如何验证列表框是否为空（客户端）

I'm working with ASP.NET 3.5. I have a list box that users must add items to (I've written the code for this). My requirement is that at least one item must be added to the listbox or they cannot submit the form. I have several other validators on the page and they all write to a ValidationSummary control. I would like this listbox validation to write to the Validation Summary control as well. Any help is greatly appreciated. Thank you.I'm working with ASP.NET 3.5. I have a list box