ObservableHashSet

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections.Specialized;

using ChangedEventArgs = System.Collections.Specialized.NotifyCollectionChangedEventArgs;
using Actions = System.Collections.Specialized.NotifyCollectionChangedAction;

namespace ConsoleApplication1
{
    class Program
    {


        static void Main(string[] args)
        {
            var rs = new ObservableHashSet<int>();
            for (int i = 0; i < 15; i++)
            {
                rs.Add(i);
            }
            rs.CollectionChanged += (s, e) =>
            {
                var r = e.OldItems;
                switch (e.Action)
                {
                    case Actions.Add: Console.WriteLine(e.NewItems[0]); break;
                    case Actions.Remove: Console.WriteLine(e.OldItems[0]); break;
                    case Actions.Reset: Console.WriteLine("rm"); break;
                }
            };
            rs.Add(18);
            rs.Remove(18);
            rs.Clear();

            Console.WriteLine(rs.Count);
            Console.Read();
        }
    }





    public class ObservableHashSet<T> : HashSet<T>, INotifyCollectionChanged
    {
        public event NotifyCollectionChangedEventHandler CollectionChanged;

        public ObservableHashSet() { }
        public ObservableHashSet(IEnumerable<T> list)
            : base(list){}

        public new bool Add(T value)
        {
            if (base.Add(value))
            {
                if (CollectionChanged != null) CollectionChanged.Invoke(this, new ChangedEventArgs(Actions.Add, value));
                return true;
            }

            return false;
        }

        public new bool Remove(T value)
        {
            if( base.Remove(value))
            {
                if (CollectionChanged != null) CollectionChanged.Invoke(this, new ChangedEventArgs(Actions.Remove, value));
                return true;
            }
            return false;
        }

        public new int Clear()
        {
            int cnt = base.Count;
            if (cnt > 0)
            {
                base.Clear();
                if (CollectionChanged != null) CollectionChanged.Invoke(this, new ChangedEventArgs(Actions.Reset, null));
                return -cnt;
            }
            return 0;
        }

        public void OnCollectionChanged(ChangedEventArgs e)
        {
            if (CollectionChanged != null) CollectionChanged.Invoke(this, e);
        }

    }
}