Cross-thread operation not valid

Question

I was doing the following to try out threadings but get the \'Cross-Threading\' error in the line txtBox.Text = DataGridView2.Rows.Item(iloop).Cells(3).Value, a

Answer 1

It's not legal to access a UI element from anything other than UI thread in .Net code. Hence when you try and use the DataGridView2 instance from a background thread it rightfully throws an exception.

In order to read or write to a UI component you need to use Invoke or BeginInvoke method to get back on the UI thread and make the update. For example

If TypeOf cCtrl Is TextBox Then
    Dim txtBox As TextBox
    txtBox = cCtrl
    txtBox.Invoke(AddressOf UpdateTextBox, txtBox, iloop)
End If

Private Sub UpdateTextBox(txtBox as TextBox, iloop as Integer) 
    If (txtBox.Name.Substring(9, 6)) = (DataGridView2.Rows.Item(iloop).Cells(0).Value.substring(0, 6)) Then
        txtBox.Text = DataGridView2.Rows.Item(iloop).Cells(3).Value 'This is the part that says "Cross-thread operation not valid: Control 'txt_Time_00_000' accessed from a thread other than the thread it was created on."
    End If
End Sub

Answer 2

@JaredPar you have the basic idea but that code itself won't compile (unless I'm missing something). For VB9 or less you need to declare an actual delegate and invoke that:

    ''//The delegate is only needed for the VB 9 or less version
    Private Delegate Sub UpdateTextBoxDelegate(ByVal txtBox As TextBox, ByVal value As String)

    If TypeOf cCtrl Is TextBox Then
        Dim txtBox As TextBox
        txtBox = cCtrl
        ''//Perform validation logic here
        If (txtBox.Name.Substring(9, 6)) = (DataGridView2.Rows.Item(iloop).Cells(0).Value.ToString().Substring(0, 6)) Then
            ''//Call the update method with our textbox and value
            UpdateTextBox(txtBox, DataGridView2.Rows.Item(iloop).Cells(3).Value.ToString())
        End If
    End If

    Private Sub UpdateTextBox(ByVal txtBox As TextBox, ByVal value As String)
        ''//Basically ask the textbox if we need to invoke
        If txtBox.InvokeRequired Then
            ''//For VB 9 or less you need a delegate
            txtBox.Invoke(New UpdateTextBoxDelegate(AddressOf UpdateTextBox), txtBox, value)
        Else
            txtBox.Text = value
        End If
    End Sub

For VB 10 we can finally use anonymous subs so we can completely get rid of the delegate:

    If TypeOf cCtrl Is TextBox Then
        Dim txtBox As TextBox
        txtBox = cCtrl
        ''//Perform validation logic here
        If (txtBox.Name.Substring(9, 6)) = (DataGridView2.Rows.Item(iloop).Cells(0).Value.ToString().Substring(0, 6)) Then
            ''//Call the update method with our textbox and value
            UpdateTextBox(txtBox, DataGridView2.Rows.Item(iloop).Cells(3).Value.ToString())
        End If
    End If

Private Sub UpdateTextBox(ByVal txtBox As TextBox, ByVal value As String)
    If txtBox.InvokeRequired Then
        ''//For VB 10 you can use an anonymous sub
        txtBox.Invoke(Sub() UpdateTextBox(txtBox, value))
    Else
        txtBox.Text = value
    End If
End Sub