public static void Main(string[] args) { Console.Write("Enter a valid 10 digit ISBN Number "); string isbn = checkIsbnClass.DestabilizeIsbn(Console.ReadLine()); // Normalizes the input and puts it on string "str" Isbn isbn = new Isbn(Console.In) if (!isbn.CheckLength()) { Console.WriteLine("The number you have entered is not a valid ISBN try again."); // Print invalid number } }