c#裁剪图片(根据鼠标画的矩形裁剪图片)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using S10806Class;
using System.IO;

namespace GetImage
{
public partial class GetScreenImgForm : Form
{

/// <summary>
/// 图像操作类
/// </summary>
ImageWork imageWork = new ImageWork();

/// <summary>
/// 保存截取到的桌面图像
/// </summary>
Image screenImg = null;

/// <summary>
/// 保存用户截取的图像区域
/// </summary>
Rectangle imgRect = new Rectangle();

/// <summary>
/// 窗体的GDI
/// </summary>
Graphics g = null;

/// <summary>
/// 区域边框画笔
/// </su