using System;

using System.Collections.Generic;

public static class Helper

    {

        public static IEnumerable<T> Distinct2<T>(this IEnumerable<T> e)

        {

            var a  = new HashSet<T>();

            foreach(var it in e)

            {

                if(!a.Contains(it))

                {

                    a.Add(it);

                    yield return it;

                }

            }

        }

    }

​

public class Program

{

    public static void Main()

    {

        var a = new List<int>{1, 2, 1, 3};

        a.Distinct2();

        foreach(var it in a.Distinct2())

        {

            Console.WriteLine(it);

        }

    }

}