最近在学习委托时发现了这篇文章写的很好：
在学习C#中的委托和事件过程中，我读了许多文章来理解他们二者究竟是怎么一回事，以及如何使用他们，现在我将整个的理解过程陈述以下，我学到的每一方面，恐怕也是你们需要掌握的 ：-）。
什么是委托？
委托和事件这两个概念是完全配合的。委托仅仅是函数指针，那就是说，它能够引用函数，通过传递地址的机制完成。委托是一个类，当你对它实例化时，要提供一个引用函数，将其作为它构造函数的参数。
每一个委托都有自己的签名，例如：Delegate int SomeDelegate(string s, bool b);是一个委托申明，在这里，提及的签名，就是说SomeDelegate 这个委托 有 string 和 bool 类型的形参，返回一个int 类型。
上面提及的：当你对委托实例化时，要提供一个引用函数，将其作为它构造函数的参数。这里要注意了：被引用的这个函数必须和委托有相同的签名。
看下面的函数：
private int SomeFunction(string str, bool bln){...}
你可以把这个函数传给SomeDelegate的构造函数，因为他们有相似的签名（in other words，他们都有相同的形参类型和个数，并且返回相同的数据类型）。
SomeDelegate sd = new SomeDelegate(SomeFunction);
sd 引用了 SomeFunction，也就是说，SomeFunction已被sd所登记注册，如果你调用 sd，SomeFunction 这个函数也会被调用，记住：我所说 SomeFunction的含义，后面，我们会用到它。
现在，你应该知道如何使用委托了，让我们继续理解事件之旅……
事件的理解
我们知道，在C#中：
l 按钮（Button）就是一个类，当我们单击它时，就触发一次click事件。
l 时钟（Timer）也是一个类，每过一毫秒，就触发一次tick事件。
让我们通过一个例子来学习，假定有这样的情节：
现在有一个Counter的类，它有一个方法 CountTo(int countTo, int reachableNum)，该方法表示：在指定的时间段内（0~~countTo），当到达指定的时间点reachableNum时，就触发一次NumberReached事件。
它还有一个事件：NumberReached，事件是委托类型的变量。意思是：如果给事件命名，用event关键字和要使用的委托类型申明它即可，如下所示：
public event NumberReachedEventHandler NumberReached;
在上面的申明中，NumberReachedEventHandle 仅是一个委托，更确切的表示应该是：NumberReachedDelegate。但是微软从不这样认为MouseDelegate或者PaintDelegate，，而是称谓：MouseEventHandler 或者 PaintEventHandler。所以
NumberReachedEventHandler 比NumberReachedDelegate听起来更方便一些，OK?好了，让我们继续，现在你知道了，在我们声明事件之前，需要象下面这样的形式来定义委托：
public delegate void NumberReachedEventHandler(object sender, NumberReachedEventArgs e);
现在声明的委托 NumberReachedEventHandle，它有一个void 返回值，和object，NumberReachedEventArgs两个形参。就像我们在第一节中强调的那样，当实例化委托时，作为实参传入的函数也必须拥有和委托同样的签名。
在你的代码中， 你是否用过PaintEventArgs 或者 MouseEventArgs来确定鼠标的移动位置？是否在触发Paint事件的对象中用过Graphics 属性？实际上，为用户提供数据的类都是继承于System.EventArgs类，就是我们常说的事件参数类，如果事件不提供参数，就不定义该类。在我们的例子中，我们通过下面的类提供预期的时间点。
public class NumberReachedEventArgs : EventArgs
{
private int _reached;
public NumberReachedEventArgs(int num)
{
this._reached = num;
}
public int ReachedNumber
{
get
{
return _reached;
}
}
}
好，有了前面的介绍，让我们到Counter类里面看看：
namespace Events
{
public delegate void NumberReachedEventHandler(object sender,
NumberReachedEventArgs e);
/// <summary>
/// Summary description for Counter.
/// </summary>
public class Counter
{
public event NumberReachedEventHandler NumberReached;
public Counter()
{
//
// TODO: Add constructor logic here
//
}
public void CountTo(int countTo, int reachableNum)
{
if(countTo < reachableNum)
throw new ArgumentException(
"reachableNum should be less than countTo");
for(int ctr=0;ctr<=countTo;ctr++)
{
if(ctr == reachableNum)
{
NumberReachedEventArgs e = new NumberReachedEventArgs(
reachableNum);
OnNumberReached(e);
return;//don't count any more
}
}
}
protected virtual void OnNumberReached(NumberReachedEventArgs e)
{
if(NumberReached != null)
{
NumberReached(this, e);//Raise the event
}
}
}
在Counter中，如果到达指定的时间点，就触发一次事件，有以下几个方面需要注意：
l 通过调用NumberReached（它是NumberReachedEventHandler委托的实例）来完成一次触发事件。
NumberReached(this, e); 通过这种方式，可以调用所有的注册函数。
l 通过 NumberReachedEventArgs e = new NumberReachedEventArgs(reachableNum); 为所有的注册函数提供事件数据。
l 看了上面的代码，你可能要问了：为什么我们直接用 OnNumberReached(NumberReachedEventArgs e)方法来调用NumberReached（this，e），而不用下面的代码呢？
if(ctr == reachableNum)
{
NumberReachedEventArgs e = new NumberReachedEventArgs(reachableNum);
//OnNumberReached(e);
if(NumberReached != null)
{
NumberReached(this, e);//Raise the event
}
return;//don't count any more
}
这个问题问得很好，那就让我们再看一下OnNumberReached 签名：
protected virtual void OnNumberReached(NumberReachedEventArgs e)
①你也明白 关键字protected限定了 只有从该类继承的类才能调用该类中的所有方法。
②关键字 virtual 表明了 在继承类中可以重写该方法。
这两点非常有用，假设你在写一个从Counter继承而来的类，通过重写OnNumberReached 方法，你可以在事件触发之前，进行一次其他的工作。
protected override void OnNumberReached(NumberReachedEventArgs e)
{
//Do additional work
base.OnNumberReached(e);
}
注意：如果你没有调用base.OnNumberReached(e), 那么从不会触发这个事件！在你继承该类而想剔出它的一些其他事件时，使用该方式是非常有用的。
l 还要注意到：委托 NumberReachedEventHandler 是在类定义的外部，命名空间内定义的，对所有类来说是可见的。
好，该我们来实际操作使用Counter类了。
在我们简单的应用程序中，我们有两个文本框，分别是：txtCountTo和txtReachable：
下面是btnRun的click事件：
private void btnRun_Click(object sender, System.EventArgs e)
{
if(txtCountTo.Text == "" || txtReachable.Text=="")
return;
oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text));
}
private void oCounter_NumberReached(object sender, NumberReachedEventArgs e)
{
MessageBox.Show("Reached: " + e.ReachedNumber.ToString());
}
初始化事件处理的语法如下：
oCounter = new Counter();
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached);
现在你明白了你刚才所做的一切，仅仅初始化 NumberReachedEventHandler 委托类型的对象（就像你实例化其他对象一样），注意到 oCounter_NumberReached 方法的签名与我前面提到的相似。
还要注意我们用的是+= 而不是=；这是因为委托是特殊的对象，它可以引用多个对象（在这里是指它可以引用多个函数）。For example 如果有另外一个
和oCounter_NumberReached一样具有相同签名的函数oCounter_NumberReached2，这两个函数都可以被引用：
oCounter = new Counter();
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached);
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached2);
现在，触发一个事件后，上面两个函数被依次调用。
视情况而定，如果你想让oCounter_NumberReached2在NumberReached事件发生后不再被调用，可以简单地这样写：oCounter.NumberReached -= new NumberReachedEventHandler(oCounter_NumberReached2);
最后
让我们看一下完整的源代码,以供参考：
using System; using System.Drawing; using System.Collections; using System.ComponentModel; using System.Windows.Forms; using System.Data; namespace Events { /// <summary> /// Summary description for Form1. /// </summary> public class Form1 : System.Windows.Forms.Form { Counter oCounter = null; private System.Windows.Forms.Button cmdRun; private System.Windows.Forms.TextBox txtReachable; private System.Windows.Forms.TextBox txtCountTo; private System.Windows.Forms.Label label1; private System.Windows.Forms.Label label2; private System.Windows.Forms.Button btnRemoveDelegate; /// <summary> /// Required designer variable. /// </summary> private System.ComponentModel.Container components = null; public Form1() { // // Required for Windows Form Designer support // InitializeComponent(); // // TODO: Add any constructor code after InitializeComponent call // oCounter = new Counter(); oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached); oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached2); } /// <summary> /// Clean up any resources being used. /// </summary> protected override void Dispose( bool disposing ) { if( disposing ) { if (components != null) { components.Dispose(); } } base.Dispose( disposing ); } #region Windows Form Designer generated code /// <summary> /// Required method for Designer support - do not modify /// the contents of this method with the code editor. /// </summary> private void InitializeComponent() { this.cmdRun = new System.Windows.Forms.Button(); this.txtReachable = new System.Windows.Forms.TextBox(); this.txtCountTo = new System.Windows.Forms.TextBox(); this.label1 = new System.Windows.Forms.Label(); this.label2 = new System.Windows.Forms.Label(); this.btnRemoveDelegate = new System.Windows.Forms.Button(); this.SuspendLayout(); // // cmdRun // this.cmdRun.Location = new System.Drawing.Point(16, 72); this.cmdRun.Name = "cmdRun"; this.cmdRun.Size = new System.Drawing.Size(48, 23); this.cmdRun.TabIndex = 2; this.cmdRun.Text = "Run"; this.cmdRun.Click += new System.EventHandler(this.cmdRun_Click); // // txtReachable // this.txtReachable.Location = new System.Drawing.Point(144, 40); this.txtReachable.Name = "txtReachable"; this.txtReachable.Size = new System.Drawing.Size(56, 20); this.txtReachable.TabIndex = 1; this.txtReachable.Text = ""; // // txtCountTo // this.txtCountTo.Location = new System.Drawing.Point(144, 16); this.txtCountTo.Name = "txtCountTo"; this.txtCountTo.Size = new System.Drawing.Size(56, 20); this.txtCountTo.TabIndex = 0; this.txtCountTo.Text = ""; // // label1 // this.label1.AutoSize = true; this.label1.Location = new System.Drawing.Point(16, 16); this.label1.Name = "label1"; this.label1.Size = new System.Drawing.Size(51, 13); this.label1.TabIndex = 3; this.label1.Text = "Count To"; // // label2 // this.label2.AutoSize = true; this.label2.Location = new System.Drawing.Point(16, 40); this.label2.Name = "label2"; this.label2.Size = new System.Drawing.Size(99, 13); this.label2.TabIndex = 4; this.label2.Text = "Reach this number"; // // btnRemoveDelegate // this.btnRemoveDelegate.Location = new System.Drawing.Point(16, 104); this.btnRemoveDelegate.Name = "btnRemoveDelegate"; this.btnRemoveDelegate.Size = new System.Drawing.Size(168, 23); this.btnRemoveDelegate.TabIndex = 5; this.btnRemoveDelegate.Text = "Remove second handler"; this.btnRemoveDelegate.Click += new System.EventHandler(this.btnRemoveDelegate_Click); // // Form1 // this.AutoScaleBaseSize = new System.Drawing.Size(5, 13); this.ClientSize = new System.Drawing.Size(224, 134); this.Controls.AddRange(new System.Windows.Forms.Control[] { this.btnRemoveDelegate, this.label2, this.label1, this.txtCountTo, this.txtReachable, this.cmdRun}); this.Name = "Form1"; this.Text = "Events"; this.ResumeLayout(false); } #endregion /// <summary> /// The main entry point for the application. /// </summary> [STAThread] static void Main() { Application.Run(new Form1()); } private void btnRun_Click(object sender, System.EventArgs e) { if(txtCountTo.Text == "" || txtReachable.Text=="") return; oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text)); } private void oCounter_NumberReached(object sender, NumberReachedEventArgs e) { MessageBox.Show("Reached: " + e.ReachedNumber.ToString()); } private void oCounter_NumberReached2(object sender, NumberReachedEventArgs e) { MessageBox.Show("Reached2: " + e.ReachedNumber.ToString()); } private void btnRemoveDelegate_Click(object sender, System.EventArgs e) { oCounter.NumberReached -= new NumberReachedEventHandler(oCounter_NumberReached2); oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text)); } } }
using System; using System.Drawing; using System.Collections; using System.ComponentModel; using System.Windows.Forms; using System.Data; namespace Events { /// <summary> /// Summary description for Form1. /// </summary> public class Form1 : System.Windows.Forms.Form { Counter oCounter = null; private System.Windows.Forms.Button cmdRun; private System.Windows.Forms.TextBox txtReachable; private System.Windows.Forms.TextBox txtCountTo; private System.Windows.Forms.Label label1; private System.Windows.Forms.Label label2; private System.Windows.Forms.Button btnRemoveDelegate; /// <summary> /// Required designer variable. /// </summary> private System.ComponentModel.Container components = null; public Form1() { // // Required for Windows Form Designer support // InitializeComponent(); // // TODO: Add any constructor code after InitializeComponent call // oCounter = new Counter(); oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached); oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached2); } /// <summary> /// Clean up any resources being used. /// </summary> protected override void Dispose( bool disposing ) { if( disposing ) { if (components != null) { components.Dispose(); } } base.Dispose( disposing ); } #region Windows Form Designer generated code /// <summary> /// Required method for Designer support - do not modify /// the contents of this method with the code editor. /// </summary> private void InitializeComponent() { this.cmdRun = new System.Windows.Forms.Button(); this.txtReachable = new System.Windows.Forms.TextBox(); this.txtCountTo = new System.Windows.Forms.TextBox(); this.label1 = new System.Windows.Forms.Label(); this.label2 = new System.Windows.Forms.Label(); this.btnRemoveDelegate = new System.Windows.Forms.Button(); this.SuspendLayout(); // // cmdRun // this.cmdRun.Location = new System.Drawing.Point(16, 72); this.cmdRun.Name = "cmdRun"; this.cmdRun.Size = new System.Drawing.Size(48, 23); this.cmdRun.TabIndex = 2; this.cmdRun.Text = "Run"; this.cmdRun.Click += new System.EventHandler(this.cmdRun_Click); // // txtReachable // this.txtReachable.Location = new System.Drawing.Point(144, 40); this.txtReachable.Name = "txtReachable"; this.txtReachable.Size = new System.Drawing.Size(56, 20); this.txtReachable.TabIndex = 1; this.txtReachable.Text = ""; // // txtCountTo // this.txtCountTo.Location = new System.Drawing.Point(144, 16); this.txtCountTo.Name = "txtCountTo"; this.txtCountTo.Size = new System.Drawing.Size(56, 20); this.txtCountTo.TabIndex = 0; this.txtCountTo.Text = ""; // // label1 // this.label1.AutoSize = true; this.label1.Location = new System.Drawing.Point(16, 16); this.label1.Name = "label1"; this.label1.Size = new System.Drawing.Size(51, 13); this.label1.TabIndex = 3; this.label1.Text = "Count To"; // // label2 // this.label2.AutoSize = true; this.label2.Location = new System.Drawing.Point(16, 40); this.label2.Name = "label2"; this.label2.Size = new System.Drawing.Size(99, 13); this.label2.TabIndex = 4; this.label2.Text = "Reach this number"; // // btnRemoveDelegate // this.btnRemoveDelegate.Location = new System.Drawing.Point(16, 104); this.btnRemoveDelegate.Name = "btnRemoveDelegate"; this.btnRemoveDelegate.Size = new System.Drawing.Size(168, 23); this.btnRemoveDelegate.TabIndex = 5; this.btnRemoveDelegate.Text = "Remove second handler"; this.btnRemoveDelegate.Click += new System.EventHandler(this.btnRemoveDelegate_Click); // // Form1 // this.AutoScaleBaseSize = new System.Drawing.Size(5, 13); this.ClientSize = new System.Drawing.Size(224, 134); this.Controls.AddRange(new System.Windows.Forms.Control[] { this.btnRemoveDelegate, this.label2, this.label1, this.txtCountTo, this.txtReachable, this.cmdRun}); this.Name = "Form1"; this.Text = "Events"; this.ResumeLayout(false); } #endregion /// <summary> /// The main entry point for the application. /// </summary> [STAThread] static void Main() { Application.Run(new Form1()); } private void btnRun_Click(object sender, System.EventArgs e) { if(txtCountTo.Text == "" || txtReachable.Text=="") return; oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text)); } private void oCounter_NumberReached(object sender, NumberReachedEventArgs e) { MessageBox.Show("Reached: " + e.ReachedNumber.ToString()); } private void oCounter_NumberReached2(object sender, NumberReachedEventArgs e) { MessageBox.Show("Reached2: " + e.ReachedNumber.ToString()); } private void btnRemoveDelegate_Click(object sender, System.EventArgs e) { oCounter.NumberReached -= new NumberReachedEventHandler(oCounter_NumberReached2); oCounter.CountTo(Convert.ToInt32(txtCountTo.Text), Convert.ToInt32(txtReachable.Text)); } } }
using System; namespace Events { public delegate void NumberReachedEventHandler(object sender, NumberReachedEventArgs e);
/// <summary> /// Summary description for Counter. /// </summary> public class Counter { public event NumberReachedEventHandler NumberReached; public Counter() { // // TODO: Add constructor logic here // } public void CountTo(int countTo, int reachableNum) { if(countTo < reachableNum) throw new ArgumentException("reachableNum should be less than countTo"); for(int ctr=0;ctr<=countTo;ctr++) { if(ctr == reachableNum) { NumberReachedEventArgs e = new NumberReachedEventArgs(reachableNum); OnNumberReached(e); return;//don't count any more } } } protected virtual void OnNumberReached(NumberReachedEventArgs e) { if(NumberReached!=null) { NumberReached(this, e); } } } public class NumberReachedEventArgs : EventArgs { private int _reached; public NumberReachedEventArgs(int num) { this._reached = num; } public int ReachedNumber { get { return _reached; } } } }
举个例子: 假如有四种动物，分别是老虎，小鹿，老鼠，蛇。现在小鹿逃跑了，这就是一个”事件“，我们称之为“逃跑事件”，对这个事件的处理当然是“追”个事件处理方法，但是有谁去执行”追“这个事件处理呢，当然是老虎，为什么是老虎不是其他动物呢，这是因为自然法则,那么自然法则就是一个"委托"。自然法则并没有把它委托给其他三种动物，同样老鼠跑是要蛇去追的，加入没有这个自然法则，上述行为是不会产生的，狮子要是在场的话，老虎吃饱了不饿，也可以委托给狮子，让狮子去追。
不知道这个例子恰不恰当，欢迎交流:binbin_wu@msn.com
--------------------------------------------------------
这个例子，，，更容易理解~~~
逃跑事件：OnNumberReached:Counter
自然法则：NumberReachedEventHandle
（老虎/狮子）追的具体事件处理：oCounter_NumberReached ，oCounter_NumberReached2 <---（可以蹦着追，也可以大喊，再跑开枪了，具体看怎么实现）
因此：
自然法则（说）：（如果）逃跑事件发生，那么我拜托 oCounter_NumberReached;oCounter_NumberReached2（去追）
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached);
oCounter.NumberReached += new NumberReachedEventHandler(oCounter_NumberReached2);
OnNumberReached <---逃跑
NumberReached（this,e） <---老虎/狮子（准备了，准备追！！）
注明：本文章转自http://www.cnblogs.com/finesite/articles/255884.html