Cross-Threaded ComboBox???

Cross-Threaded ComboBox???

• boyapati

  Hello RandomBlink

  As a Guess, you are running procedures on threads other than the one that the containing form is on. This is usually done so that the interface remains responsive even during long running processes.

  You can not update display data from a thread other than the thread that the dsplay control is contained on. On of the easiest ways to accomplish this in Net 2005 is to use the background worker.

  Here's an example:

  Imports system.ComponentModel

  Public Class Form1

  Private WithEvents MyThread As BackgroundWorker

  Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

  MyThread = New BackgroundWorker

  End Sub

  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

  Dim Myvalue As String = "Hello World"

  If Not MyThread.IsBusy Then

  MyThread.RunWorkerAsync(MyValue)

  End If

  End Sub

  Private Sub MyThread_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles MyThread.DoWork

  Dim TestString As String = e.Argument

  '------------------------------------

  'call long running procedure here

  '------------------------------------

  e.Result = TestString

  End Sub

  Private Sub MyThread_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles MyThread.RunWorkerCompleted

  '-----------------------

  'return value from the

  'backgrounder thread

  'now you can access the

  'display thread

  '-----------------------

  MsgBox(e.Result)

  End Sub

  End Class

  Later :)

  Ibrahim

  
  

• Ruurd Boeke

  Hi

  As already stated, it sounds like the UpdateAttachmentList method is running on a thread that differs from the thread that was used to create the combo box. This means that anytime you interact with the combo box, you need to coersce the other thread to run your code for you.

  The simplest way to do this is to use the Invoke method of the control and pass it a delegate (a callback to a method). When the callback is invoked it will be running on the appropriate thread and your current thread will be suspended.

  Lets try a simple example of how you might architect your solution ...

  private delegate sub fn1(of T)(byval arg as T) : private delegate sub fn2(of T, T1)(byval arg1 as T, byval arg2 as T1)

  private shared sub clearcombobox(byval cb as combobox)

  if cb.invokerequired then

  cb.invoke(new fn1(of combobox)(addressof clearcombobox), cb)

  else : cb.items.clear

  end if

  end sub

  private shared sub addcomboitem(byval cb as combobox, byval txt as string)

  if cb.invokerequired then

  cb.invoke(new fn2(of combobox, string)(addressof addcomboitem), cb, txt)

  else : cb.items.add(txt)

  end if

  end sub

  now instead of interacting direct with the combo box in your updateattachmentlist method, you'd call the clearcombobox method instead. When you want to add to the combo box, call the addcomboitem method and let it do the work for you.

  Hope this helps, but if you need some further help, please le me know.

  Richard

  
  

• Gloria123

  Hi,

   Is cmb_References on another FORM

   If so remove the Me. reference and replace it with the relevant location or try leaving it out....

   

  I'm suprised the line>>

  Dim myCmb As ComboBox 'isn't giving a squiggly line under the word ComboBox

  Have you got an Imports line in your code

   

   

  Dim myCmd As System.Windows.Forms.ComboBox

  myCmb=ComboBox1

  myCmb.Items.Add("Hello") ' works for me and so does.>>

  Dim myStr As String = "there!!"

  myCmb.Items.Add(myStr)

  ''''myCmb.Items.Clear()   '   This line works too if you take the 1st four comment marks out. ;-) 

   

   

   

  Regards,

  S_DS

   

  
  
  

• xulei

  You cannot call UI methods and properties directly from another thread than the one the form was created on. You need to use Invoke to marshall the call to the control's thread.

  (If you're using .Net 2.0, consider using the BackgroundWorker class and communicate with the UI via the ProgressChanged event).
  

  http://msdn2.microsoft.com/en-us/library/b2zk6580(vs.80).aspx

  Something which may also be useful reading.

  http://www.informit.com/guides/content.asp g=dotnet&seqNum=194&rl=1

  If you are using just simple forms and not specifically using different threads then the following works and should mention anything about thread.   It basically sets up a listbox on one form and allows you to create a different form within the same application and as long as you have a reference to the original form/control and your one the same thread then you can modify the listbox items without a problem,

  Public Class Form1
    
      Private Sub CreateASecondForm(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
          Dim x As New form2
          x.Form1Var = Me '//Reference to this form
          x.Show()
      End Sub

      Private Sub BtnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnAdd.Click
          Me.ListBox1.Items.Add("abc")
      End Sub
  End Class

   

  Public Class Form2
      Public Form1Var As Form1

      Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
          If Form1Var IsNot Nothing Then
              Form1Var.ListBox1.Items.Clear()
          End If
      End Sub

      Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
          If Form1Var IsNot Nothing Then
              Form1Var.ListBox1.Items.Add("xyz")
          End If
      End Sub
  End Class

  
  

• Sushisource

  Additional Help ( heck, you offered! )

  I actually do.

  Two questions...

  #1) The program now hangs once it has entered the clearComboBox function... or the addComboBoxItem functions and does the Invoke option. I can't figure out why

  #2) I want to add column'd items to the ComboBox in question. I want to pass an itemID as well as an ITEM. That way when the user selects an item from the list I can pass the itemID to an appropriate function. However, columns are no longer part of the ComboBox as I can tell. Do you know how to do this

  
  
  
• Cross-Threaded ComboBox???