พหุสัณฐาน

using System;

abstract class Shape { public abstract double Area(); }
class Circle(double radius) : Shape { public override double Area() => Math.PI * radius * radius; }
class Square(double side) : Shape { public override double Area() => side * side; }

class Program
{
    static void Main()
    {
        Shape[] shapes = { new Circle(2), new Square(3) };
        foreach (Shape shape in shapes) Console.WriteLine($"{shape.Area():F2}");
    }
}