limit selections in a listbox in vb.net

Question

I am creating a web control in vb.net that contains a list box. I know that I am able to choose a selection mode of Single or Multiple, but I am interested in limiting a Multiple selection to a predetermined size. For example preventing a user from selecting more than 3 items.

I have tried adding a Validator to the list box, which might be a way forward, but was wondering if there was a way of changing the selection mode to something a bit different than just the two specified ones.

Any ideas or suggestions would be greatly appreciated

Edit: Unfortunately due to project limitations I am unable to use Javascript on this problem. very annoying I know!

Answer 1

You could try handling one of the SelectedIndexChange events. With multiple selection, each time this event fires you can check how many items have already been selected. If it is more than you want, have the handler deselect the one that was just selected. You could also put up a little red label under the listbox saying "Sorry, no more than [n] selections allowed."

EDIT: Just noticed you said WEB. Same theory applies, but it would have to be done using Javascript.

Answer 2

On the SelectedIndexChanged event, write a short piece of code that will check the number of selected items and if it is greater than your desired amount, unselect the new selection (or unselect the oldest one, or however you wish this to operate).