C#中委托，事件理解入门-.NET教程,C#语言-IDC资讯中心

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;//dont 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;//dont 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;

        public form1()
        {
            //
            // required for windows form designer support
            //
            initializecomponent();

}
#endregion

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;

        public form1()
        {
            //
            // required for windows form designer support
            //
            initializecomponent();

}
#endregion

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;//dont 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);

        protected virtual void onnumberreached(numberreachedeventargs e)
        {
            if(numberreached!=null)
            {
                numberreached(this, e);
            }
        }
    }

using system;

namespace events
{
public delegate void numberreachedeventhandler(object sender, numberreachedeventargs e);

        protected virtual void onnumberreached(numberreachedeventargs e)
        {
            if(numberreached!=null)
            {
                numberreached(this, e);
            }
        }
    }

C#中委托，事件理解入门-.NET教程,C#语言

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