Why does ItemContainerGenerator return null?

Question

I have a ListBox, and I need to set its ControlTemplate to a Virtualizing WrapPanel which is a class that extends VirtualizingPanel, using a style that looks like this:

Answer 1

Most probably this is a virtualization-related issue so ListBoxItem containers get generated only for currently visible items (e.g. https://msdn.microsoft.com/en-us/library/system.windows.controls.virtualizingstackpanel(v=vs.110).aspx#Anchor_9)

I'd suggest switching to ListView instead of ListBox - it inherits from ListBoxand it supports ScrollIntoView() method which you can utilize to control virtualization;

targetListView.ScrollIntoView(itemVM);
DoEvents();
ListViewItem itemContainer = targetListView.ItemContainerGenerator.ContainerFromItem(itemVM) as ListViewItem;

(the example above also utilizes the DoEvents() static method explained in more detail here; WPF how to wait for binding update to occur before processing more code?)

There are a few other minor differences between the ListBox and ListView controls (What is The difference between ListBox and ListView) - which should not essentially affect your use case.