using System;

using System.Collections.Generic;

​

public class Program

{

    internal interface IShape

    {

        T Accept<T>(IShapeVisitor<T> calculator);

    }

    internal interface IShapeVisitor<T>

    {

        T Visit(Circle circle);

        T Visit(Square square);

    }

    public static void Main(string[] args)

    {

        var shapes = new List<IShape>()

        {

            new Square(10),

            new Circle(10)

        };

        var areaCalculator = new AreaCalculator();

        foreach (var shape in shapes)

        {

            var area = shape.Accept(areaCalculator);

            Console.WriteLine(area);

        }

    }

    internal sealed class AreaCalculator : IShapeVisitor<double>

    {

        public double Visit(Circle circle)

        {

            return System.Math.PI * circle.R * circle.R;

        }

        public double Visit(Square square)

        {

            return square.A * square.A;

        }

    }

    internal sealed class Circle : IShape

    {

        private double _r;

        public Circle(double r)

        {

            _r = r;

        }

        public double R { get { return _r; } }

        public T Accept<T>(IShapeVisitor<T> calculator)

        {

            return calculator.Visit(this);

        }

    }

    internal sealed class Square : IShape

    {

        private readonly double _a;

        public Square(double a)

        {

            _a = a;

        }

        public double A { get { return _a; } }

        public T Accept<T>(IShapeVisitor<T> calculator)

        {

            return calculator.Visit(this);

        }

    }

}