equivalent of OnSelect javascript hook in multi-select asp listbox
<p>I have a multi-select <code>&lt;asp:listbox&gt;</code> and a button.  I want to disable the button unless >= 1 item in the listbox is selected.  How can I do this client-side?  I can't find some sort of <code>OnSelect</code> callback, only <code>OnSelectedIndexChanged</code> which from googling seems complicated to use for this behavior, possibly impossible.</p>
