private static void EnterGrades(IBook book) { string input = ""; double grade = 0.0; bool IsDouble = false; while (input.ToUpper() != "Q") { Console.WriteLine("Please Enter Grade or Q for exit :"); input = Console.ReadLine(); IsDouble = double.TryParse(input, out grade); if (IsDouble) { book.AddToTextFileGrades(grade); } } }