Even if that was possible, you need a linker. Also recent versions of MSVC plug-in a pragma that contains the version number of the MSVCRT dll and once a .c/.cpp file was compiled with specific version it has this version in the .obj file, so if you try to link it with another version it'll complain. Recently they've added this for debug/release builds (NDEBUG, _DEBUG) (so you can't mix them - but I think this was done only for C++ - because of STL)