Not Connecting to website in New Thread

[aricwhi]

I have hit a road bump. I have been piecing together bits of code that I have found and customized it to work with my app. Now the following code works find out of a separate thread, but as soon as I put it into its on threads it doesn't want to connect.

Any suggestions?

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click




        getAlexaNow = New Thread(AddressOf ThreadTask)
        getAlexaNow.IsBackground = True
        getAlexaNow.Start()


    End Sub
    Private Sub ThreadTask()
        Dim intList As Integer
        Dim strAlexa As String

        intList = "0"
        strSearch = txtSearch.Text


        Do Until intList = linksList.Items.Count

            Dim linkUrl As String = "http://www.alexa.com/search?q=" & linksList.Items.Item(intList)
            Try
                Dim myResponse As Net.HttpWebResponse = Net.HttpWebRequest.Create(linkUrl).GetResponse '// connect.
                Dim myStream As IO.Stream = myResponse.GetResponseStream() '// get.
                Dim myReader As New IO.StreamReader(myStream) '// read.
                Dim myWebFileInfo As String = myReader.ReadToEnd '// get innerHtml of Page.
                myReader.Close() : myStream.Close() : myResponse.Close() '// close reader, information stream, and connection.


                RichTextBox3.Text = myWebFileInfo



                If InStr(myWebFileInfo, "No regional data") Then
                    alexaList.Items.Add((linksList.Items.Item(intList)) & ":" & "No Alexa rank")
                    Exit Try
                End If

                '// get location of <pre> by index.
                Dim myStartIndex As Integer = myWebFileInfo.IndexOf(">US</a>:") + 8 '// add +5 to not add "<pre>" to final result.
                '// get location of </pre> by index.
                Dim myEndIndex As Integer = myWebFileInfo.IndexOf("          </li>")
                '// subtract to get a total number for the substring length result.
                Dim myStringLengthToExtract As Integer = myEndIndex - myStartIndex
                '// display result.
                strAlexa = myWebFileInfo.Substring(myStartIndex, myStringLengthToExtract)
                If InStr(strAlexa, "XHTML") Then
                    alexaList.Items.Add((linksList.Items.Item(intList)) & ":" & "No Alexa rank")

                Else
                    If InStr(strAlexa, "%%") Then

                    Else

                        alexaList.Items.Add((linksList.Items.Item(intList)) & ":" & strAlexa)
                    End If
                End If
            Catch ex As Exception
                MsgBox("Connection Error.", MsgBoxStyle.Critical, "myCool Error Message")
            End Try

            intList = intList + 1
        Loop
    End Sub

[Grimfort]

If you are using a control on a thread that is different from the one it was created upon, you will usually get an exception. ie. alexaList.Items.Add

You must do all work for a control on the thread it was created upon.

To add, this line here: Catch ex As Exception, tells you what the problem is, you are actually ignoring it. It tells you the real problem. Use this instead of your existing MsgBox.

MessageBox.Show("Connection Error - " & ex.Message)

[aricwhi]

If you are using a control on a thread that is different from the one it was created upon, you will usually get an exception. ie. alexaList.Items.Add

You must do all work for a control on the thread it was created upon.

To add, this line here: Catch ex As Exception, tells you what the problem is, you are actually ignoring it. It tells you the real problem. Use this instead of your existing MsgBox.

MessageBox.Show("Connection Error - " & ex.Message)

Ok great! Thanks so much for your response.


This is the detailed error message.

Connection error - crosss-thread operation not valid: Control 'alexList' accessed from a thread other than the thread it was created on.

I'm wondering what I have to change because alexaList is a simlpe list box.

Here is my entire code.

Imports System.Text.RegularExpressions
Imports System.Net
Imports System.Text
Imports System.Threading
       
Public Class Form1

    Private Property str33 As Object
    Dim strNum As Integer
    Dim strSearch As String
    Private getAlexaNow As Thread


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

        ' Dim linkurl As String

        'linkurl = "http://www.860designs.com/check.txt"
        ' Dim myResponse As Net.HttpWebResponse = Net.HttpWebRequest.Create(linkUrl).GetResponse '// connect.
        '' Dim myStream As IO.Stream = myResponse.GetResponseStream() '// get.
        ' Dim myReader As New IO.StreamReader(myStream) '// read.
        ' Dim myWebFileInfo As String = myReader.ReadToEnd '// get innerHtml of Page.
        ' Dim strUser As String


        '// close reader, information stream, and connection.
        ' myReader.Close() : myStream.Close() : myResponse.Close()

        ' strUser = InputBox("username")
        ' If InStr(myWebFileInfo, strUser) Then

        '  Else
        '  Me.Close()
        '   Exit Sub

        '  End If
    End Sub


    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

       
        WebBrowser1.Navigate("http://www.google.com/search?tbm=blg&hl=en&source=hp&biw=&bih=&q=" & strSearch & "&btnG=Search")
        
        strNum = 1
        strNum = 10
        tmrBroswer.Enabled = True

    End Sub


    Private Sub WebBrowser1_DocumentCompleted(ByVal sender As Object, ByVal e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) Handles WebBrowser1.DocumentCompleted


        'For every link in the current document...         
        For Each ele As HtmlElement In WebBrowser1.Document.Links
            'Get whatever text there is in the 'href' attribute
            Dim eletarget As String = ele.GetAttribute("href")
            Dim eletargetNew As String
            'Add it to the listbox

            If InStr(eletarget, "google.com") Then
            ElseIf InStr(eletarget, "youtube.com") Then
            ElseIf InStr(eletarget, "wdyl.com") Then
            ElseIf InStr(eletarget, "javascript") Then
            Else

                Dim myStartIndex As Integer = eletarget.IndexOf("h") + 1 '// add +5 to not add "<pre>" to final result.
                '// get location of </pre> by index.
                Dim myEndIndex As Integer = eletarget.IndexOf("com") + 3
                '// subtract to get a total number for the substring length result.
                Dim myStringLengthToExtract As Integer = myEndIndex - myStartIndex
                '// display result.
                eletargetNew = eletarget.Substring(myStartIndex, myStringLengthToExtract)

                If eletargetNew = "t" Then

                Else

                    linksList.Items.Add("h" & eletargetNew)
                End If
            End If

            'Carry on to the next link
        Next
        'strNum = strNum + 10
        ' Dim strSearch As String
        ' strSearch = txtSearch.Text
        ' If strNum > 100 Then
        'Exit Sub
        ' End If
        If tmrBroswer.Enabled = False Then
            tmrBroswer.Enabled = False
            Exit Sub

        End If

        tmrBroswer.Enabled = True



    End Sub


    Private Sub tmrBroswer_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles tmrBroswer.Tick
        Dim intStop As Integer
        strSearch = txtSearch.Text

        If intStop >= 5 Then

            Exit Sub
            tmrBroswer.Enabled = False
        End If
        WebBrowser1.Navigate("http://www.google.com/search?tbm=blg&hl=en&source=hp&biw=&bih=&q=" & strSearch & "&btnG=Search&start=" & strNum)

        intStop = intStop + 1
        strNum = strNum + 10
    End Sub

    Private Sub cmdStop_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdStop.Click
        tmrBroswer.Enabled = False
    End Sub



    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click




        getAlexaNow = New Thread(AddressOf ThreadTask)
        getAlexaNow.IsBackground = True
        getAlexaNow.Start()


    End Sub
    Private Sub ThreadTask()
        Dim intList As Integer
        Dim strAlexa As String

        intList = "0"
        strSearch = txtSearch.Text


        Do Until intList = linksList.Items.Count

            Dim linkUrl As String = "http://www.alexa.com/search?q=" & linksList.Items.Item(intList)
            Try
                Dim myResponse As Net.HttpWebResponse = Net.HttpWebRequest.Create(linkUrl).GetResponse '// connect.
                Dim myStream As IO.Stream = myResponse.GetResponseStream() '// get.
                Dim myReader As New IO.StreamReader(myStream) '// read.
                Dim myWebFileInfo As String = myReader.ReadToEnd '// get innerHtml of Page.
                myReader.Close() : myStream.Close() : myResponse.Close() '// close reader, information stream, and connection.



                If InStr(myWebFileInfo, "No regional data") Then
                    alexaList.Items.Add((linksList.Items.Item(intList)) & ":" & "No Alexa rank")
                    Exit Try
                End If

                '// get location of <pre> by index.
                Dim myStartIndex As Integer = myWebFileInfo.IndexOf(">US</a>:") + 8 '// add +5 to not add "<pre>" to final result.
                '// get location of </pre> by index.
                Dim myEndIndex As Integer = myWebFileInfo.IndexOf("          </li>")
                '// subtract to get a total number for the substring length result.
                Dim myStringLengthToExtract As Integer = myEndIndex - myStartIndex
                '// display result.
                strAlexa = myWebFileInfo.Substring(myStartIndex, myStringLengthToExtract)
                If InStr(strAlexa, "XHTML") Then
                    alexaList.Items.Add((linksList.Items.Item(intList)) & ":" & "No Alexa rank")

                Else
                    If InStr(strAlexa, "%%") Then

                    Else

                        alexaList.Items.Add((linksList.Items.Item(intList)) & ":" & strAlexa)
                    End If
                End If
            Catch ex As Exception
                MsgBox("Connection Error - " & ex.Message)
            End Try

            intList = intList + 1
        Loop
    End Sub


    Private Sub alexaList_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles alexaList.SelectedIndexChanged

        Dim aLink As String
        Dim aLinkNew As String
        aLink = alexaList.SelectedItem.ToString

        Dim myStartIndex As Integer = aLink.IndexOf("h") + 1 '// add +5 to not add "<pre>" to final result.
        '// get location of </pre> by index.
        Dim myEndIndex As Integer = aLink.IndexOf("com") + 3
        '// subtract to get a total number for the substring length result.
        Dim myStringLengthToExtract As Integer = myEndIndex - myStartIndex
        '// display result.
        aLinkNew = aLink.Substring(myStartIndex, myStringLengthToExtract)

        If aLinkNew = "t" Then

        Else

            alexaBrowser.Navigate("h" & aLinkNew)
        End If

    End Sub





End Class

[Grimfort]

So the exception tells you that you are trying to change a control on a different thread then the one it was created upon .

Here is an example on how to start doing this here, or you could use an extension (advanced, but makes it easier after).

[aricwhi]

So the exception tells you that you are trying to change a control on a different thread then the one it was created upon .

Here is an example on how to start doing this here, or you could use an extension (advanced, but makes it easier after).

Thank you so much for all of your help. I have taken a look at the thread, but couldn't be more confused.

I will review it again, but I fear this may cause me to hit a road block.

[Grimfort]

Do not be too worried. Instead of directly using threads, M$ have created the BackgroundWorker control that can do most of this stuff for you. It has special events where you can access controls without this error.

[aricwhi]

Do not be too worried. Instead of directly using threads, M$ have created the BackgroundWorker control that can do most of this stuff for you. It has special events where you can access controls without this error.

Ok great! I will definetly read into the M$ solution. Thanks for all of your help!

[aricwhi]

I have moved my task into a background worker sub, but I am still having the same error.

Imports System.Text.RegularExpressions
Imports System.Net
Imports System.Text
Imports System.Threading
       
Public Class Form1

    Private Property str33 As Object
    Dim strNum As Integer
    Dim strSearch As String
    Private getAlexaNow As Thread


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

        ' Dim linkurl As String

        'linkurl = "http://www.860designs.com/check.txt"
        ' Dim myResponse As Net.HttpWebResponse = Net.HttpWebRequest.Create(linkUrl).GetResponse '// connect.
        '' Dim myStream As IO.Stream = myResponse.GetResponseStream() '// get.
        ' Dim myReader As New IO.StreamReader(myStream) '// read.
        ' Dim myWebFileInfo As String = myReader.ReadToEnd '// get innerHtml of Page.
        ' Dim strUser As String


        '// close reader, information stream, and connection.
        ' myReader.Close() : myStream.Close() : myResponse.Close()

        ' strUser = InputBox("username")
        ' If InStr(myWebFileInfo, strUser) Then

        '  Else
        '  Me.Close()
        '   Exit Sub

        '  End If
    End Sub


    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

       
        WebBrowser1.Navigate("http://www.google.com/search?tbm=blg&hl=en&source=hp&biw=&bih=&q=" & strSearch & "&btnG=Search")
        
        strNum = 1
        strNum = 10
        tmrBroswer.Enabled = True

    End Sub


    Private Sub WebBrowser1_DocumentCompleted(ByVal sender As Object, ByVal e As System.Windows.Forms.WebBrowserDocumentCompletedEventArgs) Handles WebBrowser1.DocumentCompleted


        'For every link in the current document...         
        For Each ele As HtmlElement In WebBrowser1.Document.Links
            'Get whatever text there is in the 'href' attribute
            Dim eletarget As String = ele.GetAttribute("href")
            Dim eletargetNew As String
            'Add it to the listbox

            If InStr(eletarget, "google.com") Then
            ElseIf InStr(eletarget, "youtube.com") Then
            ElseIf InStr(eletarget, "wdyl.com") Then
            ElseIf InStr(eletarget, "javascript") Then
            Else

                Dim myStartIndex As Integer = eletarget.IndexOf("h") + 1 '// add +5 to not add "<pre>" to final result.
                '// get location of </pre> by index.
                Dim myEndIndex As Integer = eletarget.IndexOf("com") + 3
                '// subtract to get a total number for the substring length result.
                Dim myStringLengthToExtract As Integer = myEndIndex - myStartIndex
                '// display result.
                eletargetNew = eletarget.Substring(myStartIndex, myStringLengthToExtract)

                If eletargetNew = "t" Then

                Else

                    linksList.Items.Add("h" & eletargetNew)
                End If
            End If

            'Carry on to the next link
        Next
        'strNum = strNum + 10
        ' Dim strSearch As String
        ' strSearch = txtSearch.Text
        ' If strNum > 100 Then
        'Exit Sub
        ' End If
        If tmrBroswer.Enabled = False Then
            tmrBroswer.Enabled = False
            Exit Sub

        End If

        tmrBroswer.Enabled = True



    End Sub


    Private Sub tmrBroswer_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles tmrBroswer.Tick
        Dim intStop As Integer
        strSearch = txtSearch.Text

        If intStop >= 5 Then

            Exit Sub
            tmrBroswer.Enabled = False
        End If
        WebBrowser1.Navigate("http://www.google.com/search?tbm=blg&hl=en&source=hp&biw=&bih=&q=" & strSearch & "&btnG=Search&start=" & strNum)

        intStop = intStop + 1
        strNum = strNum + 10
    End Sub

    Private Sub cmdStop_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdStop.Click
        tmrBroswer.Enabled = False
    End Sub



    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click



Me.BackgroundWorker1.RunWorkerAsync()


    End Sub


    Private Sub BackgroundWorker1_DoWork(ByVal sender As Object, _
                                     ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
        Dim worker As System.ComponentModel.BackgroundWorker = DirectCast(sender, System.ComponentModel.BackgroundWorker)

        For i As Integer = 1 To 100
            'Raise the ProgressChanged event in the UI thread.
            worker.ReportProgress(i, i & " iterations complete")

            Dim intList As Integer
            Dim strAlexa As String

            intList = "0"
            strSearch = txtSearch.Text


            Do Until intList = linksList.Items.Count

                Dim linkUrl As String = "http://www.alexa.com/search?q=" & Me.linksList.Items.Item(intList)
                Try
                    Dim myResponse As Net.HttpWebResponse = Net.HttpWebRequest.Create(linkUrl).GetResponse '// connect.
                    Dim myStream As IO.Stream = myResponse.GetResponseStream() '// get.
                    Dim myReader As New IO.StreamReader(myStream) '// read.
                    Dim myWebFileInfo As String = myReader.ReadToEnd '// get innerHtml of Page.
                    myReader.Close() : myStream.Close() : myResponse.Close() '// close reader, information stream, and connection.



                    If InStr(myWebFileInfo, "No regional data") Then
                        Me.alexaList.Items.Add((Me.linksList.Items.Item(intList)) & ":" & "No Alexa rank")
                        Exit Try
                    End If

                    '// get location of <pre> by index.
                    Dim myStartIndex As Integer = myWebFileInfo.IndexOf(">US</a>:") + 8 '// add +5 to not add "<pre>" to final result.
                    '// get location of </pre> by index.
                    Dim myEndIndex As Integer = myWebFileInfo.IndexOf("          </li>")
                    '// subtract to get a total number for the substring length result.
                    Dim myStringLengthToExtract As Integer = myEndIndex - myStartIndex
                    '// display result.
                    strAlexa = myWebFileInfo.Substring(myStartIndex, myStringLengthToExtract)
                    If InStr(strAlexa, "XHTML") Then
                        Me.alexaList.Items.Add((Me.linksList.Items.Item(intList)) & ":" & "No Alexa rank")

                    Else
                        If InStr(strAlexa, "%%") Then

                        Else

                            Me.alexaList.Items.Add((Me.linksList.Items.Item(intList)) & ":" & strAlexa)
                        End If
                    End If
                Catch ex As Exception
                    MsgBox("Connection Error - " & ex.Message)
                End Try

                intList = intList + 1
            Loop
            Threading.Thread.Sleep(250)
        Next i
    End Sub

    Private Sub BackgroundWorker1_ProgressChanged(ByVal sender As Object, _
                                              ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
        Me.ProgressBar1.Value = e.ProgressPercentage
        Me.lblStatus.Text = TryCast(e.UserState, String)
    End Sub

    'This method is executed in the UI thread.
    Private Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As Object, _
                                                     ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
        Me.lblStatus.Text = "Operation complete"
    End Sub





    Private Sub alexaList_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)

        Dim aLink As String
        Dim aLinkNew As String
        aLink = alexaList.SelectedItem.ToString

        Dim myStartIndex As Integer = aLink.IndexOf("h") + 1 '// add +5 to not add "<pre>" to final result.
        '// get location of </pre> by index.
        Dim myEndIndex As Integer = aLink.IndexOf("com") + 3
        '// subtract to get a total number for the substring length result.
        Dim myStringLengthToExtract As Integer = myEndIndex - myStartIndex
        '// display result.
        aLinkNew = aLink.Substring(myStartIndex, myStringLengthToExtract)

        If aLinkNew = "t" Then

        Else

            alexaBrowser.Navigate("h" & aLinkNew)
        End If

    End Sub




End Class

Any suggestions?

[Grimfort]

Yes, read the documentation! Don't just read it, really read it! It has a note in bright yellow that specifically states:

You must be careful not to manipulate any user-interface objects in your DoWork event handler. Instead, communicate to the user interface through the ProgressChanged and RunWorkerCompleted events.

All you have done is thrown your code into the DoWork method. You need to seperate out the links into a list/collection/array, then put the answers into another list/collection/array. When you are ready to .ReportProgress and then put the answers from the list into the controls on the form.

[Grimfort]

The OP has sent me PM asking for more help, so I will repost in here some questions to help him/her out.

In your DoWork method, this is the first time you use a control. The answer is you don't.

Do Until intList = linksList.Items.Count
Dim linkUrl As String = "http://www.alexa.com/search?q=" & Me.linksList.Items.Item(intList)


Before you even start DoWork, you need to extract the data into a list:

Private Links As New Generic.List(Of String)

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click

   For Each strItem As String Me.linksList.Items
      Links.Add(strItem)
   Next

   Me.BackgroundWorker1.RunWorkerAsync()

End Sub

'Then in the DoWork you use the Links list not the control

For Each strItem As String In Links
  .....
Next

Note, this is an example, you can use a string array instead of a List(Of) or you might use a queue instead, possibly you might use a Items.CopyTo. Different people use different methods depending on their use or preference.