Listbox box validator

[goyal.mayank]

Hi,

We have two list boxes say ListBoxA and ListBoxB.
User will be populating some of the items from ListBoxA to ListBoxB.

Now we want to attach a validator to ListBoxB will fill fail when
ListBoxB contains at least zero items.

I have tried do it through custom validator. But its not working.
Those validators are getting fired only when there is some items in
ListBoxB.

Any help will be highly appriciated.

Regards,
Mayank Goyal