Inside .vcproj files There is a list of all source files in your project.

How can we use a macro to specify the path to a source file? If we do this:

<File
   RelativePath="$(Lib3rdParty)\Qt\qtwinmigrate-2.5-commercial\src\qmfcapp.cpp">
</File>

The compiler cannot find the folder:

qmfcapp.cpp
c1xx : fatal error C1083: Cannot open source file: '.\$(lib3rdparty)\qt\qtwinmigrate- 2.5-commercial\src\qmfcapp.cpp': No such file or directory

As you can see, our project compiles in several source files from QT. QT lives inside a folder of external libraries, and we don't want hardcode the path from our project to that folder (we have a very large solution)