static void Main(string[] args) { List <Student> studentList = new List <Student>(); Student student = new Student(); bool exit = false; do { string input = Message.DisplayMenu(); if (input == "1") { Add.AddStudent(student, studentList); } else if (input == "2") { Grade.Avrg(studentList); } else if (input == "3") { exit = true; } else { Message.DisplayError(); } } while (exit == false); }