asked on

Multithreading, how to access property on the new form

Hi experts

i have a main form that launch many other forms

so i do that :

on click button to launch a new form

Dim nThread As New Thread(AddressOf newThread)
nThread.Name = Me.txtBxComputerName.Text & "_" & Me.TreeView.SelectedNode.Text
MyThreads.Add(nThread) 'arraylist
nThread.Start()

the sub :

Private Sub newThread()
Dim frm As New MainReader 'the form to load

frm.setComputerName = Me.txtBxComputerName.Text.Trim

frm.setSelectedQueue = Me.TreeView.SelectedNode.Text

Try
Application.Run(frm)
Catch ex As System.InvalidOperationException
' description :A main message loop is already running on this thread.
MsgBox("Cannot open the reader : " & ex.Message)
End Try
End Sub

i have an exception on these lines :

frm.setComputerName = Me.txtBxComputerName.Text.Trim

frm.setSelectedQueue = Me.TreeView.SelectedNode.Text

illegal cross...

so i set Control.CheckForIllegalCrossThreadCalls on False

but i'm sure that theres is a better solution to access property of a new form

can you help me please?

kind regards

Nandakumar Sakthivel

Can u post me the complete error message with stack trace

Thanks,
Nanda

Dnx_7

ASKER

System.InvalidOperationException was unhandled
Message="Cross-thread operation not valid: Control 'TreeView' accessed from a thread other than the thread it was created on."
Source="System.Windows.Forms"
StackTrace:
at System.Windows.Forms.Control.get_Handle()
at System.Windows.Forms.Control.SendMessage(Int32 msg, Int32 wparam, Int32 lparam)
at System.Windows.Forms.TreeView.get_SelectedNode()
at AutomationReader.frmMain.newThread() in C:\Projet .NET\AutomationReader\AutomationReader\frmMain.vb:line 191
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.runTryCode(Object userData)
at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()

regards

arif_eqbal

Hi Dnx_7
you can't access controls like this from a new thread, the Controls are in a separate UI thread and they are not directly accessible from outside the Thread boundary
you need to pass arguments to your Thread Function so that the Properties can be set

Try something like this

Public Class ThreadData
Public ComputerName As String
Public SelectedQueue As String

Public Sub newThread()
Dim frm As New MainReader
frm.setComputerName = ComputerName
frm.setSelectedQueue = SelectedQueue
Try
Application.Run(frm)
Catch ex As System.InvalidOperationException
' description :A main message loop is already running on this thread.
MsgBox("Cannot open the reader : " & ex.Message)
End Try
End Sub
End Class

Then to call it

Dim ThData As New ThreadData
ThData.ComputerName = Me.txtBxComputerName.Text.Trim
ThData.setSelectedQueue = Me.TreeView.SelectedNode.Text

Dim nThread As New Thread(AddressOf ThData.newThread)
nThread.Name = Me.txtBxComputerName.Text & "_" & Me.TreeView.SelectedNode.Text
MyThreads.Add(nThread)
nThread.Start()

#I have typed out the code here so take care of some Typo or some silly mistake :-)

ASKER CERTIFIED SOLUTION

Nandakumar Sakthivel

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SOLUTION

Jeff Certain

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Jeff Certain

My two cents worth? 20/80 split between Nanda and Arif.