屏幕关闭时捕获窗口

Question

I'm using Native GDI+ for capture window "over screen". It's not working when screen is off (generates black image). How can I fix it? (I'm using .Net 4.5)

public static Image CaptureWindow(IntPtr handle)
{
        IntPtr hdcSrc = User32.GetWindowDC(handle);

        RECT windowRect = new RECT();
        User32.GetWindowRect(handle, ref windowRect);

        int width = windowRect.right - windowRect.left;
        int height = windowRect.bottom - windowRect.top;

        IntPtr hdcDest = Gdi32.CreateCompatibleDC(hdcSrc);
        IntPtr hBitmap = Gdi32.CreateCompatibleBitmap(hdcSrc, width, height);

        IntPtr hOld = Gdi32.SelectObject(hdcDest, hBitmap);
        Gdi32.BitBlt(hdcDest, 0, 0, width, height, hdcSrc, 0, 0, SRCCOPY);
        Gdi32.SelectObject(hdcDest, hOld);
        Gdi32.DeleteDC(hdcDest);
        User32.ReleaseDC(handle, hdcSrc);

        Image image = Image.FromHbitmap(hBitmap);
        Gdi32.DeleteObject(hBitmap);

        return image;
 }

Answer 1

As far as i know the graphics driver stops producing an image when there is no one to receive it, to save power. This might be why you can't capture the image when the screen is "off". There is no point in using CPU and GPU and BUS power on producing an image, when there is no one to receive it.