using System;

namespace ConsoleApplication1
{

    class MyConsoleApp
    {
        static void Main(string[] args)
        {
            isbn myFavoriteBook = new isbn();

            bool isValid = CheckDigit.CheckIsbn(myFavoriteBook.GetIsbn());
            Console.WriteLine(string.Format("Your book {0} a valid ISBN", isValid ? "has" : "doesn't have"));

            Console.ReadLine();
        }
    }

    public static class CheckDigit
    {
        public static bool CheckIsbn(string isbn)
        {
            //implementation as in your question.
            return true;
        }
    }

    public class isbn
    {
        private string isbnNum;
        private int checkDigit;
        private bool isValid;

        public string GetIsbn()
        {
            return this.isbnNum;
        }

        public isbn()
        {
            Console.Write("Enter Your ISBN Number: ");
            this.isbnNum = Console.ReadLine();
        }

        public string displayISBN()
        {
            return this.GetIsbn();
        }
    }
}