using System;

public class Program

{

    public static void Main()

    {

        string zz = Encode(Guid.NewGuid());

        Console.WriteLine("-> {0}", zz);

        Console.WriteLine("-> {0}", Decode(zz));

    }

    public static string Encode(Guid guid)

    {

        string encoded = Convert.ToBase64String(guid.ToByteArray());

        encoded = encoded

        .Replace("/", "_")

        .Replace("+", "-");

        return encoded.Substring(0, 22);

    }

​

    public static Guid Decode(string value)

    {

        value = value

        .Replace("_", "/")

        .Replace("-", "+");

        byte[] buffer = Convert.FromBase64String(value + "==");

        return new Guid(buffer);

    }

}