Cancelling The ListView SelectedIndexChanged Event

I have a ListView that populates some fields whenever the user selects an item. However, if they change the data in the fields and then select a new item without saving, I want to display a message box that warns that their changes will be lost, and asks them to confirm whether to continue. Sounds simple.

Unfortunately, the ListView's SelectedIndexChanged event cannot be cancelled, and it's actually called for each change in the selection. So, for example, if item 1 is selected and the user then selects item 2, the event is called twice: once to remove the selection of item 1, and then to add the selection of item 2. If you go from having one item selected to having five (for example), the event fires to remove your original selection, and then again for each of your new selections - six hits in all.

Obviously, this makes it a bugger to determine when the event should be cancelled and reset back to the original. I obviously don't want my message box appearing six times....