C＃-picturebox，graphics.DrawImage和picturebox.Image = null，为什么？

Question

I'm trying to crop and resize an image in PictureBox1. My code:

//original image for eventually undo
undoImage = pictureBox1.BackgroundImage.Clone() as Image;
Bitmap sourceBitmap = new Bitmap(pictureBox1.BackgroundImage, pictureBox1.Width, pictureBox1.Height);
Graphics g = pictureBox2.CreateGraphics();
g.DrawImage(sourceBitmap, new Rectangle(0, 0, pictureBox2.Width, pictureBox2.Height), rectCropArea, GraphicsUnit.Pixel);
sourceBitmap.Dispose();

And it working properly on two PictureBox es. But PictureBox2.Image , PictureBox2.BackgroundImage (and any other including ErrorImage...) = null . I tried PictureBox.DrawToBitmap , and other, like g.GetHdc() found on google, but unsuccessful.

My question:
How do I properly copy the edited image from PictureBox2 to PictureBox1 ?

Answer 1

Trivial solution:

undoImage = pictureBox1.BackgroundImage.Clone() as Image;
Bitmap sourceBitmap = new Bitmap(pictureBox1.BackgroundImage, pictureBox1.Width, pictureBox1.Height);
using (Graphics g = Graphics.FromImage(sourceBitmap))
{
      g.DrawImage(sourceBitmap, new Rectangle(0, 0, pictureBox2.Width,pictureBox2.Height), rectCropArea, GraphicsUnit.Pixel);
}
pictureBox1.BackgroundImage = sourceBitmap;