I found a very good article to check that at least one chekbox is selected in a checkboxlist control. Works exactly like required field validator.
http://forums.asp.net/t/1000658.aspx (check answer from user yasserzaid)
<script type=”text/javascript”>
function ValidateRolesSelection(source, args)
{
//if your checkboxlist control is on simple asp.net page
var chkListModules = document.getElementById(‘<%= RolesCheckBoxList.ClientID %>’);
//if your checkboxlist control is inside some user control for e.g in CreateUserWizard
//var chkListModules = document.getElementById(‘<%= (CreateUserWizard1.WizardSteps[0].Controls[0].FindControl(“RolesCheckBoxList”) as CheckBoxList).ClientID %>’);
var chkListinputs = chkListModules.getElementsByTagName(“input”);
for (var i = 0; i < chkListinputs.length; i++)
{
if (chkListinputs[i].checked)
{
args.IsValid = true;
return;
}
}
args.IsValid = false;
}
</script>