using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace New_College
{
    public static class DistinctByHelper
    {

        public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> source, Func<T, TKey> keySelector)
        {
            HashSet<TKey> seenKeys = new HashSet<TKey>();
            foreach (var item in source)
            {
                if (seenKeys.Add(keySelector(item)))
                {
                    yield return item;
                }
            }
        }

    }
}