private static string CheckSum(string message)

{

    char[] chars = message.ToCharArray();

    int checksum = ;

    for (int i = ; i < chars.Length; i++)

    {

        checksum += (int)chars[i];

    }

    checksum = (~checksum & 0xFFFF) + 0x0001;

    return Convert.ToString(checksum, ).ToUpper();

}