Cross-thread operation not valid: Control 'lblCounter1' accessed from a thread other than the thread it was created on.


This is a common problem. I will provide solution with an example




This is a simple WinForm with two labels

Below is the working code to update the lables using a thread. 

Here the main point u have to see is that. controls in the main forms cant be updated from another thread. to do it we use an invoke 
Control.Invoke  Method (Delegate) (System.Windows.Forms)
Executes the specified delegate on the thread that owns the control's underlying window handle.


Here are more details on invoke 
Delegates are similar to function pointers in C or C++ languages. Delegates encapsulate a reference to a method inside a delegate object. The delegate object can then be passed to code that calls the referenced method, and the method to be invoked can be unknown at compile time. Unlike function pointers in C or C++, delegates are object-oriented, type-safe, and more secure.

The Invoke method searches up the control's parent chain until it finds a control or form that has a window handle if the current control's underlying window handle does not exist yet. If no appropriate handle can be found, the Invoke method will throw an exception. Exceptions that are raised during the call will be propagated back to the caller.


    Public oThread As System.Threading.Thread
    Public strThread As String = ""
    Public strSelector As String = ""
    Public Sub ThreadWrite()
        Select Case strSelector
            Case "Counter1"
                lblCounter1.Text = strThread
            Case "Counter2"
                lblCounter2.Text = strThread
        End Select
    End Sub


    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        oThread = New System.Threading.Thread(AddressOf ThreadCode)
    End Sub
    Public i As Integer = 0
    Public j As Integer = 0
    Public bFlag As Integer = 0
    Private Sub ThreadCode()
        Do While True
            strSelector = "Counter1"
            strThread = i.ToString
            lblCounter1.Invoke(New MethodInvoker(AddressOf ThreadWrite))
            strSelector = "Counter2"
            strThread = j.ToString
            lblCounter1.Invoke(New MethodInvoker(AddressOf ThreadWrite))
            bFlag = bFlag + 1
            If bFlag = 2 Then
                bFlag = 0
                j = j + 1
            End If
            i = i + 1
            System.Threading.Thread.Sleep(500)
        Loop
    End Sub

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