请教一个关于xml的问题

请问大侠<MileageObj>
 <_MileageTripChartID>null</_MileageTripChartID>
 <_MileageDate>20051-2</_MileageDate>
 <_MileageAddressFrom>testaddress1</_MileageAddressFrom>
 <_MileageAddressTo>testaddress2</_MileageAddressTo>
 <_MileageNo>4</_MileageNo>
 <_MileagePark>5</_MileagePark>
 <_MileageREC>3</_MileageREC>
 <_MileageReason>test reason</_MileageReason>
 <_MileageState>null</_MileageState>
  <_hasRoundTrip>false</_hasRoundTrip>
 <AllocationObj>
  <_deptid>3</_deptid>
  <_mileage>5.5</_mileage>
  <_park>3.5</_park>
  <_rec>0.5</_rec>
 </AllocationObj>
</MileageObj>我的xmlNode的outerxml如上我用这样的xnode.SelectSingleNode("//MileageObj/_MileageAddressFrom")
为什么取不到节点啊