Simple example for ReSharper 7.1 to include non-project files into the project system. This particular example will include a file called Hidden.cs that lives in the root of a C# project into the project.
This means that the declarations inside Hidden.cs are now available for ReSharper to use - types defined in Hidden.cs now appear in code completion lists, and other symbols are also resolved. Navigation also works, after a fashion.
This is useful if you have a project that declares some files as included in the project, but marked as hidden. Since ReSharper gets its project information from Visual Studio, and since the "hidden" metadata means the file is excluded from Visual Studio's file tree, ReSharper doesn't get to know about the file. This sample could be extended to interrogate the msbuild based project system and find all hidden files and expose them.
An alternative use could be providing simple support for transpiled languages, such as Less, SASS and CoffeeScript. While those languages wouldn't have first class support, since they transpile to JS or CSS, those generated files could be added as hidden files, and the declarations could then be used in HTML pages or view engines.