#Program.cs

using System;
using Lession03.Modals;

namespace Lession03
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            //Console.WriteLine("Hello World!");
            //Citizen citizen = new Citizen();
            //citizen.Input();

            //citizen.Display();

            //Student std = new Student();
            //std.Input();
            //std.Display();
            Citizen citizen = new Student();
            citizen.Input();
            citizen.Display();
            //Kiem tra object -> co phai duoc tao ra tu 1 class nao do ko
            if(citizen is Student)
            {
                ((Student)citizen).Learning();
            }
        }
    }
}

#Student.cs

using System;
namespace Lession03.Modals
{
    public class Student : Citizen
    {
        public string Rollno { get; set; }
        public string Email { get; set; }

        public Student()
        {
        }

        public Student(string fullname, string address, string cmtnd, int age, string rollno, string email) : base(fullname, address, cmtnd, age)
        {
            this.Rollno = rollno;
            this.Email = email;
        }

        public override void Input()
        {
            base.Input();

            Console.WriteLine("Nhap rollno: ");
            Rollno = Console.ReadLine();

            Console.WriteLine("Nhap email: ");
            Email = Console.ReadLine();
        }

        public override void Display()
        {
            //base.Display();
            //Console.WriteLine("Rollno: {0}, Email: {1}", Rollno, Email);
            Console.WriteLine("Fullname: {0}, Age: {1}, Cmtnd: {2}, Address: {3}, Rollno: {4}, Email: {5}", Fullname, Age, Cmtnd, Address, Rollno, Email);
        }

        public void Learning()
        {
            Console.WriteLine("Thong tin sinh vien ...");
        }
    }
}

#Citizen.cs

using System;
namespace Lession03.Modals
{
    public class Citizen
    {
        public string Fullname { get; set; }
        public string Address { get; set; }
        public string Cmtnd { get; set; }
        public int Age { get; set; }

        public Citizen()
        {
        }

        public Citizen(string fullname, string address, string cmtnd, int age)
        {
            this.Address = address;
            this.Age = age;
            this.Cmtnd = cmtnd;
            this.Fullname = fullname;
        }

        public virtual void Input()
        {
            Console.WriteLine("Nhap ten: ");
            Fullname = Console.ReadLine();

            Console.WriteLine("Nhap tuoi: ");
            Age = Int32.Parse(Console.ReadLine());

            Console.WriteLine("Nhap CMTND: ");
            Cmtnd = Console.ReadLine();

            Console.WriteLine("Nhap dia chi: ");
            Address = Console.ReadLine();
        }

        public virtual void Display()
        {
            Console.WriteLine("Fullname: {0}, Age: {1}, Cmtnd: {2}, Address: {3}", Fullname, Age, Cmtnd, Address);
        }
    }
}