using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var g = Guid.NewGuid();

        Console.WriteLine(g);

        var b = g.ToByteArray();

        var sub = b.Take(15).ToArray();

        var result = ToBase64UrlString(sub);

        Console.WriteLine(result);

        Console.WriteLine(result.Length);

        byte[] backwards = FromBase64UrlString(result);

        var guid = new Guid(backwards.Concat(new byte[1] { 0x00 }).ToArray());

        Console.WriteLine(guid);

    }

​

        private static string ToBase64UrlString(byte[] data)

        {

            return 

            Convert.ToBase64String(data)

            .Replace('+', '-')

            .Replace('/', '_')

            .Replace("=", string.Empty);

        }

​

        private static byte[] FromBase64UrlString(string s)

        {

            return Convert.FromBase64String(s.Replace('-', '+').Replace('_', '/'));

        }

}