The reason for this is stated in this Knowledge Base article[^]. Using GDI on a GDI+ Graphics Object Backed by a Bitmap When Graphics::GetHDC() is called for a Graphics object that is backed by a bitmap rather than the screen, a memory HDC is created and a new HBITMAP is created and selected into the memory HDC. This new memory bitmap is not initialized with the original bitmap's image but rather with a sentinel pattern, which allows GDI+ to track changes to the bitmap. Any changes that are made to the memory bitmap through the use of GDI code become apparent in changes to the sentinel pattern. When Graphics::ReleaseHDC() is called, those changes are copied back to the original bitmap. Because the memory bitmap is not initialized with the bitmap's image, an HDC that is obtained in this way should be considered "write only" and is therefore not suitable for use with ROPs, the use of which requires the ability to read the target, like R2_XOR. Also, there is a performance cost to this approach because GDI+ must copy the changes back to the original bitmap.
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