dropdownlist绑定数据源控件，AutoPostBack为true，选择后，值也跟着刷新

Question

---aspx文件begin---
。。。省略
<asp:DropDownList ID="DropDownList1" runat="server" DataSourceID="SqlDataSource2"
DataTextField="dept_code" DataValueField="dept_code" AutoPostBack="True">
</asp:DropDownList>
<asp:SqlDataSource ID="SqlDataSource2" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
ProviderName="<%$ ConnectionStrings:ConnectionString.ProviderName %>" SelectCommand="select distinct dept_code from foc.t8002 where rownum < 10 order by dept_code">
</asp:SqlDataSource>
。。。省略
----aspx文件end----

当我选择其它项后，页面自动刷新，这时，dropdownlist选择的项又跳到第一项