// compile with: csc -target:library -reference:abstractshape.dll shapes.cs public class Square : Shape { private int side; public Square(int side, string id) : base(id) { this.side = side; } public override double Area { get { // Given the side, return the area of a square: return side * side; } } } public class Circle : Shape { private int radius; public Circle(int radius, string id) : base(id) { this.radius = radius; } public override double Area { get { // Given the radius, return the area of a circle: return radius * radius * System.Math.PI; } } } public class Rectangle : Shape { private int width; private int height; public Rectangle(int width, int height, string id) : base(id) { this.width = width; this.height = height; } public override double Area { get { // Given the width and height, return the area of a rectangle: return width * height; } } }