最近研究XML 太迷糊了

Question

            XmlDocument xml = new XmlDocument();
            xml.Load(Server.MapPath("~/xml/xixi.xml"));
            XmlNode trackList = xml.SelectSingleNode("trackList");
            XmlElement track = xml.CreateElement("track");
            XmlElement title = xml.CreateElement("title");
            XmlElement creator = xml.CreateElement("creator");
            XmlElement location = xml.CreateElement("location");
            track.SetAttribute("indx", "11");
            track.AppendChild(title);
            track.AppendChild(creator);
            track.AppendChild(location);
            trackList.AppendChild(track);
            xml.Save(Server.MapPath("~/xml/xixi.xml"));
<?xml version="1.0" encoding="utf-8"?>
<playlist version="1" xmlns="http://xspf.org/ns/0/">
<trackList>
<track>
<title>LAN CLUB'S 2YEAR ANNIVERSARY!!</title>
<creator>Jeroen Wijering</creator>
<location>images/01.jpg</location>
<info></info>
</track>
<track>
<title>WWW.LANRENTUKU.COM</title>
<creator>Jeroen Wijering</creator>
<location>images/02.jpg</location>
<info></info>
</track>
<track>
<title>I AM XIXI</title>
<creator>Jeroen Wijering</creator>
<location>images/03.jpg</location>
<info></info>
</track>
</trackList>
</playlist>
想在trackList插入和里面一样结构的track节点
出错 未将对象引用设置到对象的实例。 
高手来！！！！

Answer 1

XmlNode trackList = xml.SelectSingleNode("playlist/trackList");

Answer 2

XmlNode trackList = xml.SelectSingleNode("playlist/trackList");
也不行

Answer 3

trackList.AppendChild(track);这行 估计就是没得到这个节点

Answer 4

XmlNodeList trackList = xml.getElementsByTagName()

Answer 5

XmlDocument xml = new XmlDocument();
            xml.Load(Server.MapPath("~/xml/xixi.xml"));
            XmlNamespaceManager xnm = new XmlNamespaceManager(xml.NameTable);
            xnm.AddNamespace("p", "http://xspf.org/ns/0/");
            XmlNode trackList = xml.SelectSingleNode("/p:playlist/p:trackList",xnm);
            XmlElement track = xml.CreateElement("track");
            XmlElement title = xml.CreateElement("title");
            XmlElement creator = xml.CreateElement("creator");
            XmlElement location = xml.CreateElement("location");
            track.SetAttribute("indx", "11");
            track.AppendChild(title);
            track.AppendChild(creator);
            track.AppendChild(location);
            trackList.AppendChild(track);
            xml.Save(Server.MapPath("~/xml/xixi.xml"));