Paint event handler stops executing after a few iterations

Question

Hi,

I've got a Windows Form that circulates through images which are displayed on the form as a slideshow. The way I'm doing this is to have a Panel control the size of the form it resides in, and add an event handler that draws an Image object that exists in memory.

void panel_Paint(object sender, PaintEventArgs e)
{
  if (_bShowImage)
  {
    Point leftCorner = new Point((this.Bounds.Width / 2) - (_image.Width / 2), (this.Bounds.Height / 2) - (_image.Height / 2));

    e.Graphics.DrawImage(_image, leftCorner);

    _bShowImage = false;
  }
}

When a new Image is loaded and referenced by _image, I'm forcing the Panel to redraw:

_bShowImage = true;
_panel.Refresh();

Immediately afterwards, the image is disposed and the dereferenced from the global variable:

_image.Dispose();
_image = null;

I've seen that it works for a while, say 5 iterations, then the panel_Paint() handler is not being called. I'm using 2-3 JPG's for the display and I know they're not corrupted as they are shown fine for the first x times. I've put debug lines around the Refresh() method of the panel which execute fine. It's as if the call to the handler has been dropped. Has anyone encountered this problem before?

Answer 1

Hello!

Wouldn't it be smarter to put your pictures in a picture box and loop through them in that way, so that you don't force a repaint on the whole window each time?

just a thought...

Tony

Answer 2

This is so completely backwards. Either you use a paint event handler like now. It's just fine (I say it's better than a picturebox) but then you need to drop that _bShowImage and _image.Dispose stuff. You should instead dispose the _image before you power it up with a new one. But not until that.

Or, if you absolutley must dispose the _image right after it's painted, then you should instead use Panel.CreateGraphics to get a Graphichs object you can use to immediately draw the _image and drop the event.

As it stands - it is just darn confusing. Also: .Invalidate() is what you almost always want -not .Refresh(). That's just something that got stuck in many minds since the VB6 era.