XML:
<?xml version="1.0" encoding="gb2312"?>
<MusicMenu>
<SingerType type="男歌手">
   <Singer name="林俊杰">
     <Music>江南</Music>
     <Music>冻结</Music>
   </Singer>
   <Singer name="张敬轩">
     <Music>断点</Music>
   </Singer>
   <Singer name="唐磊">
     <Music>丁香花</Music>
   </Singer>
</SingerType>
<SingerType type="女歌手">
   <Singer name="孙燕姿">
     <Music>坏天气</Music>
     <Music>遇见</Music>
     <Music>爱情证书</Music>
   </Singer>
   <Singer name="范玮琪">
     <Music>启程</Music>
     <Music>因为</Music>
   </Singer>
</SingerType>
</MusicMenu>

C#:
private void BTN_ShowTree_Click(object sender, System.EventArgs e)
{
XmlNodeList nodeListMenu = xmlDoc.SelectSingleNode( "MusicMenu" ).ChildNodes;

int indexType = 0;
foreach ( XmlNode nodeType in nodeListMenu )
{
   XmlElement elementType = (XmlElement)nodeType;
   this.TVW_Menu.Nodes.Add( elementType.GetAttribute("type") );

   int indexSinger = 0;
   XmlNodeList nodeListSinger = nodeType.ChildNodes;
   foreach ( XmlNode nodeSinger in nodeListSinger )
   {
     XmlElement elementSinger = (XmlElement)nodeSinger;
     this.TVW_Menu.Nodes[indexType].Nodes.Add( elementSinger.GetAttribute( "name" ) );

     XmlNodeList nodeListMusic = nodeSinger.ChildNodes;
     foreach ( XmlNode nodeMusic in nodeListMusic )
     {
       XmlElement elementMusic = (XmlElement)nodeMusic;
       if ( elementMusic.Name == "Music" )
         this.TVW_Menu.Nodes[indexType].Nodes[indexSinger].Nodes.Add( elementMusic.InnerText );
     }
     indexSinger++;
   }
   indexType++;
}
}

其中xmlDoc报错，没有找到类型及命名空间，如何解决？
开头已有using System.Xml;