有兩個picturebox控制元件，name預設沒有修改。其中picturebox1的img賦予初始值，picturebox2的visible=false；其餘不變，程式比較簡單，看以參考下。

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.Threading;  
9. using System.Threading.Tasks;  
10. using System.Windows.Forms;  
11. namespace 區域性放大  
12. {  
13.     public partial class Form1 : Form  
14.     {  
15.         Thread thDraw;  
16.         delegatevoid myDrawRectangel();  
17.         myDrawRectangel mydraw;  
18.         private Point ptBegin = new Point();  
19.         private
    bool blIsDrawRectangle = false;  
20.         public Form1()  
21.         {  
22.             InitializeComponent();  
23.         }  
24.         privatevoid pictureBox1_Paint(object sender, PaintEventArgs e)  
25.         {  
26.             //畫矩形 初始值為false否則 一開始就會顯示矩形框
27.             if (blIsDrawRectangle)  
28.             {  
29.                 e.Graphics.DrawRectangle(new
     Pen(Brushes.Black, 1), ptBegin.X, ptBegin.Y, 50, 50);  
30.             }  
31.         }  
32.         privatevoid Form1_Load(object sender, EventArgs e)  
33.         {  
34.             mydraw = new myDrawRectangel(ShowDrawRectangle);  
35.             thDraw = new Thread(Run);  
36.             thDraw.Start();  
37.         }  
38.         privatevoid Run()  
39.         {  
40.             while (true)  
41.             {  
42.                 if (pictureBox1.Image != null)  
43.                 {  
44.                     this.BeginInvoke(mydraw);  
45.                 }  
46.                 Thread.Sleep(50);  
47.             }  
48.         }  
49.         privatevoid Form1_FormClosed(object sender, FormClosedEventArgs e)  
50.         {  
51.             if (thDraw != null)  
52.             {  
53.                 thDraw.Abort();  
54.             }  
55.         }  
56.         privatevoid ShowDrawRectangle()  
57.         {  
58.             Rectangle rec = new Rectangle(ptBegin.X * pictureBox1.Image.Size.Width / 460, ptBegin.Y * pictureBox1.Image.Size.Height / 350,  
59.                                            50 * pictureBox1.Image.Size.Width / 460, 50 * pictureBox1.Image.Size.Height / 350);  
60.             Graphics g = pictureBox2.CreateGraphics();  
61.             g.DrawImage(pictureBox1.Image, pictureBox2.ClientRectangle, rec, GraphicsUnit.Pixel);  
62.             g.Flush();  
63.         }  
64.         privatevoid pictureBox1_MouseLeave(object sender, EventArgs e)  
65.         {  
66.             pictureBox2.Visible = false;  
67.             blIsDrawRectangle = false;  
68.             pictureBox1.Refresh();  
69.         }  
70.         privatevoid pictureBox1_MouseEnter(object sender, EventArgs e)  
71.         {  
72.             pictureBox2.Visible = true;  
73.             blIsDrawRectangle = true;  
74.         }  
75.         privatevoid pictureBox1_MouseMove(object sender, MouseEventArgs e)  
76.         {  
77.             if (e.X - 25 <= 0)  
78.             {  
79.                 ptBegin.X = 0;  
80.             }  
81.             elseif (pictureBox1.Size.Width - e.X <= 25)  
82.             {  
83.                 ptBegin.X = pictureBox1.Size.Width - 50;  
84.             }  
85.             else
86.             {  
87.                 ptBegin.X = e.X - 25;  
88.             }  
89.             if (e.Y - 25 <= 0)  
90.             {  
91.                 ptBegin.Y = 0;  
92.             }  
93.             elseif (pictureBox1.Size.Height - e.Y <= 25)  
94.             {  
95.                 ptBegin.Y = pictureBox1.Size.Height - 50;  
96.             }  
97.             else
98.             {  
99.                 ptBegin.Y = e.Y - 25;  
100.             }  
101.             pictureBox1.Refresh();  
102.         }  
103.     }  
104. }