Figura.cs

using System;

public abstract class Figura
{
    private string idFigura;

    public Figura (string idFigura)
    {
        IdFigura = idFigura;            // asigna el valor de idFigura a la propiedad IdFigura
    }

    public string IdFigura
    {
        get
        {
            return idFigura;
        }
        set
        {
            idFigura = value;
        }
    }

    // La propiedad Area es de sólo-lectura
    public abstract double Area
    {
        get;
    }

    public override string ToString ()
    {
        return IdFigura  + " Área = " + string.Format("{0:F2}", Area);
    }
}