update listbox in GUI from other classes - vb.net

Question

I have a listbox on my main vb.net form which I am using to display status messages from the server program I am running. My actual program consists of many different classes (in separate files) and what I would like to be able to do is to call the Sub frm.UpdateList("With Info in Here") from each of the classes to write to the listbox.
If I call the frm.UpdateList or UpdateList from the frm class, it writes to the listbox fine, but if I call it from any other class nothing happens (I don't get an error either).
I have tried with and without making it shared (and changing frm to me) but neither works as I would hope.
Would anyone be able to help me understand why this is not working, I have invoked the item, and it does get added to but just not from a separate class (which is what I need it to do).
Many Thanks!
Private Delegate Sub UpdateListDelegate(ByVal itemName As String)
Public Shared Sub UpdateList(ByVal itemName As String)
If frm.InvokeRequired Then
frm.Invoke(New UpdateListDelegate(AddressOf UpdateList), itemName)
Else
frm.ListBox1.Items.Insert(0, DateTime.Now.ToString & ": " & itemName)
End If
End Sub
Edit: Try 2, with the following thanks to Idle_Mind works on the frm class (frm is the main form and only form) but it still does not write to the listbox when called from other classes (and no errors occur):
Public Shared Sub UpdateList(ByVal itemName As String)
Dim frm As Form = My.Application.ApplicationContext.MainForm
If Not IsNothing(frm) Then
Dim matches() As Control = frm.Controls.Find("ListBox1", True)
If matches.Length > 0 AndAlso TypeOf matches(0) Is ListBox Then
Dim LB As ListBox = DirectCast(matches(0), ListBox)
LB.Invoke(New MethodInvoker(Sub() LB.Items.Insert(0, DateTime.Now.ToString & ": " & itemName)))
End If
End If
End Sub

Answer 1

I have a listbox on my main vb.net form
This will only work on the startup form, and is not really a good design. Consider other approaches as well:
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim soc As New SomeOtherClass
soc.Foo()
End Sub
End Class
Public Class SomeOtherClass
Public Sub Foo()
Dim msg As String = "Hello?!"
Helper.UpdateList(msg) ' <-- You can do this from any class...
End Sub
End Class
Public Class Helper
Public Shared Sub UpdateList(ByVal itemName As String)
Dim frm As Form = My.Application.ApplicationContext.MainForm
If Not IsNothing(frm) Then
Dim matches() As Control = frm.Controls.Find("ListBox1", True)
If matches.Length > 0 AndAlso TypeOf matches(0) Is ListBox Then
Dim LB As ListBox = DirectCast(matches(0), ListBox)
LB.Invoke(New MethodInvoker(Sub() LB.Items.Insert(0, DateTime.Now.ToString & ": " & itemName)))
End If
End If
End Sub
End Class
Other correct approaches, which would require more work on your part, might include:
(1) Pass a reference to your main form into the other classes as you create them. Then those classes can either up the ListBox directly, or possibly call a method in it as suggested by Plutonix. Here's an example of this in action:
Public Class Form1
Public Sub UpdateList(ByVal itemName As String)
ListBox1.Invoke(New MethodInvoker(Sub() ListBox1.Items.Insert(0, DateTime.Now.ToString & ": " & itemName)))
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim soc As New SomeOtherClass(Me)
soc.Foo()
End Sub
End Class
Public Class SomeOtherClass
Private _Main As Form1
Private Sub New()
End Sub
Public Sub New(ByVal MainForm As Form1)
_Main = MainForm
End Sub
Public Sub Foo()
If Not IsNothing(_Main) Then
_Main.UpdateList("Hello?!")
End If
End Sub
End Class
You'd have to modify all of your other classes in a similar fashion so that they can receive an instance of your form.
(2) Make the other classes raise a custom event that the main form subscribes to when those classes are created.