但是事件并不局限于图像界面，也可能是满足程序的某个逻辑判断触发了事件。引发事件的对象叫做事件发送方，捕获事件并对其做出响应的对象叫做事件接收方。但是事件发送方并不知道哪个对象或者方法会处理它引发的事件，所以需要在事件发送方和事件接收方之间存在一媒介，明确某个对象或者某个类型的某个方法会对这个事件进行响应。.NET用委托作为事件发送方与事件接收方之间的媒介，委托只有一个签名，只要方法签名与委托签名匹配的方法，都可以声明自己对这个委托类型的事件的感兴趣，接收并处理它。
事件发送方的对象为了给事件接收对象传递一些额外信息，就需要写一个派生于System.EventArgs的类，封装一些数据。
复制代码 代码如下:
    public class DrawEventArgs:EventArgs
    {
        privatedouble m_Size;
        public DrawEventArgs(doublesize)
        {
            m_Size = size;
        }
        public double Size
        {
            get
            {
                returnm_Size;
            }
        }
    }

下面声明一委托，该委托有两个参数，一个是参数代表事件的发送方，另一个是参数是该事件封装的数据。
public  delegate void ShapeSizeChanged(objectsender,DrawEventArgs e);
ShapeSizeChanged的实例可以绑定到任何和它方法签名匹配的方法上。
下面是自定义事件委托，
public event ShapeSizeChanged SizeChanged;
下面是一个负责引发事件的类型代码，
复制代码 代码如下:

    public class DrawManager
    {
        public event ShapeSizeChangedSizeChanged;
        protectedvoid OnSizeChanged(DrawEventArgse)
        {
            ShapeSizeChangedtemp = SizeChanged;
            //是否有委托与该事件关联
            if(temp != null)
            {
                temp(this,e);
            }
        }
        public void SizeChange(doublesize)
        {
            DrawEventArgse = new DrawEventArgs(size);
            OnSizeChanged(e);
        }
    }

而后定义两个监听事件的类型，
复制代码 代码如下:
    public class Square
    {
        publicSquare(DrawManager drawManager)
        {
            //drawManager.SizeChanged+= DrawSquare;
            //把事件关联到委托上
            drawManager.SizeChanged += new ShapeSizeChanged(DrawSquare);
        }
        public void DrawSquare(objectsender, DrawEventArgs e)
        {
            Console.WriteLine(string.Format("TheSquare'Length = {0}", e.Size));
        }
        public void Detach(DrawManagerdrawManager)
        {
            //drawManager.SizeChanged-= DrawSquare;
            //解除事件和委托的关联
            drawManager.SizeChanged -= new ShapeSizeChanged(DrawSquare);
        }
    }
    public class Rectangle
    {
        publicRectangle(DrawManager drawManager)
        {
            drawManager.SizeChanged +=DrawRectangle;
        }
        public void DrawRectangle(objectsender, DrawEventArgs e)
        {
            Console.WriteLine(string.Format("TheRectangle'length={0} and width={1}.",e.Size*2,e.Size));
        }
        public void Detach(DrawManagerdrawManager)
        {
            drawManager.SizeChanged -=DrawRectangle;
        }
    }

测试代码，
复制代码 代码如下:
    class Program
    {
        static void Main(string[]args)
        {
            DrawManagerdrawManager = new DrawManager();
            Rectanglerect = new Rectangle(drawManager);
            Squaresquare = new Square(drawManager);
            //引发事件
            drawManager.SizeChange(5);

            //解除监听事件
            square.Detach(drawManager);
            drawManager.SizeChange(10);
            Console.ReadLine();
        }
    }
/*运行结果
  The Rectangle'length=10 and width=5.
  The Square'Length = 5
  The Rectangle'length=20 and width=10.
 */

.NET中的事件模式和观察者模式非常相似，也可谓是观察者模式在.NET下的进化版吧，下面用观察者模式实现上面功能以做对比，首先定义两个接口，IObserver和IObservable，如下
复制代码 代码如下:
public interface IObserver
    {
        voidNotify(DrawEventArgs e);
    }
    public interface IObservable
    {
        voidRegister(IObserver observer);
        voidUnRegister(IObserver observer);
    }

下面是改写后的两个观察者类，
复制代码 代码如下:
    public class NewRectangle:IObserver
    {
        privateObserverManager m;
        publicNewRectangle(ObserverManager oManager)
        {
            m=oManager;
            oManager.Register(this);
        }
        public void Notify(DrawEventArgse)
        {
            Console.WriteLine(string.Format("TheRectangle'length={0} and width={1}.", e.Size * 2, e.Size));
        }
        public void Detach()
        {
            m.UnRegister(this);
        }
    }
    public class NewSquare:IObserver
    {
        privateObserverManager m;

        publicNewSquare(ObserverManager oManager)
        {
            m=oManager;
            oManager.Register(this);
        }
        public void Notify(DrawEventArgse)
        {
            Console.WriteLine(string.Format("TheSquare'Length = {0}.", e.Size));
        }
        public void Detach()
        {
            m.UnRegister(this);
        }
    }

下面是负责通知观察者的类型，
复制代码 代码如下:
    public class ObserverManager:IObservable
    {
        protectedArrayList arrList;
        publicObserverManager()
        {
            arrList = newArrayList();
        }
        public void Register(IObserverobserver)
        {
            arrList.Add(observer);
        }
        public void UnRegister(IObserverobserver)
        {
            if(arrList.Contains(observer))
            {
                arrList.Remove(observer);
            }
        }
        public void NotifyObservers(doublesize)
        {
            DrawEventArgse = new DrawEventArgs(size);
            foreach(IObserver observer inarrList)
            {
                observer.Notify(e);
            }
        }
        public void SizeChanged(doublesize)
        {
            NotifyObservers(size);
        }
    }

下面是调用代码，
复制代码 代码如下:
        static void Main(string[]args)
        {
            ObserverManageroManager = new ObserverManager();
            NewRectanglerect = new NewRectangle(oManager);
            NewSquaresquare = new NewSquare(oManager);
            oManager.SizeChanged(5);
            square.Detach();
            oManager.SizeChanged(10);
     Console.ReadLine();
        }

最好运行下代码，这样可以更easy的理解这两种模式微妙的差别了。
对事件来说，还可以显式的用add和remove编写事件访问器，事件访问器通常有编译器生成，所以可以显式的用事件访问器修改DrawManager类型，
复制代码 代码如下:
    public class DrawManager
    {
        privateevent ShapeSizeChangedm_SizeChanged;
        privatereadonly objectm_lock = new object();
        public event ShapeSizeChangedSizeChanged
        {
            add
            {
                lock(m_lock)
                {
                    m_SizeChanged += value;
                }
            }
            remove
            {
                lock(m_lock)
                {
                    m_SizeChanged -= value;
                }
            }
        }
        protectedvoid OnSizeChanged(DrawEventArgse)
        {
            ShapeSizeChangedtemp = m_SizeChanged;
            //是否有委托与该事件关联
            if(temp != null)
            {
                temp(this,e);
            }
        }
        public void SizeChange(doublesize)
        {
            DrawEventArgse = new DrawEventArgs(size);
            OnSizeChanged(e);
        }
    }