BackgroundWorker causing error when not Startup project - vb.net

Question

I have a project in VSTO/VB using a BackgroundWorker that works fine. It is a form that calls for a web page of information. The web page can take a while, so I have the form calling with the BackgroundWorker.
I then have an Excel Addin project that has added the BackgroundWorker project. When I call up the form from the Excel Addin project and use the BackgroundWorker to request the web page, it grabs the web page ok. But the work done upon completion, during the BackgroundWorker1_RunWorkerCompleted method, is resulting in an error message:
"Cross-thread operation not valid: Control 'TabPage2' accessed from a thread other than the thread it was created on."
Why does the BackgroundWorker project not work when called from the Excel Addin project?
I note that when I set the BackgroundWorker project as the "Startup Project" there is no error generated. Its something to do with calling this BackgroundWorker project from the Excel Addin project.

Answer 1

Edit: Could be that I am calling RunWorkerAsync() from a non UI thread?
I have an Excel Addin project with a Ribbon class. The Ribbon1.vb has a button click method that creates an instance of the second project, from which the backgroundworker will be called:
Private Sub Btn_Click(ByVal sender As System.Object, ByVal e As Microsoft.Office.Tools.Ribbon.RibbonControlEventArgs) Handles Btn.Click
Dim MySecondProject As SecondProject.Form1 = New SecondProject.Form1()
MySecondProject.Show()
End Sub
MySecondProject is then calling the BackgroundWorker from within its own button click method as:
BackgroundWorker1.RunWorkerAsync()
Then, after it completes, the backgroundworker is trying to update a label in MySecondProject:
Private Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As System.Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
SuccessLabel.Text = "Success!"
End Sub
When MySecondProject was the Startup Project, the backgroundworker kept track of the correct thread and updated the label successfully upon completion. With the Excel Addin as the Startup Project, and MySecondProject instantiated during runtime, the backgroundworker seems to lose track of the correct thread. Should I manually be inserting Invoke or BeginInvoke somewhere to help the backgroundworker keep track of the correct thread to update?

Answer 2

So, it turns out that a Backgroundworker launched from a Form launched from a Ribbon cannot update a control at the end of processing. I am not sure why it works from a Form launched as a Startup project while it does not work from a form launched from the Ribbon, but there it is -- turns out that you need to deal with the Backgroundworker losing track of the UI thread.
Using a MethodInvoker works, as in the following snippet:
Imports System.Windows.Forms
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
BackgroundWorker1.RunWorkerAsync()
End Sub
Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
[do nothing]
End Sub
Private myString As String = "This is my string"
Private Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As System.Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
If Label1.InvokeRequired Then
Dim mi As MethodInvoker = AddressOf UpdateFormText
Label1.BeginInvoke(mi)
Else
Label1.Text = myString
End If
End Sub
Private Sub UpdateFormText()
Label1.Text = myString + " (BeginInvoked)"
End Sub
End Class
A better answer in VS2010 would use an inline MethodInvoker instead of the second function:
Me.Invoke(CType(Sub() Me.Label1.Text = "This is my string", MethodInvoker))