event EventHandler IDrawingObject.OnDraw
        {
            add
            {
                lock (PreDrawEvent)
                {
                    PreDrawEvent += value;
                }
            }
            remove
            {
                lock (PreDrawEvent)
                {
                    PreDrawEvent -= value;
                }
            }
        }
 

//==============================>舉例

namespace EventTest
{
    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();
        }
    }
    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 Shape : IDrawingObject, IShape
    {
        // Create an event for each interface event
        event EventHandler PreDrawEvent;
        event EventHandler PostDrawEvent;

        object objectLock = new Object();

        event EventHandler IDrawingObject.OnDraw
        {
            add
            {
                lock (objectLock)
                {
                    PreDrawEvent += value;
                }
            }
            remove
            {
                lock (objectLock)
                {
                    PreDrawEvent -= value;
                }
            }
        }

        event EventHandler IShape.OnDraw
        {
            add
            {
                lock (objectLock)
                {
                    PostDrawEvent += value;
                }
            }
            remove
            {
                lock (objectLock)
                {
                    PostDrawEvent -= value;
                }
            }
        }

        public void Draw()
        {
            EventHandler handler = PreDrawEvent;
            if (handler != null)
            {
                handler(this, new EventArgs());
            }
            Console.WriteLine("Drawing a shape.");

            handler = PostDrawEvent;
            if (handler != null)
            {
                handler(this, new EventArgs());
            }
        }
    }

    public interface IDrawingObject
    {
        event EventHandler OnDraw;
    }
    public interface IShape
    {
        event EventHandler OnDraw;
    }
}
 

/* Output:
    Sub1 receives the IDrawingObject event.
    Drawing a shape.
    Sub2 receives the IShape event.
*/