[RESOLVED] [2005] How to access to an object from a separate Thread

[paraidy]

Hi all guys, i want to increase a label in Form1 using a thread, but it say "Cross-thread operation not valid: Control 'Label1' accessed from a thread other than the thread it was created on.", how can i access to the label from a thread?

Imports System.Threading
 
Public Class Form1
    Dim par As Thread
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        par.Start()
    End Sub
 
    Public Sub New()
        ' This call is required by the Windows Form Designer.
        InitializeComponent()
        ' Add any initialization after the InitializeComponent() call.
        par = New Thread(AddressOf runs)
    End Sub
    
 
Sub runs()
        Dim i As Long = 0
        While True
            For i = 0 To 300
                i += 1
                label1.Text = i
            Next
        End While
    End Sub
End Class

[gigemboy]

You have to look into using delegates and calling the delegates from the thread. Search this forum for "delegate invoke" or something to that matter and you should pull several threading questions and examples about the subject.

Its not a particularly "simple" matter and takes some time to get used to if you haven't done it, I still have problems with it.

Some links that might help:
http://msdn2.microsoft.com/en-us/lib...69(VS.80).aspx
http://msdn.microsoft.com/msdnmag/is...g/default.aspx
http://www.aspfree.com/c/a/VB.NET/De...h-VB-NET-2005/

[ComputerJy]

You should invoke your method on the correct thread, you can do this delegating the function (sync with Invoke or Async with BeginInvoke).

The restriction is because the UI thread is the main gdi thread that draws your interface, Microsoft does not want you to change things that are under that thread responsibility as you can crash, lock , a priority inversion or generate a race condition on the UI thread. .NET 2.0 enforces that rule that was overlooked on .NET 1.1.

[jmcilhinney]

E.g. setting the Text property of TextBox from a worker thread:

Private Delegate Sub SetTextBoxTextDelegate(ByVal text As String)
 
Private Sub SetTextBoxText(ByVal text As String)
    If Me.TextBox1.InvokeRequired Then
        'This is a worker thread so create a delegate to cross the thread boundary.
        Me.TextBox1.Invoke(New SetTextBoxTextDelegate(AddressOf SetTextBoxText), text)
    Else
        'This is the UI thread so access the control's members directly.
        Me.TextBox1.Text = text
    End If
End Sub

Just call the SetTextBoxText method from wherever you are and it will look after the threading issues itself.

[paraidy]

Great!, Thx a lot