Services have always run in session 0. Before Windows Vista, the first user to log on was also assigned to session 0. Now, session 0 is reserved exclusively for services and other applications not associated with an interactive user session. (The first user to log on is connected to session 1, the second user to log on is connected to session 2, and so on.) Session 0 does not support processes that interact with the user.
This change means that a service cannot post or send a message to an application and an application cannot send or post a message to a service. In addition, services cannot display a user interface item such as a dialog box directly. A service can use the
WTSSendMessage function to display a dialog box in another session.
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