namespace BaseClassEvents{    using System;    using System.Collections.Generic;    // Special EventArgs class to hold info about Shapes.     public class ShapeEventArgs : EventArgs    {        private double newArea;        public ShapeEventArgs(double d)        {            newArea = d;        }        public double NewArea        {            get { return newArea; }        }    }    // Base class event publisher     public abstract class Shape    {        protected double area;        public double Area        {            get { return area; }            set { area = value; }        }        // The event. Note that by using the generic EventHandler<T> event type         // we do not need to declare a separate delegate type.         public event EventHandler<ShapeEventArgs> ShapeChanged;        public abstract void Draw();        //The event-invoking method that derived classes can override.         protected virtual void OnShapeChanged(ShapeEventArgs e)        {            // Make a temporary copy of the event to avoid possibility of             // a race condition if the last subscriber unsubscribes             // immediately after the null check and before the event is raised.            EventHandler<ShapeEventArgs> handler = ShapeChanged;            if (handler != null)            {                handler(this, e);            }        }    }    public class Circle : Shape    {        private double radius;        public Circle(double d)        {            radius = d;            area = 3.14 * radius * radius;        }        public void Update(double d)        {            radius = d;            area = 3.14 * radius * radius;            OnShapeChanged(new ShapeEventArgs(area));        }        protected override void OnShapeChanged(ShapeEventArgs e)        {            // Do any circle-specific processing here.             // Call the base class event invocation method.             base.OnShapeChanged(e);        }        public override void Draw()        {            Console.WriteLine("Drawing a circle");        }    }    public class Rectangle : Shape    {        private double length;        private double width;        public Rectangle(double length, double width)        {            this.length = length;            this.width = width;            area = length * width;        }        public void Update(double length, double width)        {            this.length = length;            this.width = width;            area = length * width;            OnShapeChanged(new ShapeEventArgs(area));        }        protected override void OnShapeChanged(ShapeEventArgs e)        {            // Do any rectangle-specific processing here.             // Call the base class event invocation method.             base.OnShapeChanged(e);        }        public override void Draw()        {            Console.WriteLine("Drawing a rectangle");        }    }    // Represents the surface on which the shapes are drawn     // Subscribes to shape events so that it knows     // when to redraw a shape.     public class ShapeContainer    {        List<Shape> _list;        public ShapeContainer()        {            _list = new List<Shape>();        }        public void AddShape(Shape s)        {            _list.Add(s);            // Subscribe to the base class event.            s.ShapeChanged += HandleShapeChanged;        }        // ...Other methods to draw, resize, etc.         private void HandleShapeChanged(object sender, ShapeEventArgs e)        {            Shape s = (Shape)sender;            // Diagnostic message for demonstration purposes.            Console.WriteLine("Received event. Shape area is now {0}", e.NewArea);            // Redraw the shape here.            s.Draw();        }    }    class Test    {        static void Main(string[] args)        {            //Create the event publishers and subscriber            Circle c1 = new Circle(54);            Rectangle r1 = new Rectangle(12, 9);            ShapeContainer sc = new ShapeContainer();            // Add the shapes to the container.            sc.AddShape(c1);            sc.AddShape(r1);            // Cause some events to be raised.            c1.Update(57);            r1.Update(7, 7);            // Keep the console window open in debug mode.            System.Console.WriteLine("Press any key to exit.");            System.Console.ReadKey();        }    }}/* Output:        Received event. Shape area is now 10201.86        Drawing a circle        Received event. Shape area is now 49        Drawing a rectangle */