Unable to start multiple thread

Question

I'm trying to run this code, but when running only the first n listbox looping , the others remain empty. I would like to run the subroutine code (an infinite loop) for each listbox in the form at the same time

This is the Main Form:

Public Class Form1
  Private listSettings As New List(Of ListBox)
  Private nameSettings As New List(Of Label)
  Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    'Variabili Grafiche
    Dim lSettings As ListBox
    Dim i As Integer
    i = 0
    Dim n As Integer
    Dim width As Integer
    Dim height As Integer
    'Variabili Lettura File
    Dim path As New IO.DirectoryInfo("C:\BatchDomoLake\config\")
    Dim diar1 As IO.FileInfo() = path.GetFiles()
    Dim dra As IO.FileInfo
    'Lettura File e Grafica
    For Each dra In diar1
        n = n + 1
    Next
    Dim T(n) As Thread
    width = (Me.Size.Width() / n)
    height = (Me.Size.Height() / 2)
    For Each dra In diar1
        lSettings = New ListBox
        With lSettings
            .Location = New Point(10 + (width * i), 40)
            .Name = "lst" & dra.Name.Replace(".conf", "")
            .Size = New Size(width - 35, height - 40)
            .Visible = True
        End With
        Me.listSettings.Add(lSettings)
        Me.Controls.Add(lSettings)
        Dim reader As StreamReader = My.Computer.FileSystem.OpenTextFileReader(dra.FullName)
        Dim a As String
        Do
            a = reader.ReadLine
            If a IsNot Nothing Then
                lSettings.Items.Add(a)
            End If
        Loop Until a Is Nothing
        'Thread
        T(i) = New Threading.Thread(AddressOf snmpThread)
        T(i).Start(lSettings)
        i = i + 1
    Next
End Sub

This is the subroutine i would like to use for each created listbox:

Private Delegate Sub snmpThreadDelegate(ByVal list As ListBox)
Private Sub snmpThread(ByVal list As ListBox)
    If list.InvokeRequired Then
        list.Invoke(New snmpThreadDelegate(AddressOf snmpThread), New Object() {list})
    Else
        Dim count As Integer
        Dim i As Integer
        count = list.Items.Count - 1
        Do
            For i = 0 To count Step 1
                list.SetSelected(i, True)
                Thread.Sleep(100)
            Next
        Loop
    End If
End Sub

Answer 1

You cannot access/update your UI elements from background threads. Only main UI thread can do operations on your UI objects. This applies to both WinForms and WPF.

For WinForms, you can use BeginInvoke method and pass in a delegate that would be invoked in your UI thread. For WPF, you can use Dispatcher object.