SA_ID_Validator.cs

        public (bool, string) Validate(string idNumber) {
            if (!long.TryParse(idNumber, out _))
                return (false, "Not a number");
            if (idNumber.Length != 13)
                return (false, "Must be 13 digits long");
            int month = int.Parse($"{idNumber[2]}{idNumber[3]}");
            if (month < 1 || month > 12)
                return (false, "Incorrect Month Number");
            int day = int.Parse($"{idNumber[4]}{idNumber[5]}");
            if (day < 1 || day > 31)
                return (false, "Incorrect Day Number");
            if (idNumber[10] != '0' && idNumber[10] != '1')
                return (false, "Incorrect Citizenship Number");

            return LuhnChecksum(idNumber);
        }

        private (bool, string) LuhnChecksum(string idNumber) {
            int oddResult = int.Parse(idNumber[0].ToString()) +
                            int.Parse(idNumber[2].ToString()) +
                            int.Parse(idNumber[4].ToString()) +
                            int.Parse(idNumber[6].ToString()) +
                            int.Parse(idNumber[8].ToString()) +
                            int.Parse(idNumber[10].ToString());

            int evenResult = int.Parse($"{idNumber[1]}{idNumber[3]}{idNumber[5]}{idNumber[7]}{idNumber[9]}{idNumber[11]}") * 2;
            string evenResultString = evenResult.ToString();
            evenResult = 0;
            foreach (var c in evenResultString) {
                evenResult += int.Parse(c.ToString());
            }

            int both = oddResult + evenResult;
            int lastdigit = both % 10; // % 10 gets the last digit
            lastdigit = 10 - lastdigit;

            if (lastdigit.ToString().Last() == idNumber[12])
                return (true, null);
            else
                return (false, $"Checksum failed ({lastdigit.ToString().Last()} != {idNumber[12]})");
        }