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();
    }
  }
}

    Received event. Shape area is now 10201.86
    Drawing a circle
    Received event. Shape area is now 49
    Drawing a rectangle

namespace ImplementInterfaceEvents
{
  public interface IDrawingObject
  {
    event EventHandler ShapeChanged;
  }
  public class MyEventArgs : EventArgs 
  {
    // class members
  }
  public class Shape : IDrawingObject
  {
    public event EventHandler ShapeChanged;
    void ChangeShape()
    {
      // Do something here before the event…

      OnShapeChanged(new MyEventArgs(/*arguments*/));

      // or do something here after the event. 
    }
    protected virtual void OnShapeChanged(MyEventArgs e)
    {
      if(ShapeChanged != null)
      {
        ShapeChanged(this, e);
      }
    }
  }

}

namespace WrapTwoInterfaceEvents
{
  using System;

  public interface IDrawingObject
  {
    // Raise this event before drawing
    // the object.
    event EventHandler OnDraw;
  }
  public interface IShape
  {
    // Raise this event after drawing
    // the shape.
    event EventHandler OnDraw;
  }


  // Base class event publisher inherits two
  // interfaces, each with an OnDraw event
  public class Shape : IDrawingObject, IShape
  {
    // Create an event for each interface event
    event EventHandler PreDrawEvent;
    event EventHandler PostDrawEvent;

    object objectLock = new Object();

    // Explicit interface implementation required.
    // Associate IDrawingObject's event with
    // PreDrawEvent
    event EventHandler IDrawingObject.OnDraw
    {
      add
      {
        lock (objectLock)
        {
          PreDrawEvent += value;
        }
      }
      remove
      {
        lock (objectLock)
        {
          PreDrawEvent -= value;
        }
      }
    }
    // Explicit interface implementation required.
    // Associate IShape's event with
    // PostDrawEvent
    event EventHandler IShape.OnDraw
    {
      add
      {
        lock (objectLock)
        {
          PostDrawEvent += value;
        }
      }
      remove
      {
        lock (objectLock)
        {
          PostDrawEvent -= value;
        }
      }


    }

    // For the sake of simplicity this one method
    // implements both interfaces. 
    public void Draw()
    {
      // Raise IDrawingObject's event before the object is drawn.
      EventHandler handler = PreDrawEvent;
      if (handler != null)
      {
        handler(this, new EventArgs());
      }
      Console.WriteLine("Drawing a shape.");

      // RaiseIShape's event after the object is drawn.
      handler = PostDrawEvent;
      if (handler != null)
      {
        handler(this, new EventArgs());
      }
    }
  }
  public class Subscriber1
  {
    // References the shape object as an IDrawingObject
    public Subscriber1(Shape shape)
    {
      IDrawingObject d = (IDrawingObject)shape;
      d.OnDraw += new EventHandler(d_OnDraw);
    }

    void d_OnDraw(object sender, EventArgs e)
    {
      Console.WriteLine("Sub1 receives the IDrawingObject event.");
    }
  }
  // References the shape object as an IShape
  public class Subscriber2
  {
    public Subscriber2(Shape shape)
    {
      IShape d = (IShape)shape;
      d.OnDraw += new EventHandler(d_OnDraw);
    }

    void d_OnDraw(object sender, EventArgs e)
    {
      Console.WriteLine("Sub2 receives the IShape event.");
    }
  }


  public class Program
  {
    static void Main(string[] args)
    {
      Shape shape = new Shape();
      Subscriber1 sub = new Subscriber1(shape);
      Subscriber2 sub2 = new Subscriber2(shape);
      shape.Draw();

      // Keep the console window open in debug mode.
      System.Console.WriteLine("Press any key to exit.");
      System.Console.ReadKey();
    }
  }

}

  Sub1 receives the IDrawingObject event.
  Drawing a shape.
  Sub2 receives the IShape event.