public class Program

{

    public static void Main()

    {

        Shape[] shapes =

        {

            new Square(5, "Square #1"),

            new Circle(3, "Circle #1"),

            new Rectangle( 4, 5, "Rectangle #1")

        };

​

        System.Console.WriteLine("Shapes Collection");

        foreach (Shape s in shapes)

        {

            System.Console.WriteLine(s);

        }

    }

// 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{return width*height;}}

}   

    // compile with: csc /target:library abstractshape.cs

public abstract class Shape

{

    private string name;

​

    public Shape(string s)

    {

        // calling the set accessor of the Id property.

        Id = s;

    }

​

    public string Id

    {

        get

        {

            return name;

        }

​

        set

        {

            name = value;

        }

    }

​

    // Area is a read-only property - only a get accessor is needed:

    public abstract double Area

    {

        get;

    }

​

    public override string ToString()

    {

        return Id + " Area = " + string.Format("{0:F2}", Area);

    }

 }

}