Check if element does exist in WebBrowser using BackgroundWorker

Question

I'm using background worker to manipulate some of the elements on my WebBrowser using vb.net.

For example the background worker RFIDReader will check if I'm on a specific link.

ElseIf TerminalBrowser.Url.ToString.Contains(baseUrl + "box-office/ticket-verification") And rf_code <> "" Then

    ' Insert RF Code in "rf_code" hidden text field
    Me.Invoke(Sub() Me.TerminalBrowser.Document.GetElementById("rf_code").SetAttribute("value", rf_code.ToLower))

End If  

What happens here is, if I tap my RFID card. It will include that corresponding value to my rf_code element in my browser.

Now what I want to happen is, I want to check if the container itself (synchronize-rfid) does exist (Since it's a pop up). See image for reference.

Here's our code for that.

If Me.TerminalBrowser.Document.GetElementById("synchronize-rfid") IsNot Nothing Then
    ' Code here
end if

Reference: https://stackoverflow.com/a/2022120/1699388

The problem is, BackgroundWorker does not really interacts with UI as per the code above.

Is there any method for me to determine if that element exist using background worker?

I think I've done this and it does not work though.

Dim synchronize_rfid = Me.Invoke(Sub() Me.TerminalBrowser.Document.GetElementById("synchronize-rfid"))
If synchronize_rfid IsNot Nothing Then
    ' Code here
end if

Answer 1

Try declaring the variable first, then set it during the invocation (which will set it from the main thread).

The Sub() lambda behaves like a normal Sub()-End Sub method, which means you can do the same things in there as if you had a separate method.

This works for me:

Dim synchronize_rfid As HtmlElement
If Me.InvokeRequired = True Then
    Me.Invoke(Sub() synchronize_rfid = Me.TerminalBrowser.Document.GetElementById("synchronize-rfid"))
Else
    synchronize_rfid = Me.TerminalBrowser.Document.GetElementById("synchronize-rfid")
End If

If synchronize_rfid IsNot Nothing Then
    ' Code here
End If