怎么调用ajax中repeater中TextBox中的值!!!

Question

aspx 代码
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="true"></asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Always" ChildrenAsTriggers="true">
            <ContentTemplate>
<asp:Repeater ID="rptInforList" runat="server">
        <ItemTemplate>
                <asp:TextBox ID="txtId" runat="server" Text='<%# Eval("id") %>' 
       </ItemTemplate>
   </asp:Repeater>
      </ContentTemplate>
</asp:UpdatePanel>aspx.cs代码
for (int i = 0; i < this.rptInforList.Items.Count; i++)
            {
                TextBox txtId = (TextBox)rptInforList.Items[i].FindControl("txtId");
            }txtId一直为空

Answer 1

有没有判断Page_Load中的Page.IsPostBack

Answer 2

aspx.cs代码是放到一个按钮事件里的 
   protected void btnModify_Click(object sender, EventArgs e)
    {
        string flag = Request.QueryString["flag"];
        int j = 0;
        int id = 0;
        for (int i = 0; i < this.rptLinks.Items.Count; i++)
        {
            CheckBox cbx = (CheckBox)rptLinks.Items[i].FindControl("chkExport");
            TextBox tbx = (TextBox)rptLinks.Items[i].FindControl("txtCheck");
            if (cbx != null)//选中
            {
                if (cbx.Checked)
                {
                    j++;
                    id = Convert.ToInt32(tbx.Text);
                }
            }
        }