DecimalToBase64.cs

using deniszykov.BaseN;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using K4os.Text.BaseX;

namespace ConsoleApp2
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Encode");
            Console.WriteLine("========================================");
            Console.WriteLine($"Plain 0 => Encoded {EncodeValue(0)}");
            Console.WriteLine($"Plain 64 =>  Encoded {EncodeValue(64)}");
            Console.WriteLine($"Plain 714 =>  Encoded {EncodeValue(714)}");
            Console.WriteLine($"Plain 16777215 =>  Encoded {EncodeValue(16777215)}");
            Console.WriteLine($"Plain 1073741824 =>  Encoded {EncodeValue(1073741823)}");
            Console.WriteLine("========================================");

            Console.WriteLine("Decode");
            Console.WriteLine("========================================");
            Console.WriteLine($"Encode '10' => {DecodeValue("A000000")}");
            Console.WriteLine($"Encode '64' => {DecodeValue("64")}");
            Console.WriteLine($"Encode 'AA' => {DecodeValue("AA")}");
            Console.WriteLine($"Encode '____' => {DecodeValue("____")}");
            var code = EncodeValue(64);
            var value = DecodeValue(code);
        }

        public static string EncodeValue(long plainValue)
        {
            string encodedStr = "";
            while (plainValue >= 1)
            {
                int index = Convert.ToInt16(plainValue - (plainValue / 64) * 64);
                encodedStr = Base64Code[index] + encodedStr;
                plainValue = plainValue / 64;
            }
            return encodedStr;
        }

        public static long DecodeValue(string encodedStr)
        {
            long plainValue = 0;
            int power = encodedStr.Length - 1;

            for (int i = 0; i <= power; i++)
            {
                var mappedKey = encodedStr[power - i].ToString();
                var mappedValue = _Base64Code[mappedKey];
                var decimalValue = mappedValue * Convert.ToInt64(Math.Pow(64, i));
                plainValue += decimalValue;
            }

            return plainValue;
        }


        public static Dictionary<int, string> Base64Code = new Dictionary<int, string>() {
            { 0, "0" },{ 1, "1" },{ 2, "2" },{ 3, "3" },{ 4, "4" },{ 5, "5" },{ 6, "6" },{ 7, "7" },{ 8, "8" },{ 9, "9" },
            { 10, "A" },{ 11, "B" },{ 12, "C" },{ 13, "D" },{ 14, "E" },{ 15, "F" },{ 16, "G" },{ 17, "H" },{ 18, "I" },{ 19, "J" },{ 20, "K" },
            { 21, "L" },{ 22, "M" },{ 23, "N" },{ 24, "O" },{ 25, "P" },{ 26, "Q" },{ 27, "R" },{ 28, "S" },{ 29, "T" },{ 30, "U" },{ 31, "V" },
            { 32, "W" },{ 33, "X" },{ 34, "Y" },{ 35, "Z" },{ 36, "a" },{ 37, "b" },{ 38, "c" },{ 39, "d" },{ 40, "e" },{ 41, "f" },{ 42, "g" },
            { 43, "h" },{ 44, "i" },{ 45, "j" },{ 46, "k" },{ 47, "l" },{ 48, "m" },{ 49, "n" },{ 50, "o" },{ 51, "p" },{ 52, "q" },{ 53, "r" },
            { 54, "s" },{ 55, "t" },{ 56, "u" },{ 57, "v" },{ 58, "w" },{ 59, "x" },{ 60, "y" },{ 61, "z" }
            ,{ 62, "-" },{ 63, "_" },
        };

        public static Dictionary<string, int> _Base64Code
        {
            get
            {
                return Enumerable.Range(0, Base64Code.Count()).ToDictionary(i => Base64Code[i], i => i);
            }
        }
    }
}