IncreaseString

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static string IncreaseString(string value, string chars)
{
      if (string.IsNullOrEmpty(value))
           throw new ArgumentException("value can't be null or empty.", "value");
      else if (string.IsNullOrEmpty(chars))
           throw new ArgumentException("chars can't be null or empty.", "chars");

      char[] result = value.ToCharArray();

      for (int i = result.Length - 1; i >= 0; i--)
      {
            int index = chars.IndexOf(result[i]);

            if (index == -1)
                  throw new ArgumentException("Invalid Char " + result[i], "value");

            if (index < chars.Length - 1)
            {
                  result[i] = chars[index + 1];
                  break;
            }
            else
            {
                  result[i] = chars[0];
            }
      }

      return new string(result);
}