WeakDelegate demo

Program.cs

using System;
using System.Diagnostics;

namespace WeakDelegate
{
    // Containee needn't know anything about weak references, etc.
    // It just fires its events as normal.
    class Containee
    {
        public Containee(string id) { _id = id; }
        public event EventHandler MyEvent;

        public void DoIt() => MyEvent?.Invoke(this, new EventArgs());


        public string ID => _id;

        ~Containee()
        {
            Console.WriteLine(" Containee '{0}' cleaned up", ID);
        }

        private string _id;

    }

    // Container has a reference to Containee. But doesn't want to
    // force Containee to have a strong reference to it. Uses a
    // WeakContainer private nested class for this.
    class Container
    {
        public void Handler1(object sender, EventArgs args)
        {
            Console.WriteLine(" Container.Handler1 called with Containee '{0}'",_containee == null ? "<none>" : _containee.ID);
        }

        public Containee Containee
        {
            get
            {
                return _containee;
            }
            set
            {
                _weakContainer ??= new WeakContainer(this);

                // unsubscribe old
                if (_containee != null)
                {
                    _containee.MyEvent -= new EventHandler(_weakContainer.Handler1);
                }

                // subscribe new

                _containee = value;

                if (_containee != null)
                {
                    _containee.MyEvent += new EventHandler(_weakContainer.Handler1);
                }
            }
        }

        ~Container()
        {
            Console.WriteLine(" Container cleaned up");
        }

        #region WeakContainer

        private class WeakContainer : WeakReference
        {
            public WeakContainer(Container target) : base(target) { }
            public void Handler1(object sender, EventArgs args)
            {
                Console.WriteLine(" WeakContainer.Handler1 called");
                if (Target is Container b)
                {
                    b.Handler1(sender, args);
                }
                else
                {
                    if (sender is Containee c)
                    {
                        c.MyEvent -= new EventHandler(Handler1);
                        Console.WriteLine(" Removed WeakContainer handler");
                    }
                }
            }

            ~WeakContainer() => Console.WriteLine(" WeakContainer cleaned up");

        }

        #endregion
        private Containee _containee;

        private WeakContainer _weakContainer = null;

    }

    class Class1
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main(string[] args)
        {
            (new Class1()).Run();
        }

        private Containee containee1, containee2;

        private void InvokeMe()
        {
            Console.WriteLine("Invoking");
            containee1?.DoIt();
            containee2?.DoIt();

        }

        public void Run()
        {
            containee1 = new Containee("FIRST");
            containee2 = new Containee("SECOND");
            Container container = new();
            container.Containee = containee1;
            InvokeMe();
            Console.WriteLine("Switching to containee2");
            container.Containee = containee2;
            InvokeMe();
            Console.WriteLine("Switching back to containee1");
            container.Containee = containee1;
            InvokeMe();
            Console.WriteLine("Setting container to null and GC'ing");
            container = null;
            GC.Collect();
            GC.WaitForPendingFinalizers();
            InvokeMe();
            InvokeMe();
            Console.ReadLine();
        }

    }

}