In our project, we use the core editor with a custom Language Service.    
When the core editor has multiple files open, each of these opened files is
represented within the environment by a tab in the view pane.  By clicking on
a tab, it
displays the contents of that file, making it the "current view".

We use the current view successfully for a number of tasks, including
editing, colorization, setting breakpoints, etc.   We also need to retrieve
the full path name of the current opened file, or the VSITEMID of the file,
in order to access some file properties and to do other things with the file
in our project system.

We successfully get the current text view (each open file has an IVsTextView
associated with it) as follows:

      CComPtr<IVsTextManager> n_srpTextMgr;
      CComPtr<IVsTextView> n_srpTextView;
      // first get the manager
      _AtlModule.QueryService(SID_SVsTextManager,
                          IID_IVsTextManager, (void**)&n_srpTextMgr);
      // now get the current view
      hrLocal = n_srpTextMgr->GetActiveView(TRUE, NULL,
                                &n_srpTextView);

This procedure gives us a text view of the currently open file, and the
IVsTextView interface gives us access to the contents of the view: number of
lines, buffer content, caret location, doing update completion and method
tips, and so on.

How do we find the full path name of the file (or the VSITEMID of the file)
this current view is associated with?

Thanks
--Don