using System.Collections;

namespace System.Windows.Forms
{
    public partial class CheckedListBox
    {
        public class CheckedItemCollection : CollectionBase
        {
            private static string message = "Collection is read only.";

            public object this[int index] => ((Item)List[index]).Value;

            internal void Add(object obj) => List.Add(obj);

            public new void Clear() => throw new NotSupportedException(message);

            public new void RemoveAt(int index) => throw new NotSupportedException(message);

            // the reason for this method is that enumerator must return Item.Value rather than Item itself
            public new IEnumerator GetEnumerator()
            {
                foreach (Item item in List)
                    yield return item.Value;
            }
        }
    }
}