Change UI parameters using thread in separate class - vb.net

Question

I would like to be able to change an element of the main form inside a thread declared in a separated class (In this case I want to change a label text).
I tried the following code:
Form1:
Imports System.Threading
Public Class Form1
Public counter As Integer = 0
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim SecondClassObject As New SecondClass()
End Sub
End Class
SecondClass:
Imports System.Threading
Public Class SecondClass
Public Thread As New Thread(AddressOf Increment)
Public counter As Integer = 0
Sub New()
Thread.Start()
End Sub
Sub Increment()
While True
Form1.Label1.Text = counter
counter += 1
End While
End Sub
End Class
If I do the same thing using a thread but in the form code itself than the label text will change:
Imports System.Threading
Public Class Form1
Public counter As Integer = 0
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim thread As New Thread(AddressOf Increment)
thread.Start()
End Sub
Sub Increment()
While True
Label1.Text = counter
counter += 1
End While
End Sub
End Class
How should I do in order to archieve the same result using a thread in a separated class?

Answer 1

First, I do want to point out that you need to use an Invoke/Callback to safely set the label's text from the secondary thread. I don't know if you're doing that in your actual code base, but wanted to specify anyways.
Now, focused on the actual question, I believe that the easiest way to do as requested is to pass a reference to the original instance of Form1 to your SecondClass. Having a reference to the parent, means that you would be able to manipulate the parent's publicly exposed elements as needed.
Consider the below:
Public Class Form1
Public counter As Integer = 0
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim secondClass As New SecondClassObject(me)
End Sub
Delegate Sub SetTextCallback (value as String)
Public Sub SetText (value as string)
if me.Label1.InvokeRequired Then
dim d as New SetTextCallback(addressOf SetText)
Me.Invoke(d, New Object() {value})
Else
me.label1.text = value
End If
End Sub
End Class
Public Class SecondClassObject
private _parent as Form1
private myThread As New Thread(AddressOf Increment)
Public Sub New (byref p as Form1)
me._parent = p
myThread.Start()
End Sub
Sub Increment()
While True
Me._parent.SetText(counter)
counter += 1
End While
End Sub
End Class
What is happening is that the a reference to the parent is passed into the second class as a constructor, doing so allows us to interact with the parent from the second class.
Now, that is one way, but other options do exist. Things such as specialized events/handlers or wiring up databinding between the Form1.Label1 and a property exposed from the SecondClassObject. Even a singleton pattern, where the value to be incremented is shared between all instances, so when the SecondClassObject increments it, Form1 would be aware and know to update Label1.
Also, please note that the above code is for example purposes, and is missing things such as a defined declaration for Label1.

Answer 2

On windows it's not possible to change the UI from a non UI thread.
It looks like that you have to use Control.Invoke or better Control.BeginInvoke.
The problem with using Control.Invoke is that it's executed on the UI thread and the calling thread waits for completion. Which would be bad when your background worker continusly does some computations.