Checks number against Luhn algorithm/

IsValidCreditCardNumber.cs

private static bool IsValidCreditCardNumber(string creditCardNumber)
{
    // based on Luhn algorithm
    // https://en.wikipedia.org/wiki/Luhn_algorithm

    if (creditCardNumber == null) throw new ArgumentNullException(nameof(creditCardNumber));

    // credit card number length must be between [12, 19]
    if (creditCardNumber.Length < 12 || creditCardNumber.Length > 19)
    {
        return false;
    }

    int sum = 0;
    bool alternate = false;

    for (int i = creditCardNumber.Length - 1; i >= 0; i--)
    {
        var n = creditCardNumber[i] - '0';

        if (n < 0 || n > 9)
        {
            return false;
        }

        if (alternate)
        {
            n *= 2;

            if (n >= 10)
            {
                n -= 9;
            }
        }

        sum += n;
        alternate = !alternate;
    }

    return sum % 10 == 0;
}