Troubleshoot why PrintWindow is blank

Question

I am trying to capture screenshot of inactive window with PrintWindow. It works correctly for calculator and for capturing Google Chrome, but for some other applications, li

Answer 1

The documentation for PrintWindow provides information on its implementation:

The application that owns the window referenced by hWnd processes the PrintWindow call and renders the image in the device context that is referenced by hdcBlt. The application receives a WM_PRINT message or, if the PW_PRINTCLIENT flag is specified, a WM_PRINTCLIENT message. For more information, see WM_PRINT and WM_PRINTCLIENT.

Whether or not PrintWindow returns a window's content is subject to the window procedure handling the WM_PRINT or WM_PRINTCLIENT message^[1] appropriately. If a window doesn't handle either of those messages, it will not render anything into the provided device context.

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^[1]Standard window implementations provide a message handler for WM_PRINT/WM_PRINTCLIENT through DefWindowProc. Custom window class implementations need to provide their own.