DropDownList绑定数据源的情况下把Select放在首位(AppendDataBoundItems="True")

<asp:DropDownList ID="ddlStore" runat="server"  DataSourceID="SqlDataSource2" DataTextField="StoreName" DataValueField="StoreId" AppendDataBoundItems="True">

   <asp:ListItem Value="" Selected="True">--Select--</asp:ListItem>

</asp:DropDownList>