在图片框上画一个矩形

当这个窗体打开时，它有一个全屏的背景图像，用户可以用鼠标向下拖动并绘制一个矩形，稍后将用于裁剪背景图像。我遇到的问题是，当表单绘制时，它没有绘制矩形，所以用户不知道他们选择的区域

这是我的代码：

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;
namespace Quick_Screenshot
{
    public partial class Crop : Form
    {
        private Rectangle croprect = Rectangle.Empty;
        private bool mouseDown = false;
        private bool selectedArea = false;
        Point sp, ep;
        Bitmap background;
        public Rectangle CropArea
        {
            get { return croprect; }
        }
        public bool SelectedArea
        {
            get { return selectedArea; }
        }
        public Crop(Bitmap image)
        {
            InitializeComponent();
            background = image;
            picBox.MouseDown += new MouseEventHandler(Crop_MouseDown);
            picBox.MouseUp += new MouseEventHandler(Crop_MouseUp);
            picBox.MouseMove += new MouseEventHandler(Crop_MouseMove);
            picBox.Paint += new PaintEventHandler(Crop_Paint);
        }
        private void Crop_Load(object sender, EventArgs e)
        {
            picBox.Image = (Image)background;
        }
        private void Crop_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == System.Windows.Forms.MouseButtons.Right)
                this.Close();
            else
            {
                mouseDown = true;
                sp = ep = e.Location;
            }
        }
        private void Crop_MouseUp(object sender, MouseEventArgs e)
        {
            ep = e.Location;
            mouseDown = false;
            croprect = GetRectangle(sp, ep);
            if (croprect.Width > 10 && croprect.Height > 10)
            {
                selectedArea = true;
            }
            else
            {
                croprect = Rectangle.Empty;
                Taskbar.Balloon("Selected area too small", "Quick Screenshot", ToolTipIcon.Error);
            }
            this.Close();
        }
        private void Crop_MouseMove(object sender, MouseEventArgs e)
        {
            if (mouseDown)
            {
                ep = e.Location;
                Update();
            }
        }
        private void Crop_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.DrawRectangle(Pens.Red, GetRectangle(sp, ep));
            e.Graphics.Save();
        }
        private Rectangle GetRectangle(Point p1, Point p2)
        {
            return new Rectangle(
                Math.Min(p1.X, p2.X),
                Math.Min(p1.Y, p2.Y),
                Math.Abs(p1.X - p2.X),
                Math.Abs(p1.Y - p2.Y)
                );
        }
    }
}

private void Crop_MouseMove(object sender, MouseEventArgs e)
{
    if (mouseDown)
    {
        ep = e.Location;
        Update();
    }
}

private void Crop_MouseMove(object sender, MouseEventArgs e)
{
    if (mouseDown)
    {
        ep = e.Location;
        picBox.Invalidate();
    }
}