Implementation of Luhn algorithm

LuhnAlgorithm.cs

using System;

namespace Luhn
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(IsValid());
            Console.ReadLine();
        }

        static bool IsValid(string cardNumber = "1111222233334444")
        {
            int sum = 0;
            int numberOfDigits = cardNumber.Length;
            int parity = (numberOfDigits - 1) % 2;
            char[] digit = new char[] { '\0' };
            for (int index = numberOfDigits; index > 0; index--)
            {
                digit[0] = cardNumber[index - 1];
                int nDigit = int.Parse(digit);

                if (parity == index % 2)
                    nDigit *= 2;

                sum += nDigit / 10;
                sum += nDigit % 10;
            }

            return 0 == sum % 10;
        }
    }
}