c# winfrom 放大镜功能

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.Threading;

namespace test
{
    public partial class Form2 : Form
    {
        private bool blIsDrawRectangle = true;
        private Point ptBegin = new Point();
        Thread thDraw;
        delegate void myDrawRectangel();
        myDrawRectangel myDraw;
        public Form2()
        {
            InitializeComponent();
        }

        private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            if (blIsDrawRectangle)
            {
                e.Graphics.DrawRectangle(new Pen(Brushes.Black, 1), ptBegin.X, ptBegin.Y, 50, 50);
            }
        }

        private void Form2_Load(object sender, EventArgs e)
        {
            myDraw = new myDrawRectangel(ShowDrawRectangle);
            thDraw = new Thread(Run);
            thDraw.Start();
        }
        private void Run()
        {
            while (true)
            {
                if (pictureBox1.Image != null)
                {
                    this.BeginInvoke(myDraw);
                }
                Thread.Sleep(50);
            }
        }
        private void ShowDrawRectangle()
        {
            Rectangle rec = new Rectangle(ptBegin.X * pictureBox1.Image.Size.Width / 460,
                ptBegin.Y * pictureBox1.Image.Size.Height / 350, 50 * pictureBox1.Image.Size.Width / 460,
                50 * pictureBox1.Image.Size.Height / 350);
            Graphics g = pictureBox2.CreateGraphics();
            g.DrawImage(pictureBox1.Image, pictureBox2.ClientRectangle, rec, GraphicsUnit.Pixel);
            g.Flush();
        }

        private void pictureBox1_MouseLeave(object sender, EventArgs e)
        {
            blIsDrawRectangle = false;
            pictureBox1.Refresh();
        }

        private void pictureBox1_MouseEnter(object sender, EventArgs e)
        {
            blIsDrawRectangle = true;
        }

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.X - 25 <= 0)
            {
                ptBegin.X = 0;
            }
            else if (pictureBox1.Size.Width - e.X <= 25)
            {
                ptBegin.X = pictureBox1.Size.Width - 50;
            }
            else
            {
                ptBegin.X = e.X - 25;
            }

            if (e.Y - 25 <= 0)
            {
                ptBegin.Y = 0;
            }
            else if (pictureBox1.Size.Height-e.Y<=25)
            {
                ptBegin.Y = pictureBox1.Size.Height - 50;
            }
            else
            {
                ptBegin.Y = e.Y - 25;
            }
            pictureBox1.Refresh();
        }
    }
}