Listbox + Background Worker (Cross thread problem)

[HunterTTP]

Alright so I will try and make this as simple a possible. For this example, there is listbox1, and button3.

There are 10 items in listbox1 (A-T).

The user selects 1 item and then clicks Button3.

Button3 Starts the background worker.

BackgroundWorker3 downloads a file using My.Computer.Network.DownloadFile ("google.com/" & ItemSelection)

ItemSelection is "Dim(ed)" in the background worker -> Dim ItemSelection = ListBox1.SelectedItem

So what should happen is, the user selects the Item, The user clicks "go", the background worker attempts to download from (if selection was "A") google.com/A.

But what actually happens is, I get a "cross thread" error about how I cannot reference back to ItemSelection because it is not being used on the thread is was created on.

Example Code-

Imports System.IO
Imports Shell32

Public Class Form1

    Dim Desktop As String = My.Computer.FileSystem.SpecialDirectories.Desktop
    Dim Username As String = Environment.UserName
    Private P As Integer

...

Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click

        BackgroundWorker3.RunWorkerAsync()

    End Sub

...

 Private Sub BackgroundWorker3_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker3.DoWork

            Dim ItemSelection = ListBox1.SelectedItem

            My.Computer.Network.DownloadFile("http://google.com/" & ItemSelection, Desktop)

    End Sub

The Error




Any help would be great!


Hunter

[Inferrd]

Try substituting

Dim itemText = ListBox1.Text

for

Dim ItemSelection = ListBox1.SelectedItem

and ammending the rest of the code accordingly.

[HunterTTP]

Try substituting

Dim itemText = ListBox1.Text

for

Dim ItemSelection = ListBox1.SelectedItem

and ammending the rest of the code accordingly.

Don't you mean the other way around? Just clarifying.

[Inferrd]

I mean replace

Dim ItemSelection = ListBox1.SelectedItem

with

Dim itemText = ListBox1.Text

and then replace all subsequent occurences of ItemSelection with itemText

And after all that, please don't tell me it doesn't work

[HunterTTP]

Sorry bud, no dice.

[IanRyder]

Hi,

This is a Cross Thread issue which is why things are not working and nothing to do with the properties and methods which are trying to be demonstrated in the examples given so far.

When you implement a secondary thread you cannot directly access the UI thread (the initially started thread directly) since this will cause a Cross Thread exception.

The way to do this correctly is to pass, as a parameter, the information you need to use in a secondary thread from the UI thread. Have a look at this example below which passes a selected ListBox value to the BackgroundWorker:-

Public Class Form1

  Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
    With ListBox1.Items
      .Add("A")
      .Add("B")
      .Add("C")
    End With
  End Sub

  Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    If Not ListBox1.SelectedIndex = -1 Then
      BackgroundWorker1.RunWorkerAsync(ListBox1.SelectedItem.ToString)
    End If
  End Sub

  Private Sub BackgroundWorker1_DoWork(sender As System.Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
    MsgBox("Do your work here using " & e.Argument.ToString & " as a paremeter within the secondary thread")
  End Sub

  Private Sub BackgroundWorker1_RunWorkerCompleted(sender As Object, e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
    MsgBox("Done!")
  End Sub
End Class

I hope that helps.

Cheers,

Ian

[Inferrd]

You know, I swear before you edited your code, you had

Dim ItemSelection = ListBox1.SelectedItem

declared at Form level, outside the BGW's DoWork event handler.

Of course it will give problems referencing the ListBox inside the DoWork event handler, as that runs on a thread that is not the UI's thread and so you can't access any Form's Controls in there.

In any case, I prefer Ian's solution of passing the value as an argument rather than accessing a (disappeared) global variable

[HunterTTP]

Alright so this is what I have.

Imports System.IO
Imports Shell32

Public Class Form1

    Dim Desktop As String = My.Computer.FileSystem.SpecialDirectories.Desktop
    Dim Username As String = Environment.UserName

...

Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click

        BackgroundWorker3.RunWorkerAsync(ListBox1.SelectedItem.ToString)

    End Sub

...

Private Sub BackgroundWorker3_DoWork(sender As System.Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker3.DoWork

            My.Computer.Network.DownloadFile("http://google.com/" & e.Argument.ToString, Desktop)

    End Sub

And that gives me some kind of "call stack" error?

[HunterTTP]

Figured it out. I forgot the "If Not ListBox1.SelectedIndex = -1 Then". Thanks!