Using RequiredFieldValidator with RadButton (ToogleButton type), RadRadioButtons and RadCheckButtons throws a server exception that cannot be validated. 

Workaround:



ASP.NET:

<telerik:RadRadioButtonList ID="RadRadioButtonList1" runat="server" Text="RadRadioButtonList" >

    <Items>
        <telerik:ButtonListItem Text="Item 1" Value="0" />
        <telerik:ButtonListItem Text="Item 2" Value="1" />
    </Items>
</telerik:RadRadioButtonList>
 
<asp:CustomValidator ID="CustomButtonListValidator" ErrorMessage="errormessage"
    runat="server" OnServerValidate="CustomButtonListValidator_ServerValidate" />


C#:

protected void CustomButtonListValidator_ServerValidate(object source, ServerValidateEventArgs args)

{
    args.IsValid = (RadRadioButtonList1.SelectedIndex > -1);
}