void p_PreRender(object sender, EventArgs e)
        {
            if (IsInAjaxMode())
            {
                up.ContentTemplateContainer.Controls.Add(holder);
            }else {
                if(page.Form == null)
                    return; //for a page without form, there is no way to track status, and thus no need for this holder to be placed.
                //modified by Randy,the origin code:
                //page.Form.Controls.Add(holder);
               //下面的修改是处理在一些页面有<% %>时导致添加placeholder失败，这种情况下可以在页面的form中加：
                //<asp:PlaceHolder ID="NHibernate_Burrow_WebUtil_GlobalPlaceHolder" runat="server"></asp:PlaceHolder>
                //burrow会自动使用这个placeholder
                PlaceHolder existedHolder = page.FindControl(holderId) as PlaceHolder;
                if (existedHolder == null)
                {
                    page.Form.Controls.Add(holder);

                }
                else
                {
                    for (int i = holder.Controls.Count-1; i >=0 ; i--)
                    {
                        Control theControl=holder.Controls[i];
                        holder.Controls.Remove(theControl);
                        existedHolder.Controls.Add(theControl);
                    }                    
                    holder = existedHolder;
                }
            }
        }