C# Winform中WndProc函数作用

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace ControlTest
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private Demo demo = null;
        private void Form1_Load(object sender, EventArgs e)
        {
            //demo = new Demo(this.Handle.ToInt32());
        }

        private void button1_Click(object sender, EventArgs e)
        {
            demo = new Demo(this.Handle.ToInt32());
            demo.Test();
        }

        protected override void WndProc(ref Message m)
        {
            if (m.Msg == Demo.MY_MSG_BEGIN)
            {
                MessageBox.Show("类Demo for循环开始.");
            }
            else if (m.Msg == Demo.MY_MSG_END)
            {
                MessageBox.Show("类Demo for循环结束.");
            }

            base.WndProc(ref m);
        }
    }

    public class Demo
    {
        private int m_hWnd = 0;

        public Demo(int hWnd)
        {
            m_hWnd = hWnd;
        }

        private const int WM_USER = 0x0400;
        public static int MY_MSG_BEGIN = WM_USER + 100;
        public static int MY_MSG_END = WM_USER + 101;


        [DllImport("User32.DLL")]
        public static extern int SendMessage(int hWnd, int Msg, int wParam, int lParam);

        public void Test()
        {
            SendMessage(m_hWnd, MY_MSG_BEGIN, 0, 0);

            for (int i = 0; i < 100000; i++)
            {
                Application.DoEvents();
            }
            SendMessage(m_hWnd, MY_MSG_END, 0, 0);
        }
    }
   
}