using System;

using System.Security.Cryptography;

using System.Collections.Generic;

using Microsoft.AspNetCore.WebUtilities;

public class Program

{

    public static void Main()

    {

        List<string> lst = new List<string>();

        for(int i =0; i <= 500; i++)

        {

            string link = GetLinkId();

            if(lst.Contains(link))

            {

            Console.WriteLine("Duplicate found! " + link  );

            }

            lst.Add(link);

         Console.WriteLine(link);

        }

    }

     public static string GetLinkId()

        {

            using (RNGCryptoServiceProvider rngCryptoServiceProvider = new RNGCryptoServiceProvider())

            {

                byte[] randomBytes = new byte[4];

                rngCryptoServiceProvider.GetBytes(randomBytes);

                return WebEncoders.Base64UrlEncode(randomBytes);

            }

        }

}