using System;

​

public class Program

{

    delegate string kappa(int i);

    delegate string keepo(int i, string w);

    public static void Main()

    {

        var a = new A();

        a.aa();

        a.xxx = new xbase();

        a.aa();

        a.Kappa(new xchild());

        a.aa();

    }

}

​

public class A

{

    xbase xx;

    public void aa()

    {

        xx.xdo();

    }

​

    public A()

    {

        xx = new xchild();

    }

​

    public xbase xxx

    {

        get

        {

            return this.xx;

        }

​

        set

        {

            this.xx = value;

        }

    }

​

    public void Kappa(xbase e)

    {

        this.xx = e;

    }

}

​

public class xbase

{

    public string val

    {

        get;

        set;

    }

​

    public virtual void xdo()

    {

        Console.WriteLine("base\n");

    }

}

​

public class xchild : xbase

{

    public string cval

    {

        get;

        set;

    }

​

    public override void xdo()

    {

        Console.WriteLine("child");

    }

}

​

public class xgchild : xchild

{

    public string xgval

    {

        get;

        set;

    }

​

    override public void xdo()

    {

        Console.WriteLine("grand child");

    }

}

​

interface pr

{

    void print();

}

​

public abstract class machine

{

    public string id

    {

        get;

        set;

    }

​

    public void dostuff()

    {

        Console.WriteLine("Machine working\n");

    }

​

    public bool isWorking()

    {

        if (id != null)

            return true;

        else

            return false;

    }

}

​

public class Typewriter : machine, pr

{

    public Typewriter(string st)

    {

        this.xx = st;

    }

​

    public Typewriter()

    {

    }

​

    public string xx

    {

        get;

        set;

    }

​

    public void print()

    {

        Console.WriteLine("this is :" + this.xx);

    }

​

    new public void dostuff()

    {

        Console.WriteLine("Typewriter working\n");

    }

}