If called on a top-level (parent) ICommentThreaded object, it returns a top-level (parent) ICommentThreaded object that represents the previous comment. Using this method on the first comment on a sheet returns null (not the last comment on the previous sheet).

If called on a reply ICommentThreaded object, it returns a reply ICommentThreaded object that represents the previous reply of a thread. This method works only on one thread. Using this method on the first reply of a thread returns null (not its top-level comment).