vb.net - background thread issue - vb.net

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.

Answer 2

You can only access UI controls from the UI thread.
I suggest reading this first: http://www.albahari.com/threading/

Answer 3

As others have mentioned, it is forbidden (for good reasons) to update UI elements from a non-UI thread.
The canonical solution is as follows:
Test whether you are outside the UI thread
If so, request for an operation to be performed inside the UI thread
[Inside the UI thread] Update the control.
In your case:
Private Sub bgFindThread()
If lblStatus.InvokeRequired Then
lblStatus.Invoke(New Action(AddressOf bgFindThread))
Return
End If
lblStatus.Text = "test"
End Sub
The only thing that changed is the guard clause at the beginning of the method which test whether we’re inside the UI thread and, if not, requests an execution in the UI thread and returns.

Answer 4

You can use a delegate to update UI controls in a background thread.
Example
Private Delegate Sub bkgChangeControl(ByVal bSucceed As Boolean)
Private dlgChangeControl As bkgChangeControl = AddressOf ChangeControl
Private Sub threadWorker_ChangeControl(ByVal bSucceed As Boolean)
Me.Invoke(dlgChangeControl, New Object() {bSucceed})
End Sub
Private Sub ChangeControl()
Me.lable="Changed"
End Sub
'In your background thread, call threadWorker_ChangeControl.