问题
Why can't I include windows.h in afx(MFC) projects?
回答1:
Typically, MFC application code includes afx.h or afxwin.h (the latter includes former). First two lines of windows.h are
#ifndef _WINDOWS_
#define _WINDOWS_
which means that _WINDOWS_ becomes defined if this header is included.
Afx.h includes afxver_.h and this header includes afxv_w32.h which contains following code:
#ifdef _WINDOWS_
#error WINDOWS.H already included. MFC apps must not #include <windows.h>
#endif
...
#include <windows.h>
So, if you include windows.h before MFC headers, you'll get this error generated in compile time and, as you can see, if you include afxwin.h you don't need to include windows.h yourself - it will already be included by afxv_w32.h.
回答2:
Because in MFC you are not supposed to use it directly. AFAIR you should include afx.h instead, which in turn indirectly includes windows.h the proper way.
来源：https://stackoverflow.com/questions/5026799/windows-h-and-mfc