeHelpers.GetHashCode(obj);
            callbackRef = new CallbackRef(hashCode);
            callbackRef.Collected += action;
            _table.Add(obj, callbackRef);
        }

        public static void DeregisterGCEvent(this object obj, Action<int> action)
        {
            CallbackRef callbackRef;
            bool found = _table.TryGetValue(obj, out callbackRef);
            if (!found)
                throw new Exception("No events registered");

            callbackRef.Collected -= action;
        }

        private class CallbackRef
        {
            private int _hashCode;
            public event Action<int> Collected;

            public CallbackRef(int hashCode)
            {
                _hashCode = hashCode;
            }

            ~CallbackRef()
            {
                Action<int> handle = Collected;
                if (handle != null)
                    handle(_hashCode);
            }
        }
    }
}

Tested with the following code:

public partial class Form1 : Form
{
    private object _obj;

    public Form1()
    {
        InitializeComponent();

        _obj = new object();

        _obj.RegisterGCEvent(delegate(int hashCode)
        {
            MessageBox.Show("Object with hash code " + hashCode + " recently collected");
        });
    }

    private void button1_Click(object sender, EventArgs e)
    {
        _obj = null;
        GC.Collect();
        GC.WaitForPendingFinalizers();
    }
}