tommy.blog

Very cool. I've long been trying to find a way to render a Form instance to an Image without showing the form on screen. Something like:

using (Form f = new MyForm()) {
Image i = new Image(f.Width, f.Height);
Graphics g = Graphics.FromImage(i);
ControlPainter.PaintControl(g, f);
}

However, I've never been able to get this to work and I don't know why. If you have any thoughts I'd love to hear them.

Cheers,
Kent

I just tried it, and it does work for simple controls like Buttons, but apparently not for Forms. It only draws the client rectangle (not the borders and titlebar), and it doesn't draw any of the child controls.
I don't really know how to solve this (yet).

I just looked at this again and it turns out it was pretty easy to fix:

using System;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Windows.Forms;

public class ControlPainter
{
private const int WM_PRINT = 0x317, PRF_CLIENT = 4, PRF_CHILDREN = 0x10, PRF_NON_CLIENT = 2;

[DllImport("USER32.DLL")]
private static extern int SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, int lParam);

public static void PaintControl(Graphics graphics, Control control)
{ // paint control onto graphics
IntPtr hWnd = control.Handle;
IntPtr hDC = graphics.GetHdc();
SendMessage(hWnd, WM_PRINT, hDC, PRF_CLIENT | PRF_CHILDREN | PRF_NON_CLIENT);
graphics.ReleaseHdc(hDC);
}
}

The only problem is you still have to show the form for it to work (otherwise the non-client area has nothing in it). I got around this by showing the form off-screen:

using (Form f = new ToCapture())
{
f.StartPosition = FormStartPosition.Manual;
f.Location = new Point(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);
f.Show();
Image i = new Bitmap(f.Width, f.Height);

using (Graphics g = Graphics.FromImage(i))
{
ControlPainter.PaintControl(g, f);
}

this.pictureBox1.Image = i;
}

That works and it's probably good enough for my needs. I'm still trying to understand why you need to show the form for it to work.

I can't thank you enough for pointing me in the right direction . . . :)

Sorry, I meant the client area has nothing in it if the form isn't visible.

http://kentb.blogspot.com/2005/03/nice-form.html

That's cool, I'll change my post as soon as possible.

Here's also a nice article: http://www.codeproject.com/useritems/Screen_Capturing.asp.

Thanks for the link. Was wondering why I hadn't seen it before then I realised it was posted a couple of days ago ;)

That PrintWindow API function looks useful, even if the MSDN doco is a tad confusing. Does it work on 2000 or not? Will have to try later . . .

I think it doesn't: the Platform SDK documentation says: Included in Windows XP and Windows Server 2003

As of .Net 2.0, you can do this with Control.DrawToBitmap:

Bitmap bitmap = new Bitmap(control.Width,control.Height);

control.DrawToBitmap(bitmap, new Rectangle(0,0,control.Width,control.Height));

Thanks a lot (I was looking for clue and I've found it here).

A.Keylin

Wicked, thank you.

That Control.DrawToBitmap is exactly was i was looking for, is awsome.

Thank you very much...

What about painting an ActiveX control hosted insude a .Net application? None of the methods described work in case of an ActiveX.

I tried to use this method to paint a control every 50 milliseconds, and the memory is raising up quickly, an OOM exception thrown within one minute.

My code is:

System.Drawing.Image image = new Bitmap(_control.Width, _control.Height);

using (Graphics graphics = Graphics.FromImage(image))
{
ControlPainter.PaintControl(graphics, _control);
}

image.Dispose();

But if I use BitBlt instead of SendMessage, there is no such a memory leak issue. Any ideas?

"What about painting an ActiveX control hosted insude a .Net application?"
----
I would love to know an anwer to this question as well.

Thanks