1 using System;  2 using System.Collections.Generic;  3 using System.ComponentModel;  4 using System.Data;  5 using System.Drawing;  6 using System.Linq;  7 using System.Text;  8 using System.Windows.Forms;  9 using System.Threading; 10  11 namespace test 12 { 13     public partial class Form2 : Form 14     { 15         private bool blIsDrawRectangle = true; 16         private Point ptBegin = new Point(); 17         Thread thDraw; 18         delegate void myDrawRectangel(); 19         myDrawRectangel myDraw; 20         public Form2() 21         { 22             InitializeComponent(); 23         } 24  25         private void pictureBox1_Paint(object sender, PaintEventArgs e) 26         { 27             if (blIsDrawRectangle) 28             { 29                 e.Graphics.DrawRectangle(new Pen(Brushes.Black, 1), ptBegin.X, ptBegin.Y, 50, 50); 30             } 31         } 32  33         private void Form2_Load(object sender, EventArgs e) 34         { 35             myDraw = new myDrawRectangel(ShowDrawRectangle); 36             thDraw = new Thread(Run); 37             thDraw.Start(); 38         } 39         private void Run() 40         { 41             while (true) 42             { 43                 if (pictureBox1.Image != null) 44                 { 45                     this.BeginInvoke(myDraw); 46                 } 47                 Thread.Sleep(50); 48             } 49         } 50         private void ShowDrawRectangle() 51         { 52             Rectangle rec = new Rectangle(ptBegin.X * pictureBox1.Image.Size.Width / 460, 53                 ptBegin.Y * pictureBox1.Image.Size.Height / 350, 50 * pictureBox1.Image.Size.Width / 460, 54                 50 * pictureBox1.Image.Size.Height / 350); 55             Graphics g = pictureBox2.CreateGraphics(); 56             g.DrawImage(pictureBox1.Image, pictureBox2.ClientRectangle, rec, GraphicsUnit.Pixel); 57             g.Flush(); 58         } 59  60         private void pictureBox1_MouseLeave(object sender, EventArgs e) 61         { 62             blIsDrawRectangle = false; 63             pictureBox1.Refresh(); 64         } 65  66         private void pictureBox1_MouseEnter(object sender, EventArgs e) 67         { 68             blIsDrawRectangle = true; 69         } 70  71         private void pictureBox1_MouseMove(object sender, MouseEventArgs e) 72         { 73             if (e.X - 25 <= 0) 74             { 75                 ptBegin.X = 0; 76             } 77             else if (pictureBox1.Size.Width - e.X <= 25) 78             { 79                 ptBegin.X = pictureBox1.Size.Width - 50; 80             } 81             else 82             { 83                 ptBegin.X = e.X - 25; 84             } 85  86             if (e.Y - 25 <= 0) 87             { 88                 ptBegin.Y = 0; 89             } 90             else if (pictureBox1.Size.Height-e.Y<=25) 91             { 92                 ptBegin.Y = pictureBox1.Size.Height - 50; 93             } 94             else 95             { 96                 ptBegin.Y = e.Y - 25; 97             } 98             pictureBox1.Refresh(); 99         }100     }101 }