I include <windows.h> but, code completion for some API's like CreateFile() doesn't work. Although for API's like MessageBox() is does work.
Any ideas?
Win32 uses macros. Lots of them. In strange and wonderful/awful ways. Code completion doesn't like macros.
The most common culprit is the fact that a ton of Win32 functions are actually macros typedef-ed to either the ANSI or Unicode version of the actual function. Whether this is the limiting factor in your case, I couldn't say. Suffice it to be known that, as always, code completion is a work in progress with known bugs and limitations.