Professional VB 2005 with .NET 3.0 (Programmer to Programmer)

To properly design and develop a Windows Service, it’s important to understand how it differs from a typical Windows program. Here are the most important characteristics of a Windows Service:

• A Windows Service can start before a user logs on. The system maintains a list of Windows Services, which be set to start at boot time. Services can also be installed such that they require a manual startup and will not start at bootup.

• A Windows Service can run under a different account from that of the current user. Most Windows Services provide functionality that needs to be running all the time, and some load before a user logs on, so they cannot depend on a user being logged on to run.

• A Windows Service has its own process. It does not run in the process of a program communicating with it (Chapter 27 has more information on processes).

• A Windows Service typically has no user interface. This is because the service may be running under a different account from that of the current user, or the service may start at bootup, which means that the calls to put up a user interface might fail because they are out of context (it is possible to create a Windows Service with a user interface, but Visual Basic 2005 cannot be used to do it; you’ll learn why later).

• A Windows Service requires a special installation procedure; just clicking on a compiled EXE won’t run it. The program must run in a special context in the operating system, and a specific installation process is required to do the configuration necessary for a Windows Service to be run in this special context.

• A Windows Service works with a Service Control Manager (discussed shortly). The Service Control Manager is required to provide an interface to the Windows Service. External programs that want to communicate with a Windows Service (for example, to start or stop the service) must go through the Service Control Manager. The Service Control Manager is an operating-system-level program, but it has a user interface that can be used to start and stop services, and this interface can be accessed through the Computer Management section of the Control Panel.