public override void LoadData(XmlDocument doc, XmlNode node)
    {
        var objectsNodes = node.SelectNodes("xxxs/xxx");
        if (objectsNodes == null)
        {
            return;
        }

        foreach (XmlNode objectNode in objectsNodes)
        {
            xxx x = new xxx();
            x.LoadData(doc, objectNode);
            _xxxs.Add(x);
        }
    }

    public override void SaveData(XmlDocument doc, XmlElement elem)
    {
        var xxxsElem = doc.CreateElement("xxxs");
        for (int i = 0; i < _xxxs.Count(); i++)
        {
           var xxxElem = doc.CreateElement("xxx");
            _xxxs[i].SaveData(doc, xxxElem);

            xxxElem.AppendChild(xxxElem);
        }

        elem.AppendChild(xxxsElem);
    }