Hi Claudio,
Please see the documentation for the Listbox class here as it lists all of the methods, properties and events that are available to you.
If this is a single selection Listbox then the following will return the selected index and display it in a textbox.
class-id testlistselect.Form1 is partial
inherits type System.Windows.Forms.Form.
working-storage section.
method-id NEW.
procedure division.
invoke self::InitializeComponent
goback.
end method.
method-id button1_Click final private.
procedure division using by value sender as object e as type System.EventArgs.
invoke listBox1::"Items"::"Add"("Item 1")
invoke listBox1::"Items"::"Add"("Item 2")
invoke listBox1::"Items"::"Add"("Item 3")
invoke listBox1::"Items"::"Add"("Item 4")
end method.
method-id listBox1_SelectedIndexChanged final private.
procedure division using by value sender as object e as type System.EventArgs.
declare selIndex as binary-long = listBox1::SelectedIndex
set textBox1::Text to selIndex::ToString
end method.
end class.