Validating a checkListBox? - vb.net

Question

I have two forms (1 and 2). I have been battling with some code that would prevent the user from selecting an item in the checkedListBox that was not added into ListBox2 from the previous form (form1).
The code I have is kind of weird because even if the item was added to listbox2 from form1, it continues to display the msgBox. I need the msgBox to display only to those items that were not added to listbox2, form1.
Here is what I have:
Public Class Form1
Dim ActSubject As Boolean
Public Function ActivateSubject() As String
Return ActSubject
End Function
Private Sub ListBox2_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListBox2.TextChanged
Dim x As New Items
x.AvailableItems = ListBox2.Items.ToString
For Each x In ListBox2.Items
If ListBox2.Items.Contains(x) Then
ActSubject = True
Else
ActSubject = False
End If
Next
End Sub
End Class
Public Class Form2
Dim HaveActSubject As Boolean = Form1.ActivateSubject
Private Sub CheckedListBox1_SelectedValueChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles CheckedListBox1.SelectedValueChanged
If HaveActSubject = False Then
MsgBox("Sorry! Subject should be activated six month before registration.")
End If
Return
End Sub
End Class

Answer 1

Public Class Form1
Public Function ActivateSubject(itm as string) As String
Return ListBox2.Items.Contains(itm)
End Function
End Class
Public Class Form2
Dim HaveActSubject As Boolean = Form1.ActivateSubject
Private Sub CheckedListBox1_SelectedValueChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles CheckedListBox1.SelectedValueChanged
If Form1.ActivateSubject(CheckedListBox1.selectedValue) = False Then
MsgBox("Sorry! Subject should be activated six month before registration.")
End If
Return
End Sub
End Class