Unselects all list box items.
If the filter criteria is applied to the list box on the client side, the UnselectAll method affects only the filtered list box items. If an end-user implements the following actions sequentially - filtering, selecting all the filtered items and then clearing the applied filter criteria - only the previously filtered list box items remains selected. It means that the UnselectAll method is applied only to the currently visible list box items.
For more information about multiple selection, read the Multi-Selection Mode topic.