this.MouseDown += new System.Windows.Forms.MouseEventHandler(this.Form1_MouseDown1);
this.MouseDown += new System.Windows.Forms.MouseEventHandler(this.Form1_MouseDown2); private void Form1_MouseDown1(object sender, System.Windows.Forms.MouseEventArgs e)
{ if(e.Button==System.Windows.Forms.MouseButtons.Left) System.Windows.Forms.MessageBox.Show("消息被Form1_MouseDown1函数响应");
} private void Form1_MouseDown2(object sender, System.Windows.Forms.MouseEventArgs e)
{ if(e.Button==System.Windows.Forms.MouseButtons.Left) System.Windows.Forms.MessageBox.Show("消息被Form1_MouseDown2函数响应");
} //上面this.MouseDown是C#中的一个事件。它的定义public event MouseEventHandler MouseDown;
public delegate void MouseEventHandler( object sender,MouseEventArgs e); //MouseEventHandler的定义

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;namespace MessageMech3
{public class CLButtonDownFilter : IMessageFilter{//实现消息过滤器接口public bool PreFilterMessage(ref Message m){if (m.Msg == 0x0201)// WM_LBUTTONDOWN
            {System.Windows.Forms.MessageBox.Show("App中鼠标左键按下");//返回值为true， 表示消息已被处理，不要再往后传递，因此消息被截获//返回值为false，表示消息未被处理，需要再往后传递，因此消息未被截获return true;}return false;}}public class WinForm : System.Windows.Forms.Form{private System.Windows.Forms.Label label1;private System.ComponentModel.Container components = null;public WinForm(){InitializeComponent();//安装自己的过滤器
 CLButtonDownFilter MyFilter = new CLButtonDownFilter();System.Windows.Forms.Application.AddMessageFilter(MyFilter);}protected override void Dispose(bool disposing){if (disposing){if (components != null){components.Dispose();}}base.Dispose(disposing);}#region Windows Form Designer generated codeprivate void InitializeComponent(){this.label1 = new System.Windows.Forms.Label();this.SuspendLayout();this.label1.BackColor = System.Drawing.Color.Transparent;this.label1.Dock = System.Windows.Forms.DockStyle.Top;this.label1.ForeColor = System.Drawing.Color.DarkViolet;this.label1.Name = "label1";this.label1.Size = new System.Drawing.Size(440, 32);this.label1.TabIndex = 0;this.label1.Text = "演示如何在App对象中处理消息,请点鼠标左键";this.label1.TextAlign = System.Drawing.ContentAlignment.BottomCenter;this.AutoScaleBaseSize = new System.Drawing.Size(7, 22);this.BackColor = System.Drawing.Color.WhiteSmoke;this.ClientSize = new System.Drawing.Size(440, 273);this.Controls.AddRange(new System.Windows.Forms.Control[] { this.label1 });this.Font = new System.Drawing.Font("华文行楷", 15F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((System.Byte)(134)));this.Name = "WinForm";this.Text = "WinForm";//消息响应函数的调用顺序和添加委托的顺序一致,即：以下命令将先调用Form1_MouseDown1再调用Form1_MouseDown2,
        通过委托添加自己的鼠标按键消息响应函数1this.MouseDown += new System.Windows.Forms.MouseEventHandler(this.Form1_MouseDown1);//通过委托添加自己的鼠标按键消息响应函数2this.MouseDown += new System.Windows.Forms.MouseEventHandler(this.Form1_MouseDown2);this.ResumeLayout(false);}#endregion//应用程序的主入口点
[STAThread]static void Main(){Application.Run(new WinForm()); //启动当前Form线程上的应用程序消息循环
         }// 通过C#提供的事件接口添加自己的鼠标按键事件的响应函数private void Form1_MouseDown1(object sender, System.Windows.Forms.MouseEventArgs e){if (e.Button == System.Windows.Forms.MouseButtons.Left)System.Windows.Forms.MessageBox.Show("消息被Form1_MouseDown1函数响应");}private void Form1_MouseDown2(object sender, System.Windows.Forms.MouseEventArgs e){if (e.Button == System.Windows.Forms.MouseButtons.Left)System.Windows.Forms.MessageBox.Show("消息被Form1_MouseDown2函数响应");}//通过覆盖基类的事件引发函数拦截消息protected override void OnMouseDown(MouseEventArgs e){if (e.Button == System.Windows.Forms.MouseButtons.Left)System.Windows.Forms.MessageBox.Show("消息被OnMouseDown函数响应");//如果需要截获消息，可将base.OnMouseDown(e);语句注释掉base.OnMouseDown(e);}//通过覆盖基类的窗体函数拦截消息protected override void WndProc(ref System.Windows.Forms.Message e){//如果需要截获消息//if (e.Msg == 0x0201)// WM_LBUTTONDOWN//    System.Windows.Forms.MessageBox.Show("消息被WndProc函数响应");//else//    base.WndProc(ref e);if (e.Msg == 0x0201)System.Windows.Forms.MessageBox.Show("消息被WndProc函数响应");// WM_LBUTTONDOWN, 不需要截获消息则为base.WndProc(ref e);}}
}