Figura.cs

using System;

abstract class Figura
{
    public const double PI = Math.PI;
    protected double x;
    protected double y;

    public Figura (double x, double y)
    {
        this.x = x;
        this.y = y;
    }

    public abstract double CalcularArea();
}

class Circulo : Figura
{
    public Circulo(double radio)
        : base(radio, 0)
    {
    }

    public override double CalcularArea()
    {
        return PI * x * x;
    }
}

class Cilindro : Circulo
{
    public Cilindro (double radio, double altura)
        : base (radio)
    {
        y = altura;
    }

    public override double CalcularArea()
    {
        return ( 2 * base.CalcularArea()) + ( 2 * PI * x * y);
    }
}

class PruebaFiguras
{
    static void Main()
    {
        double radio = 3.7;
        double altura = 3.1;

        Circulo circulo = new Circulo(radio);
        Cilindro cilindro = new Cilindro(radio, altura);

        Console.WriteLine ("El área del círculo es: {0}.", circulo.CalcularArea());
        Console.WriteLine ("El área del cilindro es: {0}.", cilindro.CalcularArea());
    }
}