屏幕截图的一个问题

Question

    小弟做了一个小小的屏幕截图程序，程序基本运行正常。其基本思路是：在按下notifyIcon的弹出菜单的截图后，截取当前屏幕，并显示在Form中的PictureBox中，拖动鼠标选择截图区域，然后保存，然后隐藏窗体。
    但是在拖动鼠标，显示选择区域时（想实时更新选择框），矩形框有时只有在松开鼠标后才显示，有时又可正常显示，不知问题所在。
    源码如下，求高手相助！
 
public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }        bool flag = false;
        Point firstpoint;
        Point secondpoint;
        Image cachImage = null;        //加载窗体时
        private void Form1_Load(object sender, EventArgs e)
        {
            this.Visible = false;
            this.WindowState = FormWindowState.Minimized;
        }        //按下鼠标时
        void pictureBox1_MouseUp(object sender, System.Windows.Forms.MouseEventArgs e)
        {
            if (flag)
            {
                flag = false;                int x1 = Math.Min(firstpoint.X, secondpoint.X);
                int x2 = Math.Max(firstpoint.X, secondpoint.X);
                int y1 = Math.Min(firstpoint.Y, secondpoint.Y);
                int y2 = Math.Max(firstpoint.Y, secondpoint.Y);                if (x1 != x2 && y1 != y2)
                {
                    //方法一，有红边
                    //Image img = new Bitmap(x2 - x1, y2 - y1);
                    //Graphics g = Graphics.FromImage(img);
                    //g.CopyFromScreen(x1, y1, 0, 0, new Size(x2 - x1, y2 - y1));
                    //img.Save("C:\\test.jpg");                    //方法二，这个方法没有红边
                    Image img = new Bitmap(x2 - x1, y2 - y1);
                    Graphics g = Graphics.FromImage(img);
                    g.DrawImage(cachImage, 0, 0, new Rectangle(x1, y1, x2 - x1, y2 - y1), GraphicsUnit.Pixel);
                    img.Save("C:\\test.jpg");                    //存到剪贴板, 将 System.Windows.Forms.DataFormats.Bitmap 格式的 System.Drawing.Image 添加到剪贴板中
                    Clipboard.SetImage(img);                    MessageBox.Show("保存在C:\\test.jpg\r\n" + "宽：" + (x2 - x1).ToString() + "\r\n" + "高：" + (y2 - y1).ToString(), "截图成功", MessageBoxButtons.OK, MessageBoxIcon.Information);                    this.Visible = false;
                    this.WindowState = FormWindowState.Minimized;   
                }
            }    
        }        //拖动鼠标时
        void pictureBox1_MouseMove(object sender, System.Windows.Forms.MouseEventArgs e)
        {
            if (flag)
            {
                secondpoint  = new Point(e.X, e.Y);                int x1 = Math.Min(firstpoint.X, secondpoint.X);
                int x2 = Math.Max(firstpoint.X, secondpoint.X);
                int y1 = Math.Min(firstpoint.Y, secondpoint.Y);
                int y2 = Math.Max(firstpoint.Y, secondpoint.Y);                Image img = new Bitmap(cachImage);
                Graphics g = Graphics.FromImage(img);
                //g.Clear(Color.Beige);
                g.DrawRectangle(Pens.Red, new Rectangle(x1, y1, x2 - x1, y2 - y1));
                pictureBox1.BackgroundImage = img;
            } 
        }        //松开鼠标时
        void pictureBox1_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
        {
            flag = true;
            firstpoint= new Point(e.X, e.Y);
        }        //退出
        private void 退出ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.Close();
        }        //截图
        private void 截图ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            //Rectangle r = Screen.PrimaryScreen.Bounds;            //只截取工作区域
            Rectangle r = Screen.PrimaryScreen.WorkingArea;
            Image img = new Bitmap(r.Width, r.Height);
            Graphics g = Graphics.FromImage(img);
            g.CopyFromScreen(new Point(0, 0), new Point(0, 0), r.Size);            this.Visible = true;
            this.WindowState = FormWindowState.Normal;
           
            //窗体最大化，及相关处理
            this.Width = r.Width;
            this.Height = r.Height;
            this.Left = 0;
            this.Top = 0;            pictureBox1.Width = r.Width;
            pictureBox1.Height = r.Height;
            pictureBox1.BackgroundImage = img;            //将当前截得的图保存到缓存
            cachImage = img;        }        private void 隐藏ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.Visible = false;
            this.WindowState = FormWindowState.Minimized;      
        }       
    }