ASP.NET动态控件的保存

[meceky]

页面有动态生成的FileUpLoad,有Buuton,每次点Button后动态生成的FileUpload就消失了。。我想在页面回传后在页面还能保存那些动态生成的FileUpload,怎么办啊！
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 <form id="form1" runat="server">
    <div>
    <table>
      <tr><td>选择上传文件个数</td>
      <td><asp:DropDownList runat="server" AutoPostBack="True" 
               ID="numbers" onselectedindexchanged="numbers_SelectedIndexChanged">
         <asp:ListItem Text="1" Value="1"></asp:ListItem>
         <asp:ListItem Text="2" Value="2"></asp:ListItem>
         <asp:ListItem Text="3" Value="3"></asp:ListItem>
         <asp:ListItem Text="4" Value="4"></asp:ListItem>
         <asp:ListItem Text="5" Value="5"></asp:ListItem>
      </asp:DropDownList></td></tr>
      <tr><td>上传文件</td><td>
          <asp:PlaceHolder ID="pl" runat="server"></asp:PlaceHolder>
          <asp:Panel runat="server" ID="board"></asp:Panel></td></tr>
      <tr><td><asp:Button runat="server" ID="bt" Text="上传文件" onclick="bt_Click"/></td><td></td></tr>
    </table>
    <hr />
    <asp:Label runat="server" ID="msg"></asp:Label>
        <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
    </div>
    </form>

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 public partial class _Default : System.Web.UI.Page 
{
    static string selectNum;
    protected void Page_Load(object sender, EventArgs e)
    {
        if(Page.IsPostBack)
        {
            //CreateControls(selectNum);
            msg.Text = selectNum==null?"true":"false"+selectNum;
            //CreateControls(selectNum);

        }
    }
    protected override void OnPreLoad(EventArgs e)
    {
       // CreateControls((string)Session["nums"]);
       // msg.Text = "selectValue=" + Session["nums"].ToString() ;
        base.OnPreLoad(e);
    }
    protected void numbers_SelectedIndexChanged(object sender, EventArgs e)
    {
       // msg.Text = numbers.SelectedItem.Value;
        //Request.Cookies ck = new HttpCookieCollection
       // Session["nums"] = numbers.SelectedItem.Value;
        selectNum = numbers.SelectedItem.Value;
        CreateControls(selectNum);
        
    }
    private void CreateControls(string numbers)
    {
        int num = Int32.Parse(numbers); 
        
        FileUpload uploadFile = null;
        for (int i = 0; i < num; i++)
        {
            uploadFile = new FileUpload();
            uploadFile.ID = "upFile" + i.ToString();
            LiteralControl lc = new LiteralControl("<br/>");
            board.Controls.Add(lc);
            board.Controls.Add(uploadFile);
            //pl.
        }
         
    }
    protected void bt_Click(object sender, EventArgs e)
    {
        //        }    
    }
}

求救！！