C# Edition of Base62 integer encode/decode originally from https://gist.github.com/sebastien-p/1170594

Program.cs

using System;
using System.Linq;
using System.Text;

namespace ConsoleApp1
{
    class Program
    {
        public static string Base62EncodeInt(int Original)
        {
            string Result = "";
            
            for (decimal Temp = Original; Temp > 0; Temp = Math.Floor((Temp / 62)))
            {
                // Base62 Encode source https://gist.github.com/sebastien-p/1168420/bc98b64ef1e6f18ee4379ad57df1a65d205b634f
                // a%62 -> 0-61
                // 0-9   | 36-61 | 10-35
                // 48-57 | 65-90 | 97-121
                // 0-9   | A-Z   | a-z
                var Modulus = Temp % 62;
                Result = ((char)((
                    Modulus > 9 ?
                    (Modulus > 35 ? 29 : 87) :
                    48
                ) + Modulus)) + Result;
            }
            return Result;
        }

        public static int Base62DecodeInt(string Encoded)
        {
            // Base62 decode source https://gist.github.com/sebastien-p/1170594 - base62 decode
            int Result = 0;

            int[] ASCIIPad = { 0, 48, 29, 87 };
            foreach (var Char in Encoded)
            {
                Result = Result * 62 + Char - ASCIIPad[Char >> 5];
            }

            return Result;
        }
        
        public static string Base62EncodeLong(ulong Original)
        {
            string Result = "";
            
            for (decimal Temp = Original; Temp > 0; Temp = Math.Floor((Temp / 62)))
            {
                // https://gist.github.com/sebastien-p/1168420/bc98b64ef1e6f18ee4379ad57df1a65d205b634f - base62 encode
                // a%62 -> 0-61
                // 0-9   | 36-61 | 10-35
                // 48-57 | 65-90 | 97-121
                // 0-9   | A-Z   | a-z
                var Modulus = Temp % 62;
                Result = ((char)((
                    Modulus > 9 ?
                    (Modulus > 35 ? 29 : 87) :
                    48
                ) + Modulus)) + Result;
            }
            return Result;
        }

        public static ulong Base62DecodeLong(string Encoded)
        {
            // https://gist.github.com/sebastien-p/1170594 - base62 decode
            ulong Result = 0;

            int[] ASCIIPad = { 0, 48, 29, 87 };
            foreach (var Char in Encoded)
            {
                Result = Result * 62ul + Char - (ulong)ASCIIPad[Char >> 5];
            }

            return Result;
        }

        static void Main(string[] args)
        {
            string intA = "BD45A", intB, ulongA = "140bytes", ulongB;
            int intValue;
            ulong ulongValue;
            
            Console.WriteLine("string: " + intA);
            Console.WriteLine("decoded: " + (intValue = Base62DecodeInt(intA)));
            Console.WriteLine("encoded: " + (intB = Base62EncodeInt(intValue)));
            Console.WriteLine("match: " + (intB == intA ? "Yes" : "No"));
            
            Console.WriteLine("");
            
            Console.WriteLine("string: " + ulongA);
            Console.WriteLine("decoded: " + (ulongValue = Base62DecodeLong(ulongA)));
            Console.WriteLine("encoded: " + (ulongB = Base62EncodeLong(ulongValue)));
            Console.WriteLine("match: " + (ulongB == ulongA ? "Yes" : "No"));
            
            Console.ReadLine();
        }
    }
}