Hooking into Forms redrawing

Question

I'm looking for a way to overlay the graphical output of a third-party application with some lines, arcs etc. The applications accepts a handle of a window in which it will then display its output.

Using VC++ I put together a Windows Forms app in Visual Studio that draws (non-static) stuff in the onPaint-method of a form. Passing this form's handle to the other app, of course, overwrites my graphics stuff every time the other app redraws.

Can I somehow hook into this redrawing process to add my graphics after the other app redraws? Overlaying the form with a transparent panel onto which I draw could be an alternative. But real transparency for controls seem to be a problem of its own in Windows ...

Answer 1

You can't do this easily without getting notifications from the app. Which, if it doesn't provide them, would require setting a global hook with SetWindowsHookEx() so you can see the WM_ERASEBKGND and WM_PAINT messages. That's hard to get right, you cannot write such a hook in managed code. Since it requires injecting a DLL into the target process.

The only other option is that you put a transparent overlay on top of your form. Another form that has its TransparencyKey property set. The basic code you need to get that right is available in my answer in this thread. You just need to tweak it so it is permanent.