private static void EnterGrades(IBook Br) { var input = ""; Double Grade; while (true) { Console.WriteLine("Enter Grades or q to quit"); input = Console.ReadLine(); if (input == "q") { break; } try { Grade = Convert.ToDouble(input); Br.AddGrades(Grade); } catch (ArgumentException ex) { Console.WriteLine(ex.Message); } catch (FormatException ex) { Console.WriteLine(ex.Message); } } }