crossthreading cross class vb.net issue - vb.net

Question

i have my form class, and a second module with some special functions,
when i click a button on my form i run a public function from the second module(which run later other public functions from the second module) in a separated thread, i set SetApartmentState(ApartmentState.STA), and i tried using deletage sub and CheckForIllegalCrossThreadCalls = False, but the problem stays the same, my thread functions (which are on the second module) can't access my form controls, but when i move the functions to the form class everything work again, what do you suggest to solve this issue?
Public Class Form1
Dim T0 As Thread
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
CheckForIllegalCrossThreadCalls = False
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
T0 = New Thread(AddressOf sub1)
T0.SetApartmentState(ApartmentState.STA)
T0.start()
End Sub
End Class
Module Module1
Public Sub Sub1()
msgbox(form1.textbox1.text) 'even if the textbox contains content it returns ""
Function2()
End Sub
Public Function Function1()
'SomeInstructions
msgbox(form1.textbox1.text) 'same problem here
End Function
End module
PS: it dosn't give any error or stop the code while debugging, and i tried to put the sub1 on the form class and the other functions in the module, but then, only he sub1 can access the controls, i tried delegate but i don't know if i have done it right, can anyone make any suggestions please

Answer 1

There are two issues at play here.
One of them is due to the way default forms work in VB.NET. See, in C# you need a concrete instance of type Form1 in order to access its non-static members, whereas in VB.NET Form1 looks like an instance of the form allowing you to write things like Form1.TextBox1.Text. This works fine while you're accessing members of Form1 from the UI thread, but when you try to get it from a background thread, a new instance of Form1 is created, and Form1.TextBox1 seen by that thread actually points to a completely different instance of TextBox.
Another way saying this is that default form instances in VB.NET are thread static.
A way to get around this is to keep a concrete reference to Form1 so that you can pass it around.
When you go
Dim myForm As New Form1
... or
Dim myForm As Form1 = Me
... the 'myForm' variable pointing to that specific Form1 instance can then be passed between threads like any other reference and will not change its meaning.
This, however, brings us to issue #2:
You should not be accessing a UI control (which means any type derived from Control, and that includes Form , from a thread other than the thread that it was created on.
If you absolutely have to, you have to marshal the calls accessing the Control to the thread that it was created on, for example by using Invoke/BeginInvoke (there are other ways too).
Here's a modification of your code to achieve what you want, plus a more complex example which demonstrates multiple thread "switches": gathering interesting state on the UI thread, performing work with it on the background thread, then displaying results on the UI thread.
Imports System.Threading
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
' We don't need this anymore.
' We'll do things right and access
' the UI on the UI thread only.
' CheckForIllegalCrossThreadCalls = False
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
' Note that we're passing a reference
' to THIS instance of Form1 to Sub1 and Sub2.
Dim form As Form1 = Me
' Let's spin up some threads.
Dim T0 As New Thread(Sub() Module1.Sub1(form))
T0.Start()
Dim T1 As New Thread(Sub() Module1.Sub2(form))
T1.Start()
End Sub
End Class
Module Module1
' Note that this sub now accepts
' a reference to an instance of Form1.
Public Sub Sub1(form As Form1)
' This is what we want to do:
Dim action As New Action(Sub() MsgBox(form.TextBox1.Text))
' See if we're on the right thread.
If form.InvokeRequired Then
' Invoke on the thread which created this Form1 instance.
form.Invoke(action)
Else
' Invoke on the current thread.
action.Invoke()
End If
End Sub
' This is a more complex example.
Public Sub Sub2(form As Form1)
' This function will get the text from TextBox1 when invoked.
' It still needs to be invoked on the UI thread though.
Dim getText As New Func(Of String)(Function() form.TextBox1.Text)
Dim text As String
If form.InvokeRequired Then
text = CStr(form.Invoke(getText))
Else
text = getText() ' Shorthand syntax.
End If
' Now that we have the text, let's do some
' intensive work with it while we're on
' the background thread.
For i = 0 To 5
text &= i
Thread.Sleep(100)
Next
' Now we want to show the message box - again, on the UI thread.
Dim showMessageBox As New Action(Sub() MsgBox(text))
If form.InvokeRequired Then
form.Invoke(showMessageBox)
Else
showMessageBox()
End If
End Sub
End Module