求救！！！

Question

      //创建一个data.xml
    private void CreateXml(string basedir,string id)
{
                XmlDocument xmldoc = new XmlDocument();
                XmlDeclaration xn = xmldoc.CreateXmlDeclaration("1.0", "utf-8", null);
                xmldoc.AppendChild(xn);
                XmlElement xmlelem = xmldoc.CreateElement("FileList");
                //加入另外一个元素
                XmlElement xmlelem2 = xmldoc.CreateElement("File");
                string time = System.DateTime.UtcNow.Date.ToShortDateString();
                xmlelem2.SetAttribute("DateTime", time);
                xmlelem.AppendChild(xmlelem2);                XmlElement xmlelem3 = xmldoc.CreateElement("Guid");
                xmlelem3.InnerText = id;
                xmlelem2.AppendChild(xmlelem3);
                xmldoc.AppendChild(xmlelem);
                xmldoc.Save(basedir + "data.xml");  
}我调这个方法时，把数据传进去，总是把以前的数据覆盖掉了....
我想通过if判断,如:    
        private static void CreateXml(string basedir,string id)
        {            
            XmlDocument xmldoc = new XmlDocument();
            if (System.IO.File.Exists(basedir + "data.xml"))
            {
                xmldoc.Load("data.xml");
                XmlNode root = xmldoc.SelectSingleNode("Filelist");
                XmlElement el = xmldoc.CreateElement("File");
                string time = System.DateTime.UtcNow.Date.ToShortDateString();
                el.SetAttribute("DateTime", time);                XmlElement eid = xmldoc.CreateElement("Guid");
                eid.InnerText = id;
                el.AppendChild(eid);
                root.AppendChild(el);       //这里引发System.NullReferenceException
                xmldoc.Save(basedir + "data.xml");
            }
            else
            {               
                XmlDeclaration xn = xmldoc.CreateXmlDeclaration("1.0", "utf-8", null);
                xmldoc.AppendChild(xn);
                XmlElement xmlelem = xmldoc.CreateElement("FileList");
                //加入另外一个元素
                XmlElement xmlelem2 = xmldoc.CreateElement("File");
                string time = System.DateTime.UtcNow.Date.ToShortDateString();
                xmlelem2.SetAttribute("DateTime", time);
                xmlelem.AppendChild(xmlelem2);                XmlElement xmlelem3 = xmldoc.CreateElement("Guid");
                xmlelem3.InnerText = id;
                xmlelem2.AppendChild(xmlelem3);
                xmldoc.AppendChild(xmlelem);
                xmldoc.Save(basedir + "data.xml");
            }请高手帮帮忙,如何解决啊??