If you are trying to retrieve attributes in the assemblyinfo file, it’s better to use the ProjectItem.FileCodeModel rather than the Project.CodeModel:

BUG: EnvDTE.Project.CodeModel doesn’t retrieve attributes in the AssemblyInfo file
http://www.mztools.com/articles/2009/MZ2009014.aspx

The bug report for Microsoft is here:

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=471708