You need to learn about the CustomValidator.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfsystemwebuiwebcontrolscustomvalidatorclasstopic.aspMost of the work is building a method to detect the state of the RadioButtonList or CheckBoxList. These controls document that you use the SelectedIndex property to determine their state. If SelectedIndex is not equal to -1, it indicates a specific radiobutton or checkbox that is checked.
So your customvalidator will evaluate that SelectedIndex is not -1. If it is, your evaluation method will report IsValid is false.
FYI: While my Professional Validation And More Suite is $200, you can get its "
VAM: Essential Validators" module for $50. It gives you 14 validators including my RequiredListValidator, and all of the enhancements I've created to the ASP.NET validation system.
--- Peter Blum
Creator of Professional Validation And More Suite, Peter's Date Package, and Peter's Polling Package
www.PeterBlum.com