genereteCodes.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var list = new HashSet<string>();
            using (TextWriter writer = File.CreateText("codes.txt"))
            {

                while(list.Count!= 100)
                {
                    var code = generateCode(8);
                    if (!list.Contains(code))
                    {
                        Console.WriteLine(code);
                        writer.WriteLine("BT-"+code);
                        list.Add(code);
                        
                    }
                }
            }

            Console.ReadLine();
        }

        private static string _digits = "23456789";
        private static string _lowers = "abcdefghjkmnpqrstuvwxyz";
        private static string _uppers = "ABCDEFGHJKMNPQRSTUVWXYZ";

        // Number of characters in _digits.
        private static int _digitLen
        {
            get
            {
                return _digits.Length;
            }
        }


        // Number of characters in _lowers.
        private static int _lowersLen
        {
            get
            {
                return _lowers.Length;
            }
        }

        // Number of characters in _uppers.
        private static int _uppersLen
        {
            get
            {
                return _uppers.Length;
            }
        }

        public static string generateCode(int pLength)
        {
            // unchecked truncates the value to fit int if it overflows (long -> int).
            Random rnd = new Random(); //unchecked((int)DateTime.Now.Ticks)

            string password = string.Empty;

            for (int i = 0; i < pLength; ++i)
            {
                // Randomly select from which character set to take next character from.
                // Letters are twice as likely compared to digits as there are more of them.
                // Currently does not force all character sets to be included to the password.
                switch (rnd.Next(5))
                {
                    case 0:
                        {
                            password += _digits[rnd.Next(_digitLen)];
                            break;
                        }

                    case 1:
                    case 2:
                        {
                            password += _lowers[rnd.Next(_lowersLen)];
                            break;
                        }

                    case 3:
                    case 4:
                        {
                            password += _uppers[rnd.Next(_uppersLen)];
                            break;
                        }
                }
            }

            return password;
        }

    }
}