onkeypress="return filtraPassa(this, event, 'THIS_VAR_CHANGES_EVERYPAGELOAD');">

DOMEventHandler domEvent = delegate(object sender, DOMEventArgs e)
{
     DOMEventType eventType = e.Type;
     Console.Out.WriteLine("handled event = " + eventType);
};

element.AddEventListener(DOMEventType.OnKeyPress, domEvent, false);