I tried ListSelectionNotificationReceiver on ListBox1 in Window1. In the Opening event of Window1 I added the following statement:
ListBox1.AddListSelectionNotificationReceiver(MyReceiver1)
MyReceiver1 is part of Windows1 and refers to MyReceiver with the selected interface ListSelectionNotificationReceiver. The class MyReceiver defines the methods SelectionChanged (MessageBox “Hello1”)) and SelectionChanging (MessageBox(“Hello2”)).
A new selection of a row in ListBox1 doesn’t display a message. What is wrong in my example?
How are you calling the MyReciever1.SelectionChanged()? and MyReciever1.Changing()?
@brian_franco I try to understand the working from the description of the methods in the documentation . The ListBox1.AddListSelectionNotificationReceiver(MyReceiver1) makes the connection with the class MyReceiver (no super) but with the interface ListSelectionNotificationReceiver (documentation: “The receiver passed is notified when the list selection changes”. The methods SelectionChanged and SelectionChanging are called by the receiver (documentation:" SelectionChanged : The method called for this receiver to run its action." “SelectionChanging: The method called for this receiver to run its action.” ). Both methods must be provided.
But, I must admit that I doubt about my solution (Although there are no compilation errors …).
@Carlos_De_Backer
Learning is good 
You created the class MyReciever, set it to include the listSelectionNotificationReciever Interface, added to those 2 methods.
In Window1 Open you Instantiated MyReciever1 As New MyReciever, correct?
Just for test purposes under where you instantiated MyReciever1 add below MyReciever1.SelectionChanging
Try and let me know if you get the MessageBox
@brian_franco No problem to make MyReceiver1 an instance of MyReceiver (a compilation error would appear if this were not correct). Now, I even dropped the class MyReceiver on the window.
Of course, the instruction MyReceiver1.SelectionChanging shows a message (“Hello2”): you simply call the class method SelectionChanging directly. But this is not the solution. The purpose of the notifier is to send a ‘changed’ notifier signal when you select a new row in the ListBox.
@Carlos_De_Backer
OK and I fully understand about what would cause a compilation error.
I was just trying to diagnose and help
Anyway I replied to your second replicate question https://forum.xojo.com/t/listselectionnotificationreceiver-a-frustrating-problem/84319/9