Program.cs

namespace Luhn
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Please enter your card number: ");
            string? cardNumber = Console.ReadLine();

            if (string.IsNullOrWhiteSpace(cardNumber))
            {
                Console.WriteLine("Invalid card number!");
                return;
            }

            int len = cardNumber.Length;

            if (IsValid(len, cardNumber))
            {
                CheckCardType(len, cardNumber);
            }
            else
            {
                Console.WriteLine("Invalid");
            }

        }

        // Check card type
        static void CheckCardType(int len, string cardNumber)
        {
            if (cardNumber[0] == '4' && (len == 13 || len == 16))
                Console.WriteLine("VISA");
            else if (cardNumber[0] == '3' && (cardNumber[1] == '4' || cardNumber[1] == '7') && len == 15)
                Console.WriteLine("AMEX");
            else if (cardNumber[0] == '5' && (cardNumber[1] >= '1' && cardNumber[1] <= '5') && len == 16)
                Console.WriteLine("Mastercard");
            else
                Console.WriteLine("Invalid");
        }

        // Check whether the card is valid
        static bool IsValid(int len, string cardNumber)
        {
            int sum = 0;
            bool doubleDigit = false;

            for (int i = len - 1; i >= 0; i--)
            {
                if (!char.IsDigit(cardNumber[i]))
                {
                    return false;
                }

                int digit = cardNumber[i] - '0';

                if (doubleDigit)
                {
                    digit *= 2;

                    if (digit > 9)
                    {
                        digit -= 9;
                    }
                }

                sum += digit;

                doubleDigit = !doubleDigit;
            }
            return sum % 10 == 0;
        }
    }
}