dialog beforeClose Problem

dialog beforeClose Problem

Hi everybody,
can someone please give an effective example for preventing the
closing of a dialog?
Sadly, the official jQuery UI documentation gives only the global
syntax for the beforeClose event.
I tried with this code :
$('#preload').dialog({
                    autoOpen: false,
                    width: 300,
                    bgiframe:true,
                    modal:true,
                    resizable:false,
                    closeOnEscape:false,
                    draggable:false,
                    title:'Chargement en cours ...',
                    beforeClose :function(event,ui){ return false;}
});
but it didn't work ... :(
please help ...
--