/FU (Name Forced #using File)
Gives you a compiler option alternative to passing a file name to The #using Directive in source code.
Specify one file per compiler option.
If you are referencing metadata to use the Friend Assemblies (C++) feature, you cannot use /FU; you must reference the metadata in code with #using (and the [as_friend] attribute).
See/clr (Common Language Runtime Compilation) for information on how to create an assembly or module for the common language runtime.
To set this compiler option in the Visual Studio development environment
Open the project's Property Pages dialog box. For details, see How to: Open Project Property Pages.
Click the C/C++ folder.
Click the Advanced property page.
Modify the Force #using property.
To set this compiler option programmatically