using System;

​

public class Program

{

    private const string Base36Chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

​

    // See https://stackoverflow.com/a/35004409

    public static string LongToBase(long value)

    {

       long targetBase = Base36Chars.Length;

       // Determine exact number of characters to use.

       char[] buffer = new char[Math.Max( 

                  (int) Math.Ceiling(Math.Log(value + 1, targetBase)), 1)];

​

       var i = buffer.Length;

       do

       {

           buffer[--i] = Base36Chars[(int)(value % targetBase)];

           value = value / targetBase;

       }

       while (value > 0);

​

       return new string(buffer, i, buffer.Length - i);

    }   

​

    public static string LongToBaseReversed(long value)

    {

       long targetBase = Base36Chars.Length;

       // Determine exact number of characters to use.

       char[] buffer = new char[Math.Max( 

                  (int) Math.Ceiling(Math.Log(value + 1, targetBase)), 1)];

​

       var i = 0;

       do

       {

           buffer[i++] = Base36Chars[(int)(value % targetBase)];

           value = value / targetBase;

       }

       while (value > 0);

​

       return new string(buffer, 0, i);

    }

    public static void Main()

    {   

        Random random = new Random();

        for(int i=0; i<10; i++)

        {

            Console.WriteLine(LongToBase(DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()));

            System.Threading.Thread.Sleep(random.Next(10, 500));

        }

        for(int i=0; i<10; i++)

        {

            Console.WriteLine(LongToBaseReversed(DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()));

            System.Threading.Thread.Sleep(random.Next(10, 500));

        }

    }

}

​