RequiredFieldValidator

<asp:DropDownList ID="selStates" runat="server" Width="250px" AppendDataBoundItems="True" AutoPostBack="true" CausesValidation="true" ValidationGroup="BranchSp">
<asp:ListItem Value="-1">----Select-----</asp:ListItem>
</asp:DropDownList>




<asp:RequiredFieldValidator ID="RequiredFieldValidator7" runat="server" ControlToValidate="selStates" CssClass="red_text"
  ErrorMessage="Please select state" InitialValue="
-1" SetFocusOnError="True"  Text="Please select state" ValidationGroup="BranchSp"></asp:RequiredFieldValidator>