using System; using System.Collections.Generic; public interface IShape { double Area { get; } } public class Circle : IShape { public Circle( double radius ) { this.radius = radius; } public double Area { get { return 3.14 * radius * radius; } } private double radius; } public class Rect : IShape { public Rect( double width, double height ) { this.width = width; this.height = height; } public double Area { get { return width*height; } } private double width; private double height; } public class Shapes<T> where T: IShape { public double TotalArea { get { double acc = 0; foreach( T shape in shapes ) { acc += shape.Area; } return acc; } } public void Add( T shape ) { shapes.Add( shape ); } private List<T> shapes = new List<T>(); } public class Test { static void Main() { Shapes<IShape> shapes = new Shapes<IShape>(); shapes.Add( new Circle(3) ); shapes.Add( new Rect(7, 5) ); Console.WriteLine( "Total Area: {0}", shapes.TotalArea ); } }