Q:

System.Windows.Interactivity must be referenced in the main project

So I have a WPF project and my interface in a different project than the main one.  I went to reference System.Windows.Interactivity, but it would not work.  Not until I added it as a reference to the main project as well...
This is the first time this has ever happened with any reference I have added.  What makes this one so special?
EDIT:
By wouldn't work... This is the error.
{"Could not load file or assembly 'System.Windows.Interactivity, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified."}

A:

System.Windows.Interactivity.dll is not in the GAC, so .Net doesn't know where to find it.
By adding a reference to your main project, you make the build system copy the DLL to the output folder.
This lets the runtime find it.

