I create a project, and I set "Properties>Configuration Properties>C/C++>General>Additonal Include Directories " to "D:\work\test\ConsoleApplicationTest\ConsoleApplicationTest",that folder contains many files include "lib1.h",in my project I write: #include "lib1.h", when build solution, the Error list tells: Can not open sourcefile "lib1.h". But when I write: #include "D:\work\test\ConsoleApplicationTest\ConsoleApplicationTest\lib1.h" , it's OK, I don't know why?
Thanks a lot!

is it in different folder than main ?

yes, lib1.h and main.cpp are in the different folder.

I'm running an older version of Visual Studio but this probably still works the same in later versions. When you open the properties page for the project it will show at the top the active mode, either Debug or Release. Any settings you make apply only to that mode, although you can use the drop down list box to switch to All Configurations, in which case changes will be applied to both Debug and Release modes. Is it possible you set the include directories in one mode and now have switched to the other mode? Just a suggestion.