namespace UniqueIdGeneration
{
    using System;
    using System.Security.Cryptography;
    using System.Text;

    class Program
    {
        static string GenerateUniqueId(int size, string symbols)
        {
            char[] chars = symbols.ToCharArray();
            var data = new byte[1];
            var crypto = new RNGCryptoServiceProvider();
            crypto.GetNonZeroBytes(data);
            data = new byte[size];
            crypto.GetNonZeroBytes(data);
            var result = new StringBuilder(size);

            foreach (byte b in data)
            {
                result.Append(chars[b % (chars.Length - 1)]);
            }

            return result.ToString();
        }

        static void Main(string[] args)
        {
            var size = 6;
            var symbols = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

            for (int i = 0; i < 24; i++)
            {
                Console.WriteLine(GenerateUniqueId(size, symbols));
            }

            Console.ReadKey();
        }
    }
}