C#委托,事件理解

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