Is there a way to capture a bitmap from a WPF window using native C++?

Question

Imagine a document window in a MDI application which contains a child WPF window, say a sidebar for example. How can one get a bitmap containing both the WPF pixels AND the GDI (non-wpf) pixels?

I've discovered that when making my thumbnail preview for the Win7 taskbar app icon hover, I get black in the parts of the preview where the WPF pixels should be. My current method simply grabs a bitmap capture of the document window. Then I get a DC for the preview, make a memory DC from it and select my bitmap into it. Then I do some size adjustments and bitblt the memory dc to the real dc. I'm guessing that the BitBlt operation doesn't take into account the fact that the WPF pixels are hardware accelerated and therefore need to be grabbed from the graphics hardware. All the stuff in GDI is managed just fine, though and when there's no WPF child windows, the preview image looks fine.

I'm wondering if it's at all possible to grab a bitmap of the WPF window from native C++. Then I can blt that onto the black area of the previous preview.

Answer 1

Maybe I'm not understanding your current approach correctly, but could you do a BitBlt() from the screen DC to your memory DC? You'd need to get the screen rect of your window, but that shouldn't be too bad.

Answer 2

To solve this, I had to create an abstract class in native code containing a virtual method to get the bitmap that was implemented in C++/CLI. In the managed implementation, I used .NET's RenderTargetBitmap class to get a bitmap capture of the WPF window and then I filled up the passed in CBitmap object (see http://stackoverflow.com/questions/2625170/how-to-get-an-bitmap-struct-from-a-rendertargetbitmap-in-c-cli). In the unmanaged caller routine, I used the virtual method to obtain the Bitmap.

In short, there was no way to get the bitmap by simply using unmanaged C++ since WPF and GDI really don't work together for all practical purposes.