Is there a way to access and use CustomDocumentProperties in the Word Object
Library other than using the InvokeMember of the Type?  Any attempt to cast
the object into Microsoft.Office.Core.DocumentProperties fails with an
InvalidCastException.  Sample:

Microsoft.Office.Interop.Word.DocumentClass doc = new
    Microsoft.Office.Interop.Word.DocumentClass();
Microsoft.Office.Core.DocumentProperties docCustomProps =
    (Microsoft.Office.Core.DocumentProperties)(doc.CustomDocumentProperties);