Question

Is it possible to use an UnhandledException Handler in a Windows Service?

Normally I would use a custom built Exception Handling Component that does logging, phone home, etc. This component adds a handler to System.AppDomain.CurrentDomain.UnhandledException but as far as I can tell this doesn’t achieve anything win a Windows Service so I end up with this pattern in my 2 (or 4) Service entry points:

      Protected Overrides Sub OnStart(ByVal args() As String)         ' Add code here to start your service. This method should set things         ' in motion so your service can do its work.         Try             MyServiceComponent.Start()         Catch ex As Exception             'call into our exception handler             MyExceptionHandlingComponent.ManuallyHandleException (ex)             'zero is the default ExitCode for a successfull exit, so if we set it to non-zero             ExitCode = -1             'So, we use Environment.Exit, it seems to be the most appropriate thing to use             'we pass an exit code here as well, just in case.             System.Environment.Exit(-1)         End Try     End Sub  

Is there a way my Custom Exception Handling component can deal with this better so I don’t have to fill my OnStart with messy exception handling plumbing?

Answer 1

Ok, I’ve done a little more research into this now. When you create a windows service in .Net, you create a class that inherits from System.ServiceProcess.ServiceBase (In VB this is hidden in the .Designer.vb file). You then override the OnStart and OnStop function, and OnPause and OnContinue if you choose to. These methods are invoked from within the base class so I did a little poking around with reflector. OnStart is invoked by a method in System.ServiceProcess.ServiceBase called ServiceQueuedMainCallback. The vesion on my machine ‘System.ServiceProcess, Version=2.0.0.0’ decompiles like this:

  Private Sub ServiceQueuedMainCallback(ByVal state As Object)     Dim args As String() = DirectCast(state, String())     Try          Me.OnStart(args)         Me.WriteEventLogEntry(Res.GetString('StartSuccessful'))         Me.status.checkPoint = 0         Me.status.waitHint = 0         Me.status.currentState = 4     Catch exception As Exception         Me.WriteEventLogEntry(Res.GetString('StartFailed', New Object() { exception.ToString }), EventLogEntryType.Error)         Me.status.currentState = 1     Catch obj1 As Object         Me.WriteEventLogEntry(Res.GetString('StartFailed', New Object() { String.Empty }), EventLogEntryType.Error)         Me.status.currentState = 1     End Try     Me.startCompletedSignal.Set End Sub  

So because Me.OnStart(args) is called from within the Try portion of a Try Catch block I assume that anything that happens within the OnStart method is effectively wrapped by that Try Catch block and therefore any exceptions that occur aren’t technically unhandled as they are actually handled in the ServiceQueuedMainCallback Try Catch. So CurrentDomain.UnhandledException never actually happens at least during the startup routine. The other 3 entry points (OnStop, OnPause and OnContinue) are all called from the base class in a similar way.

So I ‘think’ that explains why my Exception Handling component can’t catch UnhandledException on Start and Stop, but I’m not sure if it explains why timers that are setup in OnStart can’t cause an UnhandledException when they fire.