linq更新xml字段，更新总是失败（没有效果）。

    protected void AddQuestion(int consultID, string content)
    {
        try
        {
            using(OLGlassesDataContext dc = new OLGlassesDataContext())
            {
                ExpertConsult item = dc.ExpertConsult.Single(p => p.ConsultID == consultID);
                if (item != null)
                {
                    item.ConsultContent.Add(new XElement("ConsultItem",
                    new XElement("ConsultTime", DateTime.Now.ToString()),
                    new XElement("ConsultContent", content)));
                    item.Time = DateTime.Now;

                    dc.SubmitChanges();
                }
            }
        }
        catch (System.Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
    }