目录
l 导论
l 什么是委托
l 事件的理解
l 事件 关键字
l 最后
导论
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：
form1.cs
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 );
}
windows form designer generated code#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 );
}
windows form designer generated code#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 );
}
windows form designer generated code#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));
}
}
}
counter.cs
counter.cs
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;
}
}
}
}
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;
}
}
}
}
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;
}
}
}
}
