The OnClientItemsRequesting client-side event occurs when EnableLoadOnDemand is True and the listbox is about to send a server-side request to load more items. This event fires before the items are added to the listbox' Items collection.
The event handler receives two parameters:
The instance of the listbox firing the event.
An eventArgs parameter containing the following methods:
get_context returns a context object (implements IDictionary) that is passed to the server-side code that handles the request for items.
set_cancel lets you prevent the items request.
This event can be used to cancel the load-on-demand request.