Why does my delegate called from backgroundworker runs in main thread - vb.net

Question

I am writing a program that will allow me to run a number of different hardware test routines.
The routines can be quite time consuming, lasting up to 30mins. During this time, I have to control a selection of test equipment to set up conditions and take measurements.
I was thinking that using a background worker to carry out the tasks would be ideal, and allow the UI to stay responsive. This worked well until one of the routines requires me to take measurements every 1.5seconds. I am using a system timer to trigger these events. The timer is created and started in the doWork sub of the background worker, however, I find that the delegate is running in the main(UI) thread and not in the background worker thread as I thought.
Am I doing something wrong? I have attached the main parts of a simplified program that has the same structure.
Private Sub getMeasurement()
' Runs in backgroundWorker thread
Me.TextBox2.Text = (System.DateTime.Now - startTime).TotalSeconds.ToString
startTime = System.DateTime.Now
Debug.Print("Thread name is " & Thread.CurrentThread.Name & ", ID = " & Thread.CurrentThread.ManagedThreadId)
End Sub
Private Sub OnTimedEvent()
'Runs in own thread, Calls getMeasurement which runs in BackgroundWorker thread
Thread.CurrentThread.Name = "OTE"
Debug.Print("In OnTimedEvent, thread = " & Thread.CurrentThread.Name & ", ID = " & Thread.CurrentThread.ManagedThreadId)
Dim ServiceTimerDelegate As New ServiceTimerDelegate(AddressOf getMeasurement)
Me.BeginInvoke(ServiceTimerDelegate)
End Sub
Private Sub backgroundWorker1_DoWork(ByVal sender As System.Object, _
ByVal e As DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Thread.CurrentThread.Name = "BW1"
mTimer = New Timers.Timer(1490) ' 14.9secs (allow for some latency)
AddHandler mTimer.Elapsed, New Timers.ElapsedEventHandler(AddressOf OnTimedEvent)
Dim worker As BackgroundWorker = CType(sender, BackgroundWorker)
Dim i As Integer
'main timer for measurements every 1.5secs (may change to take interval from UI)
Select Case e.Argument
Case "Sunday"
mTimer.Start()
Debug.Print("Thread in DoWork = " & Thread.CurrentThread.Name & ", ID = " & Thread.CurrentThread.ManagedThreadId)
startTime = System.DateTime.Now
'main loop for temperature ramping
For i = 20 To 70
If (worker.CancellationPending = True) Then
e.Cancel = True
Else
Thread.Sleep(500)
worker.ReportProgress((i - 19) * (100 / 50))
i += 1
End If
Next
Case Else
e.Cancel = True
End Select
End Sub

Answer 1

Three things are confusing here, and make me think that the code you've posted above doesn't actually use the BackgroundWorker for its function.
The DoWork event always runs in a separate thread from the UI. That's the entire point of it. ReportProgress runs in the UI thread, but you're not making use of that.
You don't actually appear to be running the BackgroundWorker at all in the above code. Rather, you seem to be using a ServiceTimerDelegate - not 100% sure I know what that is. You've defined a BackgroundWorker within your DoWork handler (which makes no sense), but you never seem to be actually hooking a BackgroundWorker up to that handler nor calling RunWorkerAsync on it.
Even if you were running getMeasurement() in a BackgroundWorker, it should fail - you're altering the contents of a UI item, which should cause an exception due to InvalidCrossThreadAccess. If you need to alter a UI control, use the ReportProgress event, which occurs within the UI thread.