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:

<ListBox VirtualizingStackPanel.IsVirtualizing="False" SelectionMode="Extended" ItemsSource="{Binding DataContext.ResultList, RelativeSource={RelativeSource AncestorType=Window}}">

or

<ListBox ...>
  ...
  <ListBox.ItemsPanel>
    <ItemsPanelTemplate>
      <StackPanel />
    </ItemsPanelTemplate>
  </ListBox.ItemsPanel>
</ListBox>

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.