VB.Net BackgroundWorker1 Error: InvalidCastException was unhandled by user code - WebBrowser1.Document.GetElementById

Question

I have the below code, works but when I move it to a BackgroundWorker1 I get an error: InvalidCastException was unhandled by user code.  Thoughts?

Working code: but don't want code in UI

 Private Sub ButtonSend_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonSend.Click

     
        Dim var As Integer
        Dim startVal As Integer
        Dim endVal As Integer
        startVal = 1
        endVal = (TextBoxRepeat.Text)

        For var = startVal To endVal
            WebBrowser1.Document.GetElementById("Name").SetAttribute("value", TextBoxName.Text)
            WebBrowser1.Document.GetElementById("Email").SetAttribute("value", TextBoxeMail.Text)
            WebBrowser1.Document.GetElementById("Country").SetAttribute("value", TextBoxContry.Text)
            WebBrowser1.Document.GetElementById("Comments").SetAttribute("value", TextBoxComments.Text)
            WebBrowser1.Document.GetElementById("ctl10").InvokeMember("click")
            System.Threading.Thread.Sleep(TextBoxRepeat.Text * 1000)
        Next var
    End Sub

Non-Working Code, with Error:

Any Help is greatly appreciated!

Answer 1

First step is switching Option Strict On, look at each compile error and think about how to fix it.

Second step: Controls shall only be access from the Thread that created them. It is invalid to access them from the BackgroundWorker. If it's really necessary to access a WebBrowser from the BGW, you must create a new one in the worker. Looking at your code, it may be a better solution to do without the worker. Instead, add a (Winforms) Timer and do the job in it's Tick event.

---

Armin

Answer 2

Thanks Armin, can you give me a quick example. I turned Option Strict On. Fixed all errors other than the sleep, is that your suggestion to use a timer rather than sleep? and my concern would be take out the BGW would have the code in the UI. I thought this would hang the application on the repeat.

Answer 3

Thanks Armin, can you give me a quick example. I turned Option Strict On. Fixed all errors other than the sleep, is that your suggestion to use a timer rather than sleep? and my concern would be take out the BGW would have the code in the UI. I thought this would hang the application on the repeat.

Drop a Timer from the toolbox on the Form. Double-click on it and handle it's Tick event. There you can add the 5 lines you currently have inside the loop.

Whenever TextBoxRepeat changes (handle it's TextChanged event), you can update the Interval of the Timer. Instead of a Textbox you can use a NumericUpDown control so that input validation is not required (handle the ValueChanged event then).

You're right the UI would hang, however, how long do these 5 lines take to execute? 1 ms? That's not a noticable hang.

---

Armin

Answer 4

Thanks,  but I had a repeate and a delay or sleep.  so if doing the repeate 100 times with a sleep of 60 seconds the app would hang. 

you have to remember I have working code in the UI,  and my sleep.  your suggestion is to just change out the sleep for timer.  correct?   I really think I need a BGW or a thread. 

Answer 5

"I really think I need a BGW or a thread."

"your suggestion is to just change out the sleep for timer"

You're not getting it.  When you get rid of sleep and use a timer instead, your UI won't hang.

Answer 6

If you've never used a timer as a part of a loop before, it can seem a little strange.  Here's your code in a timer tick event:

  Private Sub ButtonSend_Click(ByVal sender As Object, ByVal e As EventArgs) Handles ButtonSend.Click
    Doc = WebBrowser1.Document
    Timer1.Interval = 1
    Timer1.Start()
  End Sub
  Dim Cnt As Integer
  Dim Doc As HtmlDocument
  Private Sub Timer1_Tick(ByVal sender As Object, ByVal e As EventArgs) Handles Timer1.Tick
    Timer1.Stop()
    Cnt += 1
    If Cnt > CInt(TextBoxRepeat.Text) Then Cnt = 0 : Return
    Dim Elements() As String = {"Name", "Email", "Country", "Comments"}
    Dim Values() As String = {TextBoxName.Text, TextBoxMail.Text, TextBoxCountry.Text, TextBoxComments.Text}
    For I As Integer = 0 To TBs.Length - 1
      Doc.GetElementById(Elements(I)).SetAttribute("value", Values(I))
    Next
    Doc.GetElementById("ct110").InvokeMember("click")
    Timer1.Interval = CInt(TextBoxRepeat.Text) * 1000
    Timer1.Start()
  End Sub

Answer 7

Thanks,  I rewote to use the timer:

 Private Sub ButtonSend_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonSend.Click

        Dim var As Integer
        Dim startVal As Integer
        Dim endVal As Integer
        startVal = 1
        endVal = CInt((TextBoxRepeat.Text))

        For var = startVal To endVal
            WebBrowser1.Document.GetElementById("Name").SetAttribute("value", TextBoxName.Text)
            WebBrowser1.Document.GetElementById("Email").SetAttribute("value", TextBoxeMail.Text)
            WebBrowser1.Document.GetElementById("Country").SetAttribute("value", TextBoxContry.Text)
            WebBrowser1.Document.GetElementById("Comments").SetAttribute("value", TextBoxComments.Text)
            'WebBrowser1.Document.GetElementById("ctl10").InvokeMember("click")
            Timer1.Interval = CInt((CDbl(TextBoxDelay.Text) * 1000)) 'In Seconds

        Next var



    End Sub

and it does not hang the UI.  thanks,  unsure why anyone would use a sleep then.  anyway to test I added a simple writeline to see the loop is working and get the same Error: InvalidCastException was unhandled.  thoughts?

Answer 8

Thanks,  but I had a repeate and a delay or sleep.  so if doing the repeate 100 times with a sleep of 60 seconds the app would hang. 

you have to remember I have working code in the UI,  and my sleep.  your suggestion is to just change out the sleep for timer.  correct?   I really think I need a BGW or a thread. 

Remove Thread.Sleep. The action is already done in intervals by using a Timer. It's executed each time the Timer ticks. Between the ticks, the UI is responsive.

---

Armin

Answer 9

Right, the string "Line26 0" can not be converted to an Integer. "26" or "0" could be converted but not "Line26 0".

EDIT: I'd use Integer.Parse() instead of CInt() because the exception (FormatException) is more appropriate then. CInt() does a lot, and the last thing it obviously attempts after parsing fails, is casting to Integer.

---

Armin

Answer 10

Thank you,  Armin!!!