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Cross-thread operation not valid: Control

Posted on 2010-11-26
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Last Modified: 2012-05-10

I am trying to load a combo box from a web service.  I am doing this through a callback Display lists.  Display lists receives the Lists parameter fine when called. So the web service call itself is working fine.   But when i tries to bind the list to the combo box i get the error  "Cross-thread operation not valid: Control '' accessed from a thread other than the thread it was created on."  

I have tried re-factoring this in many ways using background workers etc I even tried to create my own custom control who sole purpose was to load this data.  All methods seem to give me the same error.




 
Private WithEvents M_List As New IContact.List

  
Private Sub ctrl_ImportGroups_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Me.M_List.LoadLists()
End Sub



Public Sub DisplayLists(ByVal Lists As List(Of IContact.List)) Handles M_List.onLoadListsCompleted
        Me.cmbIContactList.DataSource = Lists
        Me.cmbIContactList.ValueMember = "listid"
        Me.cmbIContactList.DisplayMember = "name"
End Sub

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Question by:Kevin Robinson
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2 Comments
 
LVL 13

Accepted Solution

by:
Naman Goel earned 500 total points
ID: 34218607
The Reason:
If you are from the old COM/COM+ Days, you must remember that an STA (Single Threaded Apartment) can only be accessed by the thread that originally created it. All the controls on the form are created by the main thread of the application, so only the main thread can update it. When your asynch webservice call returns it is calling the DisplayLists(ByVal Lists As List(Of IContact.List)) Handles M_List.onLoadListsCompleted
event handler in the context of a different thread. This thread is not allowed to touch any controls on the form

Please refer to modified code
Private WithEvents M_List As New IContact.List

  
Private Sub ctrl_ImportGroups_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Me.M_List.LoadLists()
End Sub

Private Delegate Sub UpdateUI(ByVal Lists As List(Of IContact.List))


Public Sub DisplayLists(ByVal Lists As List(Of IContact.List)) Handles M_List.onLoadListsCompleted
        Me.Invoke(New UpdateUI(AddressOf UpdateUIMethod), Lists)
End Sub

Public Sub UpdateUIMethod(ByVal Lists As List(Of IContact.List))
	Me.cmbIContactList.DataSource = Lists
        Me.cmbIContactList.ValueMember = "listid"
        Me.cmbIContactList.DisplayMember = "name"
End Sub

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LVL 3

Author Closing Comment

by:Kevin Robinson
ID: 34225141
Great.  Almost had that actully
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