public void ReadFromFile(string FileName)
        {
            StreamReader reader = new StreamReader(FileName);
            string       line;

            listStudent.Clear();
            while ((line = reader.ReadLine()) != null)
            {
                string[] items = line.Split('|');
                Student  s     = null;
                if (items[0].Equals("1"))
                {
                    s = new Student(
                        Convert.ToInt32(items[1]),
                        items[2],
                        Convert.ToDateTime(items[3])
                        );
                }
                else if (items[0].Equals("2"))
                {
                    s = new ForeignStudent(Convert.ToInt32(items[1]),
                                           items[2],
                                           Convert.ToDateTime(items[3]), items[4]);
                }
                else if (items[0].Equals("3"))
                {
                    s = new VNStudent(Convert.ToInt32(items[4]), Convert.ToInt32(items[1]),
                                      items[2],
                                      Convert.ToDateTime(items[3]));
                }
                else
                {
                    throw new Exception("Invalid student type");
                }
                listStudent.Add(s);
            }
            reader.Close();
        }
        public bool AddStudent()
        {
            Console.WriteLine("=============================");
            Console.WriteLine("Input student. Choose one of below");
            Console.WriteLine("1 add Student: ");
            Console.WriteLine("2 add Foreign Student: ");
            Console.WriteLine("3 add Vietnamese student: ");
            Console.WriteLine("4 to exit: ");
            Console.WriteLine("=============================");
            Console.Write("Enter your choice : ");
            int choice = Convert.ToInt32(Console.ReadLine());

            switch (choice)
            {
            case 1:
                Student student = new Student();
                student.Input();
                listStudent.Add(student);
                return(true);

            case 2:
                Student foreignStudent = new ForeignStudent();
                foreignStudent.Input();
                listStudent.Add(foreignStudent);
                return(true);

            case 3:
                Student vietnameseStudent = new VNStudent();
                vietnameseStudent.Input();
                listStudent.Add(vietnameseStudent);
                return(true);

            default:
                return(false);
            }
        }