using System;

public class Circle

{

    private double _R;

    private PropertyDelegate _handler;

    public Circle() : this(0)

    {

    }

    public Circle(float R)

    {

        this._R = R;

    }

    public double R

    {

        get

        {

            return _R;

        }

​

        set

        {

            _R = value;

        }

    }

    public PropertyDelegate handler

    {

        get

        {

            return _handler;

        }

​

        set

        {

            _handler = value;

        }

    }

    public delegate void PropertyDelegate();

    public void Area()

    {

        Console.WriteLine("Area: " + Math.PI * Math.Pow(R, 2));

    }

    public void Length()

    {

        Console.WriteLine("Length: " + 2 * Math.PI * R);

    }

    public void Property(PropertyDelegate callback)

    {

        callback();

    }

}

​

public class Program

{

    public static void Main()

    {

        Circle circle = new Circle(4);

        circle.handler = circle.Area;

        circle.Property(circle.handler);

        circle.handler = circle.Length;

        circle.Property(circle.handler);

    }

}