table不能维持数据的问题
我写的下单页面PlaceOrder.aspx：
<%@ Page Title="" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true" CodeFile="PlaceOrder.aspx.cs" Inherits="PlaceOrder" %>

<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<h2>下单页面：</h2>
 产品列表：
    
    <asp:GridView ID="GridView2" ......>
    <asp:SqlDataSource .......>   
        已经选择的产品：<br />

        <asp:Table ID="items" runat="server" EnableViewState="true">
        </asp:Table>
      
        <br />
        <asp:Button ID="Button1" runat="server" Text="提交" />
        <br />
</asp:Content>

后台代码：

public partial class PlaceOrder : System.Web.UI.Page

{

    protected void Page_Load(object sender, EventArgs e)

    {

 

    }

    protected void GridView2_SelectedIndexChanged(object sender, EventArgs e)

    {

        string productInfoId = GridView2.SelectedRow.Cells[0].Text.Trim();

        string productName = GridView2.SelectedRow.Cells[1].Text.Trim();

  

        items.Rows.Add(getNewRow(productInfoId, productName));



    }



    private TableRow getNewRow(string productInfoId,string productName )

    {

        TableRow tr = new TableRow();

        tr.BorderWidth = 1;

        //1

        TableCell tc = new TableCell();

        tc.Visible = false;

        tc.Text = productInfoId;

        tr.Cells.Add(tc);

        //2

        tc = new TableCell();

        tc.Width = 150;

        tc.Text = productName;

        tr.Cells.Add(tc);

        //3

        tc = new TableCell();

        tc.Width = 70;

        TextBox tb = new TextBox();

        tb.Width = 30;

        tc.Controls.Add(tb);

        Label