为什么后台给TextBox赋值没有作用呀？

<form id="form1" runat="server">

    <asp:TextBox ID="txtKeMu" runat="server" />

    <ext:PageManager ID="PageManager1" runat="server" />

    <ext:Form ID="frmSearch" ShowBorder="False" BodyPadding="10px" EnableBackgroundColor="false" ShowHeader="False"  runat="server" LabelWidth="80px">

         <Rows>

            <ext:FormRow>

                <Items>

                    <ext:TextBox ID="txtChengJi"     Label="成绩" Regex="^[1-9]\d*$" RegexMessage="只允许输入数字！" width="150px"   MaxLength="20"   runat="server" />

                    <ext:DatePicker ID="startDate"  Label="考试日期"  EmptyText="请选择日期" width="150px" runat="server" DateFormatString="yyyy-MM-dd" />

                    <ext:DropDownList runat="server"  ID="drpKeMu"    width="150px" Label="科目" OnSelectedIndexChanged="drpKeMu_SelectedIndexChanged" AutoPostBack="true"/>

                    <ext:Button ID="btnSearch"  Icon="ZoomOut" Text="查询" Cssclass="inlineR"   runat="server" OnClientClick="test()"/>

                </Items>

            </ext:FormRow>

        </Rows>

    </ext:Form>

    

    <div id="container" style="width:880px; height: 340px; margin: 0 auto"></div>

</form>

 protected void drpKeMu_SelectedIndexChanged(object sender, EventArgs e)

 {

    txtKeMu.Text = drpKeMu.SelectedValue;

 }