If version control is enabled for a project, you must check out some entities before changing them in your code, and check them in after posting the changes.
For QCResource and Req, use the VersionControl object to check in and out. Get the VersionControl object from the VC property of the IVersioned Entity interface.
For Test objects, use the VCS object to check in and out. Get the VCS object from the test's IVCS property.