Is it illegal for your landlord to just walk in with no notice/no request?

Actually it’s the property manager, not the landlord, but he has this habit of just coming into our apartment, or knocking and then immediately unlocking our door, or just showing up randomly and pushing his way in with his workers to do something or other. We feel this is really inappropriate and an invasion of privacy. We’ve asked him to give notice but he has ignored us. What do you think?