Writing Your First Service in Visual Basic 2008 (part 2)

Writing Our First Code

Now we need to modify the service so that it does something useful. Initially this will not be a major change, but later on we will be expanding the service.

In Visual Studio, click the Tutorials.vb tab and you will again see the <OnStart> and <OnStop> code. We will modify both methods to write an entry into the Application log.

This simple application demonstrates the process of creating a service in Visual Basic 2008; modifying the design template provided by Visual Studio 2008; and creating, building, and then installing the service.

Click the <OnStart> method. Under the comments section, we’ll first add an error handler:

Try
Catch ex As Exception
End Try

You should never leave your code unprotected from exceptions, especially in a service where the message box that appears when an unhandled exception occurs cannot be processed without special implementation.

You use a Try/Catch block to capture exceptions that are thrown by your code if it generates custom exceptions, or to catch exceptions thrown by internal .NET classes. The important thing to notice here is that we are capturing an exception called Exception. In Visual Basic terms, this means any exception—or in other words, a catchall. Although using a catchall here is fine, you should always code your applications to catch only the specific exceptions that you can expect and then do what you need to based on that specific exception. We will deal more with this as we progress.

Modifying the <OnStart> Method

In the <OnStart> method we are going to write an event to the Application log when the service is told to start the “Hello, World” of beginner services. We plan to implement other code in this method in the future, so this will help you understand how to standardize code in your services.

In Listing 1 we will modify the <OnStart> method to log an event as the user starts the service. If there is an error, we will do nothing for now. If the attempt to write the startup message fails, most likely an attempt to write a failure message to the Application log would also fail.

Listing 1. Modifications to <OnStart> to write to the Application log.

Try
   Dim StartLog As EventLog = New EventLog ("Application")
   StartLog.Source = "Tutorials"
   StartLog.WriteEntry("Tutorials Starting", EventLogEntryType.Information, 1000)
   StartLog.Dispose()
   Catch ex As Exception
   'We Catch the Exception
   'to avoid any unhandled errors
   'and we will stop the service if any occur here
   Me.Stop()
End Try

					    

You will note that in Catch we aren’t actually doing anything. For now this is okay. However, in the future—and for real services—you may well want to shut down the service if this method fails, thereby limiting the work load of the <OnStart>.

Modifying the <OnStop> Method

In the <OnStop> method, shown in Listing 2, we are going to modify the method to write a single event to the Application log when the user stops the service. As with the <OnStart> method, we won’t try to log a failure of the service to write the stop message.

Listing 2. Modifications to <OnStop> to write to the Application log.

Try
Dim StopLog As EventLog = New EventLog ("Application")
StopLog.Source = "Tutorials"
StopLog.WriteEntry("Tutorials Stopping", EventLogEntryType.Information, 1001)
StopLog.Dispose()
Catch ex As Exception

   'We Catch the Exception
   'to avoid any unhandled errors
   'since we are stopping and
   'logging an event is what failed
   'we will merely write the output
   'to the debug window
   Debug.WriteLine("Error stopping service: " + ex.ToString())
End Try

					    

You will notice that the only real difference here between <OnStart> and <OnStop> is that we are sending a different message with a different EventID so that we can easily distinguish what the service is doing and validate that the code is working properly.

Modifying the <OnPause> Method

Once again in the <OnPause> method (shown in Listing 1-3) we are going to modify the method to write a single event to the Application log when the user pauses the service. As with <OnStart> we won’t try to log a failure of the service to write the pause message.

Listing 3. Modifications to <OnPause> to write to the Application log.

Try
Dim PauseLog As EventLog = New EventLog ("Application")
PauseLog.Source = "Tutorials"
PauseLog.WriteEntry("Tutorials Pausing", EventLogEntryType.Information, 1002)
StopLog.Dispose()
Catch ex As Exception

   'We Catch the Exception
   'to avoid any unhandled errors
   'since we are pausing and
   'logging an event is what failed
   'we will merely write the output
   'to the debug window
   Debug.WriteLine("Error pausing service: " + ex.ToString())
   Me.Stop()
End Try

					    

Modifying the <OnContinue> Method

Finally, in the <OnContinue> method (shown in Listing 1-4) we are going to modify the method to write a single event to the Application log when the user continues the service after it has been paused. As with <OnStart> we won’t try to log a failure of the service to write the continue message.

Listing 4. Modifications to <OnContinue> to write to the Application log.

Try
   Dim ContinueLog As EventLog = New EventLog ("Application")
   ContinueLog.Source = "Tutorials"
   ContinueLog.WriteEntry("Tutorials Continuing", EventLogEntryType.Information, 1003)
   StopLog.Dispose()
Catch ex As Exception

   'We Catch the Exception
   'to avoid any unhandled errors
   'since we are resuming and
   'logging an event is what failed
   'we will merely write the output
   'to the debug window
   Debug.WriteLine("Error resuming service: " + ex.ToString())
   Me.Stop()
End Try