using System;

using System.Collections.Generic;

using System.Linq;

​

public class Person 

{

    public string name;

    public int age;

    public Person()

    {

        // Blank Constructor exists to allow inheritance

    }

    public Person(string name, int age)

    {

        // Regular Constructor takes in 2 args: name and age

        this.name = name;

        this.age = age;

    }

    public virtual void Intro() 

    {

        // Virtual so it can be overridden

        Console.WriteLine("Hi, my name is {0} and I am {1} years old", new Object[2]{ this.name, this.age });

    }

}

​

public class Student : Person

{

    // No need for name or age; inherited from Person

    public int grade;

    public Student(string name, int age, int grade) 

    {

        // Student constructor takes all 3 args

        this.name = name;

        this.age = age;

        this.grade = grade;

    }

    // Override of Virtual function

    public override void Intro()

    {

        base.Intro(); // super()

        Console.WriteLine("I am in year {0}", this.grade); // Only one format, so we don't need an array[]

    }

    public void AddToRegister(List<Person> register) 

    {

        register.Add(this);

    }

}

​

public class Teacher : Person

{

    public string subject;

    public Teacher(string name, int age, string subject) 

    {

        this.name = name;

        this.age = age;

        this.subject = subject;

    }

    public override void Intro()

    {

        base.Intro();

        Console.WriteLine("My name is {0}, I am {1} I teach {2}", this.name, this.age, this.subject);

    }

}

​

​

public class Program

{   

    public static void Main()

    {

        List<Person> register = new List<Person>();

        Person jacob = new Teacher("Jacob", 22, Software Developer);

        Student joao = new Student("Joao", 16, 12);

        Student maja = new Student("Maja", 16, 12);

        jacob.Intro();

        joao.AddToRegister(register);

        maja.AddToRegister(register);

        // "p => p.name" is the same as "foreach (Person p in register)"

        string output = String.Join(", ", register.Select(p => p.name));

        Console.WriteLine(output);

    }

}