无法使用XDocument从XML正确加载子记录

我有两个实体

public class DbServer
    {
        public int ServerId { set; get; }
        public string Name { set; get; }
        public string Address { set; get; }
        public List<DbDetail> DbDetails = new List<DbDetail>();
    }
    public class DbDetail
    {
        public int DbId { set; get; }
        public string DbName { set; get; }
        public string UserName { set; get; }
        public string PassWord { set; get; }
        public List<string> ConfiguredRules = new List<string>();
    }

和XML文件在

下面

<DataBaseServers>
      <DataBaseServer id ="1" title="IISDevdb1" address="iisdevdb1.mcafee.int'iisdevdb1,1515">
        <DataBases>
          <database id="1" name="ebiz" userID="se_user" password="se_user_pw!"> </database>
          <database id="2" name="oneclick" userID="se_user" password="se_user_pw!"> </database>
          <database id="3" name="EbizStats" userID="se_user" password="se_user_pw!"> </database>
          <database id="4" name="ebizlite" userID="se_user" password="se_user_pw!"> </database>
          <database id="5" name="EbizErrors" userID="se_user" password="se_user_pw!"> </database>
          <database id="6" name="machinetracker" userID="se_user" password="se_user_pw!"> </database>
          <database id="7" name="mast" userID="se_user" password="se_user_pw!"> </database>
          <database id="8" name="subscribe" userID="se_user" password="se_user_pw!"> </database>
          <database id="9" name="OFFER" userID="se_user" password="se_user_pw!"> </database>
          <database id="10" name="Session" userID="se_user" password="se_user_pw!"> </database>
          <database id="11" name="Reconciliation" userID="se_user" password="se_user_pw!"> </database>
          <database id="12" name="smartmessaging" userID="se_user" password="se_user_pw!"> </database>
          <database id="13" name="Payment" userID="se_user" password="se_user_pw!"> </database>
          <database id="46" name="SQLEye" userID="se_user" password="se_user_pw!"> </database>
          <database id="50" name="VCAPI" userID="se_user" password="se_user_pw!"> </database>         
        </DataBases>
      </DataBaseServer>
      <DataBaseServer id ="3" title="172.16.216.232" address="172.16.216.232">
        <DataBases>
          <database id="34" name="ebiz" userID="ebizsol" password="ebizsol_pw"> </database>
          <database id="35" name="oneclick" userID="ebizsol1click" password="ebizsol1click_pw"> </database>
          <database id="36" name="EbizStats" userID="ebizsol" password="ebizsol_pw"> </database>
          <database id="37" name="ebizlite" userID="ebizlite" password="ebizlite_pw"> </database>
          <database id="38" name="EbizErrors" userID="ebizsol" password="ebizsol_pw"> </database>
          <database id="39" name="machinetracker" userID="mtrackuser" password="mtrackUser_pw"> </database>
          <database id="40" name="mast" userID="mast_user" password="mast_user_pw"> </database>
          <database id="41" name="subscribe" userID="subscribeuser" password="subscribeuser"> </database>
          <database id="42" name="OFFER" userID="offer" password="offer_pw"> </database>
          <database id="43" name="Session" userID="Session" password="session_pw"> </database>
          <database id="44" name="smartmessaging" userID="ebizsol" password="ebizsol_pw"> </database>
          <database id="45" name="Payment" userID="ebizsol" password="ebizsol_pw"> </database>
          <database id="47" name="monitoring" userID="ebizsol" password="ebizsol_pw"> </database>          
        </DataBases>
      </DataBaseServer>      
    </DataBaseServers>

我试图填充

下的两个实体

XDocument xdoc = XDocument.Load(dbServerPath);
var res = from dbServer in xdoc.Descendants("DataBaseServer")
                     select new DbServer
                     {
                         ServerId = Convert.ToInt32(dbServer.Attribute("id").Value),
                         Name = dbServer.Attribute("title").Value,
                         Address = dbServer.Attribute("address").Value,
                          DbDetails = (from dbDetail in xdoc.Descendants("DataBases")
                                       select new DbDetail
                                       {
                                           DbId = Convert.ToInt32(dbDetail.Attribute("id").Value),
                                           DbName = dbDetail.Attribute("name").Value
                                       }).ToList()
                     };

但是DBDetails没有被正确填充。(对象引用错误抛出).

如何纠正?

XElement xdoc = XDocument.Load("dbServerPath").Element("DataBaseServers");
var res = from dbServer in xdoc.Elements("DataBaseServer")
    select new DbServer
    {
        ServerId = Convert.ToInt32(dbServer.Attribute("id").Value),
        Name = dbServer.Attribute("title").Value,
        Address = dbServer.Attribute("address").Value,
        DbDetails = (from dbDetail in dbServer.Element("DataBases").Elements("database")
                     select new DbDetail
                     {
                         DbId = Convert.ToInt32(dbDetail.Attribute("id").Value),
                         DbName = dbDetail.Attribute("name").Value 
                     }).ToList()
    };