Luhn Formula Demo C#, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public static class Luhn
    {
        public static bool LuhnCheck(this string cardNumber)
        {
            return LuhnCheck(cardNumber.Select(c => c - '0').ToArray());
        }
 
        private static bool LuhnCheck(this int[] digits)
        {
            return GetCheckValue(digits) == 0;
        }
 
        private static int GetCheckValue(int[] digits)
        {
            return digits.Select((d, i) => i % 2 == digits.Length % 2 ? ((2 * d) % 10) + d / 5 : d).Sum() % 10;
        }
    }
    
    public class Program
    {
        public static void Main(string[] args)
        {            
            long[] testNumbers = {800112794657, 6011000990139424};
            foreach (var testNumber in testNumbers)
                Console.WriteLine("{0} is {1}valid", testNumber, testNumber.ToString().LuhnCheck() ? "" : "not ");
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,19 sec, absolute running time: 0,11 sec, cpu time: 0,09 sec, average memory usage: 13 Mb, average nr of threads: 3