protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            HttpCookie cookie = Request.Cookies["userSName"];
            if (cookie != null )
            {
               
                Response.Write(cookie + "  ：1");
            }
            else
            {
                Response.Write(cookie +"  ：2");
            }
        }
    }

        HttpCookie cookie = new HttpCookie("UserName", "test");
        Response.Cookies.Add(cookie);
        HttpCookie ck = Request.Cookies["UserName"];
        if (ck != null)
        {
            Response.Write(ck.Value + ":1");
        }
        else
        {
            Response.Write("cookie不存在");
        }

------解决方案--------------------

IE清除缓存后需要关掉重新打开。