using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        HashSet<int> set = new HashSet<int>();

        set.Add(1);

        set.Add(2);

        set.Add(2);

        //Cannot insert duplicates

        foreach (var item in set)

        {

            Console.WriteLine(item);

        }

    }

}