Hello Keven,
You can use the built in ASP.NET validation with
RadListBox. To validate for selected item of a listbox, just specify the ID of the listbox in the
ControlToValidate property of the
RequiredFieldValidator. Validation checks to see if there is at least one selected item.
For more information:
http://www.telerik.com/help/aspnet-ajax/listbox-validation.html
Thanks,
Princy.