EventHandlerList.cs

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namespace System.ComponentModel;
 
public sealed class EventHandlerList : IDisposable
{
    private sealed class ListEntry
    {
        internal readonly ListEntry _next;
 
        internal readonly object _key;
 
        internal Delegate _handler;
 
        public ListEntry(object key, Delegate handler, ListEntry next)
        {
            _next = next;
            _key = key;
            _handler = handler;
        }
    }
 
    private ListEntry _head;
 
    private readonly 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);
            }
        }
    }
 
    internal EventHandlerList(Component parent)
    {
        _parent = parent;
    }
 
    public EventHandlerList()
    {
    }
 
    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)
    {
        if (listToAddFrom == null)
        {
            throw new ArgumentNullException("listToAddFrom");
        }
        for (ListEntry listEntry = listToAddFrom._head; listEntry != null; listEntry = listEntry._next)
        {
            AddHandler(listEntry._key, listEntry._handler);
        }
    }
 
    public void Dispose()
    {
        _head = null;
    }
 
    private ListEntry Find(object key)
    {
        ListEntry listEntry = _head;
        while (listEntry != null && listEntry._key != key)
        {
            listEntry = listEntry._next;
        }
        return listEntry;
    }
 
    public void RemoveHandler(object key, Delegate? value)
    {
        ListEntry listEntry = Find(key);
        if (listEntry != null)
        {
            listEntry._handler = Delegate.Remove(listEntry._handler, value);
        }
    }
}