Listbox “IsSelected” binding only partially working

Question

I have a ListBox that I populate dynamically via a binding (this is defined in a DataTemplate, which is why the binding is somewhat unusual):

Answer 1

I think the behavior you're seeing is to due to VirtualizingStackPanel.IsVirtualizing which is True by default when binding to ItemsSource of ListBox

if you for eg set your ListBox such as:

or

  ...
  
    
      
    
  

then you should see all your bound items have their IsSelected updated accordingly with Ctrl+A or Shift + ...

Properties such as Count of the collection even with virtualization would report the correct value to accommodate for things like computing the required ScrollBar.Height. Items which are outside the View-port do not get rendered hence no bindings are in effect on them until they actually get used.