[2008] Cross Thread issuses with WebBrowser control

[DC123]

I am loading a web page into a WebBrowser control so that I can access it's HTML later in the code using WebBrowser1.DocumentText. My problem is that I can access the HTML from outside the BackgroundWorker1 thread but I can not access the HTML from within the thread. Instead I get the following error: "Specified cast is not valid" refering to the WebBrowser1.DocumentText line of code. I have checked that it is not a document loaded issue the document has loaded.

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

WebBrowser1.Navigate("http://www.google.com/")
BackgroundWorker1.RunWorkerAsync()

End Sub

[Deathader]

Try adding this:

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Me.CheckForIllegalCrossThreadCalls = False
    End Sub

it might work, might not

[DC123]

Try adding this:

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Me.CheckForIllegalCrossThreadCalls = False
    End Sub

it might work, might not

It did not work, still getting the error.

[Deathader]

Ok, don't use backgroundworker. Just put the code into a normal sub, then excecute the sub

yoursub()

[Deepak Sakpal]

Can you post the BackgroundWorker code from where you are trying to access the WebBrowser's text.

[DC123]

Can you post the BackgroundWorker code from where you are trying to access the WebBrowser's text.

Private Sub BackgroundWorkerMonitor_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorkerMonitor.DoWork

If Browser1NavMode = "HomePageLoaded" Then
WebBrowser1HTML = WebBrowser1.Document.Body.InnerHtml
End If

End Sub

[DC123]

Ok, don't use backgroundworker. Just put the code into a normal sub, then excecute the sub

yoursub()

I want to keep the GUI functional.

[Deepak Sakpal]

Why don't you use the DocumentCompleted Event of WebBrowser control? It occurs when the WebBrowser control finishes loading a document.

[DC123]

Why don't you use the DocumentCompleted Event of WebBrowser control? It occurs when the WebBrowser control finishes loading a document.

I tried this and it worked, but then I wanted to use the WebBrowser's Document Object Model (listed below) and the issue was again a problem.

Private Sub DoLogIn()

Dim args As Object() = {"en"}

WebBrowser1.Document.GetElementById("username").SetAttribute("value", "Username")
WebBrowser1.Document.GetElementById("password").SetAttribute("value", "Password")
WebBrowser1.Document.GetElementById("code").SetAttribute("value", "Code")
WebBrowser1.Document.InvokeScript("doLogin", args)

End Sub

[Deepak Sakpal]

What problem?

[DC123]

What problem?

"Specified cast is not valid"

My original question has the details.

[Deepak Sakpal]

Is your web page accessible to out side world?

[DC123]

Is your web page accessible to out side world?

No I am loading a website from the www into a WebBrowser control in a windows app.

[Deepak Sakpal]

and what is that www address?

[DC123]

and what is that www address?

I get the "Specified cast is not valid" error from any website I try even Google.
This is a cross threading issue on a windows application not a website issue.

[Atheist]

"Specified cast is not valid" isnt a cross-thread issue. On what line does it occur?

[DC123]

"Specified cast is not valid" isnt a cross-thread issue. On what line does it occur?

WebBrowser1HTML = WebBrowser1.Document.Body.InnerHtml

When I access the DOM via the above method from within the same thread all is OK but when I try to access the DOM from code in another thread I get the "Specified cast is not valid" error.

[DC123]

When I access the GUI controls across threads I use Delegates. Is there a way of using a Delegate to access the WebBrowser control across a thread?

[Atheist]

Here's an example on how to use safe cross-threading to retrieve the property value of a control:

'This is the method that runs in a separate thread
    Private Sub doSomething()
        Dim innerHTML As String = GetInnerHTML()
        MessageBox.Show(innerHTML)
    End Sub
 
    Private Delegate Function GetInnerHTMLCallBack() As String
    Private Function GetInnerHTML() As String
        If WebBrowser1.InvokeRequired Then
            Return CStr(WebBrowser1.Invoke(New GetInnerHTMLCallBack(AddressOf GetInnerHTML)))
        Else
            Return WebBrowser1.Document.Body.InnerHtml
        End If
    End Function

[DC123]

Here's an example on how to use safe cross-threading to retrieve the property value of a control:

Thanks that has fixed it.