c#位图到picturebox中

`

    public Bitmap catchFullScreen()
    { Bitmap r = new Bitmap(SystemInformation.VirtualScreen.Width ,SystemInformation.VirtualScreen.Height);
        Rectangle bounds = new Rectangle (0,0,SystemInformation.VirtualScreen.Width ,SystemInformation.VirtualScreen.Height);
        using (Bitmap bitmap = new Bitmap(SystemInformation.VirtualScreen.Width ,SystemInformation.VirtualScreen.Height))
        {
            using (Graphics g = Graphics.FromImage(bitmap))
            {
                g.CopyFromScreen(Point.Empty, Point.Empty, bounds.Size);
                r = bitmap;
                pictureBox1.Image = r;
                pictureBox1.Update();
                pictureBox1.Refresh();
            }
            pictureBox2.Image = r; // breakpoint 1 
            pictureBox2.Update();  // breakpoint 2
            pictureBox2.Refresh();
        }
        pictureBox3.Image = r;
        pictureBox3.Update();
        pictureBox3.Refresh();
        return r;
    }

`这是我的捕获截图，但发生了一些奇怪的事情，picturebox1和2可以捕获，但picturebox3不能。此外，断点1有效，但断点2从未到达，

为什么我不能在使用例程之外使用此位图？？更重要的是它不会返回r？请提出建议！