gistfile1.txt

using System;
using System.Collections.Generic;

static class P
{
    static void Main()
    {
        var bytes = new List<byte>();
        bytes.AddRange(new byte[] { 0, 1, 2, 3 });
        bytes.AddRange(new byte[] { 4, 5 });
        bytes.AddRange(new byte[] { 6, 7, 8 });
        var arr = bytes.ToArray();

        Console.WriteLine(BitConverter.ToString(arr));
        // ^^ 00-01-02-03-04-05-06-07-08

        var b64 = Convert.ToBase64String(arr);
        Console.WriteLine(b64);
        // ^^ AAECAwQFBgcI

        var backAgain = Convert.FromBase64String(b64);
        Console.WriteLine(BitConverter.ToString(backAgain));
        // ^^ 00-01-02-03-04-05-06-07-08
    }
}