What can i do with "WordprocessingCommentPart == null"?

I need to add comments to a WordprocessingDocument,however:


WordprocessingDocument. MaindocumentPart.WordprocessingCommentPart has no public constructor nor does Maindocument class provide any method to add or create an instance of WordprocessingCommentPart ...

what can i do with "WordprocessingCommentPart == null"? also to ask: how to create a WordprocessingCommentPart ?!

Problem Solved:
by Using Maindocument.AddNewPart();
with no root elements pre-difined.

this is kind of different with adding a Headerpart or Footerpart, ImagePart ,etc.