Question

For some reason a background thread in my app can’t change any labels, textbox values, etc on my main form. There is no compile errors, when the thread executes nothing happens.

Here is some example code:

Imports System.Threading

Public Class Class1
    Dim tmpThread As System.Threading.Thread

    Private Sub bgFindThread()
        Form1.lblStatus.Text = "test"
    End Sub

    Public Sub ThreadAction(ByVal Action As String)
        If Action = "Start" Then
            tmpThread = New System.Threading.Thread(New System.Threading.ThreadStart(AddressOf bgFindThread))
            tmpThread.Start()
        ElseIf Action = "Abort" Then
            If tmpThread.IsAlive = True Then tmpThread.Abort()
        End If
    End Sub

End Class

Can someone let me know what I’m doing wrong?

Answer 1

AFAIK code above will throw an exception IllegalCrossThreadException, it is because the background thread is not the same as UI thread and background try to set value on other thread. So windows form check every thread that work properly.
You can set Control.CheckForIllegalCrossThreadCalls to false to make it works.

Code below is when setting property is not run

Add into your code
------------------------------
Delegate Sub MyDelegate()
Private Sub RunMyControl()
  lblStatus.Text = "test"
End Sub

Change your code
------------------------------
Private Sub bgFindThread
  lblStatus.BeginInvoke (New MyDelegate(AddressOf RunMyControl))
End Sub

The method asyncronsly run code from background thread to UI thread.