using System;

public class Program

{

    public static void Main()

    {

        // double

        double doubleValue = 123.456d;

        byte[] doubleAsBytes = BitConverter.GetBytes(doubleValue);

        string doubleBase64 = Convert.ToBase64String(doubleAsBytes, 0, doubleAsBytes.Length);

​

        Console.WriteLine(doubleValue); 

        Console.WriteLine(BitConverter.ToString(doubleAsBytes));

        Console.WriteLine(doubleBase64);

    }

}