private static string ToName(int index)

        {

            if (index < 0) { throw new Exception("invalid parameter"); }

            List<string> chars = new List<string>();

            do

            {

                if (chars.Count > 0) index--;

                chars.Insert(0, ((char)(index % 26 + (int)'A')).ToString());

                index = (int)((index - index % 26) / 26);

            } while (index > 0);

            return String.Join(string.Empty, chars.ToArray());

        }