vb.NET You are not allowed to perform an operation across different threads - vb.net

Question

I am running code to read text from pdf files and than create a WordCloud. To inform the user the process is on going I add a BackgroundWorker to my form that shows an image saying Loading. I get an error for operating across different threads.
Private Sub ButtonCreate_Click(sender As Object, e As EventArgs) Handles ButtonCreate.Click
bTextEmpty = False
ListView1.Items.Clear()
ResultPictureBox.Image = Nothing
If ListBox1.SelectedIndex > -1 Then
For Each Item As Object In ListBox1.SelectedItems
Dim ItemSelected = CType(Item("Path"), String)
Dim myTempFile As Boolean = File.Exists(ItemSelected + "\Words.txt")
If myTempFile = False Then
'when we load the form we first call the code to count words in all files in a directory
'lets check if the folder exists
If (Not System.IO.Directory.Exists(ItemSelected)) Then
Dim unused = MsgBox("The archive " + ItemSelected.Substring(ItemSelected.Length - 5, Length) + " was not found",, Title)
Exit Sub
Else
Call CreateWordList(ItemSelected)
End If
End If
'if the words file is empty we cant create a cloud so exit the sub
If bTextEmpty = True Then Exit Sub
'then we fill the wordcloud and Listview from the created textfile
Call CreateMyCloud(ItemSelected + "\Words.txt")
Next
Else
Dim unused = MsgBox("You have to choose an Archive to create the Word Cloud",, Title)
End If
Size = New Drawing.Size(1400, 800)
End Sub
I put above code in a Private Sub and called it from my BackgroundWorker. The error occured at this line: If ListBox1.SelectedIndex > -1 Then
After trying the suggested code I get the same error again:
Public Sub CreateMyCloud(ByVal sourcePDF As String)
Dim WordsFreqList As New List(Of WordsFrequencies)
For Each line As String In File.ReadLines(sourcePDF)
Dim splitText As String() = line.Split(","c)
If splitText IsNot Nothing AndAlso splitText.Length = 2 Then
Dim wordFrq As New WordsFrequencies
Dim freq As Integer
wordFrq.Word = splitText(0)
wordFrq.Frequency = If(Integer.TryParse(splitText(1), freq), freq, 0)
WordsFreqList.Add(wordFrq)
End If
Next
If WordsFreqList.Count > 0 Then
' Order the list based on the Frequency
WordsFreqList = WordsFreqList.OrderByDescending(Function(w) w.Frequency).ToList
' Add the sorted items to the listview
WordsFreqList.ForEach(Sub(wf)
error -> ListView1.Items.Add(New ListViewItem(New String() {wf.Word, wf.Frequency.ToString}, 0))
End Sub)
End If
Dim wc As WordCloudGen = New WordCloudGen(600, 400)
Dim i As Image = wc.Draw(WordsFreqList.Select(Function(wf) wf.Word).ToList, WordsFreqList.Select(Function(wf) wf.Frequency).ToList)
ResultPictureBox.Image = i
End Sub

Answer 1

Should look something more like:
Private Sub ButtonCreate_Click(sender As Object, e As EventArgs) Handles ButtonCreate.Click
If ListBox1.SelectedItems.Count > 0 Then
Dim data As New List(Of Object)
For Each Item As Object In ListBox1.SelectedItems
data.Add(Item)
Next
bTextEmpty = False
ListView1.Items.Clear()
ResultPictureBox.Image = Nothing
BackgroundWorker1.RunWorkerAsync(data)
Else
MessageBox.Show("You have to choose an Archive to create the Word Cloud",, Title)
End If
End Sub
Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Dim data As List(Of Object) = DirectCast(e.Argument, List(Of Object))
For Each Item As Object In data
Dim ItemSelected = CType(Item("Path"), String)
Dim myTempFile As Boolean = File.Exists(ItemSelected + "\Words.txt")
If myTempFile = False Then
'when we load the form we first call the code to count words in all files in a directory
'lets check if the folder exists
If (Not System.IO.Directory.Exists(ItemSelected)) Then
MessageBox.Show("The archive " + ItemSelected.Substring(ItemSelected.Length - 5, Length) + " was not found",, Title)
Exit Sub
Else
Call CreateWordList(ItemSelected)
End If
End If
'if the words file is empty we cant create a cloud so exit the sub
If bTextEmpty = True Then Exit Sub
'then we fill the wordcloud and Listview from the created textfile
Call CreateMyCloud(ItemSelected + "\Words.txt")
Next
End Sub
Private Sub BackgroundWorker1_RunWorkerCompleted(sender As Object, e As RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
Size = New Drawing.Size(1400, 800)
End Sub

Answer 2

To fix the error in your second, edited post:
WordsFreqList.ForEach(Sub(wf)
ListView1.Invoke(Sub()
ListView1.Items.Add(New ListViewItem(New String() {wf.Word, wf.Frequency.ToString}, 0))
End Sub)
End Sub)

Answer 3

You cannot simply access controls outside of the current thread. You first need to call the Invoke method on the target control and then pass a delegate containing the instructions intended to modify the control outside of the current thread. See this article on MSDN on how to do this: https://learn.microsoft.com/en-us/dotnet/desktop/winforms/controls/how-to-make-thread-safe-calls-to-windows-forms-controls?view=netframeworkdesktop-4.8