EventHandlerList.cs

using System.Security.Permissions;

namespace System.ComponentModel
{
    [HostProtection(SecurityAction.LinkDemand, SharedState = true)]
    public sealed class EventHandlerList : IDisposable
    {
        private sealed class ListEntry
        {
            internal ListEntry next;

            internal object key;

            internal Delegate handler;

            public ListEntry(object key, Delegate handler, ListEntry next)
            {
                this.next = next;
                this.key = key;
                this.handler = handler;
            }
        }

        private ListEntry head;

        private Component parent;

        public Delegate this[object key]
        {
            get
            {
                ListEntry listEntry = null;
                if (parent == null || parent.CanRaiseEventsInternal)
                {
                    listEntry = Find(key);
                }
                return listEntry?.handler;
            }
            set
            {
                ListEntry listEntry = Find(key);
                if (listEntry != null)
                {
                    listEntry.handler = value;
                }
                else
                {
                    head = new ListEntry(key, value, head);
                }
            }
        }

        public EventHandlerList()
        {
        }

        internal EventHandlerList(Component parent)
        {
            this.parent = parent;
        }

        public void AddHandler(object key, Delegate value)
        {
            ListEntry listEntry = Find(key);
            if (listEntry != null)
            {
                listEntry.handler = Delegate.Combine(listEntry.handler, value);
            }
            else
            {
                head = new ListEntry(key, value, head);
            }
        }

        public void AddHandlers(EventHandlerList listToAddFrom)
        {
            for (ListEntry next = listToAddFrom.head; next != null; next = next.next)
            {
                AddHandler(next.key, next.handler);
            }
        }

        public void Dispose()
        {
            head = null;
        }

        private ListEntry Find(object key)
        {
            ListEntry next = head;
            while (next != null && next.key != key)
            {
                next = next.next;
            }
            return next;
        }

        public void RemoveHandler(object key, Delegate value)
        {
            ListEntry listEntry = Find(key);
            if (listEntry != null)
            {
                listEntry.handler = Delegate.Remove(listEntry.handler, value);
            }
        }
    }
}