[SOLVED] ASP.Net RequiredFieldValidator not working with AJAX Cascading DropDown

hi 

I used dropdownlist in page and use ajax CascadingDropDown below are codes

    <asp:DropDownList ID="DDLstate" runat="server" CssClass="ddlE">
                                                        </asp:DropDownList>
 <cc1:CascadingDropDown ID="CDLstate" TargetControlID="DDLstate" PromptText="Select Country"
           PromptValue="" ServicePath="../ServiceCS.asmx" ServiceMethod="GetCountries" runat="server"                                    Category="state" />
<asp:RequiredFieldValidator Display="Dynamic" ID="R1" runat="server" ErrorMessage="*لطفاٌ نام استان را وارد نمایید."                                                          ControlToValidate="DDLstate" CssClass="valid7M" InitialValue="0"></asp:RequiredFieldValidator>

here when I use cascadingDropDown   RequiredFieldValidator  didn't work I need to use  RequiredFieldValidator  how I should change codes to use it?

Best Regards

Neda