后台获取 iframe 对象

<iframe
id ="mainFrame" frameborder="0" scrolling="no"  style="height: 580px; width: 700px; margin-left: 0px;">
</iframe>



  public partial class Main : System.Web.UI.Page
    {
        protected static System.Web.UI.HtmlControls.HtmlContainerControl mainFrame;
        protected void Page_Load(object sender, EventArgs e)
        {
            mainFrame = (HtmlContainerControl)this.FindControl("mainFrame");
                  
        }
………………。
总是提示mainFrame为空，求大神帮忙！