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    C#中委托,事件理解入门_.NET教程_编程技术

    时间:2019-09-11|栏目:ASP.NET|点击:

  • 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;
                }
            }
        }
    }



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