如何使用xPath從命名空間xml中檢索特定元素

Question

我有一個關於從命名空間xml中檢索節點的問題。只要我在查詢中指定xml的單個元素，我就可以檢索所有匹配的元素，並且可以遍歷他們的子元素。我嘗試通過指定更詳細的查詢來檢索單個唯一元素時，出現問題。

XML文件，我使用測試

<?xml version="1.0" encoding="utf-8"?> 
<<MTConnectStreams xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:mtconnect.org:MTConnectStreams:1.2 http://mtconnect.org/schemas/MTConnectStreams_1.2.xsd" xmlns="urn:mtconnect.org:MTConnectStreams:1.2"> 
    <Streams> 
    <Samples> 
     <Position dataItemId="xs4" timestamp="2012-11-29T12:30:19Z" sequence="1718269" name="x_pos_cmd" subType="COMMANDED" units="MILLIMETER">1</Position> 
     <Position dataItemId="xs5" timestamp="2012-11-29T12:30:19Z" sequence="1718270" name="x_pos_act" subType="ACTUAL" units="MILLIMETER">2</Position> 
     <Position dataItemId="xs4" timestamp="2012-11-29T12:30:19Z" sequence="1718269" name="y_pos_cmd" subType="COMMANDED" units="MILLIMETER">3</Position> 
     <Position dataItemId="xs4" timestamp="2012-11-29T12:30:19Z" sequence="1718269" name="y_pos_act" subType="COMMANDED" units="MILLIMETER">4</Position> 
     <Position dataItemId="xs4" timestamp="2012-11-29T12:30:19Z" sequence="1718269" name="z_pos_cmd" subType="COMMANDED" units="MILLIMETER">5</Position> 
    </Samples> 
    <Condition> 
     <Unavailable dataItemId="xc1" type="POSITION" timestamp="2012-11-27T23:19:40Z" sequence="7" name="x_cond_pos">UNAVAILABLE</Unavailable> 
     <Unavailable dataItemId="xc2" type="LOAD" timestamp="2012-11-27T23:19:40Z" sequence="8" name="x_cond_load">UNAVAILABLE</Unavailable> 
    </Condition> 
    </Streams> 
</MTConnectStreams> 

考慮下面的代碼：

Dim xIt As XPathNodeIterator 
Dim xDoc As XmlDocument = New XmlDocument 
xDoc.Load("c:\Download\Test.xml") 
Dim xNav As XPathNavigator = xDoc.CreateNavigator 
Dim nsMgr = New XmlNamespaceManager(xNav.NameTable) 
nsMgr.AddNamespace("ns", "urn:mtconnect.org:MTConnectStreams:1.2") 

sQuery = "//ns:Samples" 
xIt = xNav.Select(sQuery, nsMgr) 
MsgBox(xIt.Count.ToString) 

When sQuery = "//ns:Samples" (1 result) 
When sQuery = "//ns:Position" (5 results) 
When sQuery = "//ns:Samples/Position (0 results why?) 
When sQuery = "//ns:Samples/Position[@name='x_pos_act'] (0 results) 
When sQuery = "//ns:Position[@name='x_pos_act']" (0 results) 
When sQuery = "//ns:Position/[@name='x_pos_act']" (0 results) 

等等等等，只要查詢比單個節點名稱我沒有得到任何結果的更多。我無法弄清楚如何指定更詳細的查詢。

Answer 1

它應該使用//ns:Samples/ns:Position

AFAIK XPath中，你必須指定每個分部命名空間前綴。這也適用於屬性！

所以嘗試：

When sQuery = "//ns:Samples" 
When sQuery = "//ns:Position" 
When sQuery = "//ns:Samples/ns:Position 
When sQuery = "//ns:Samples/ns:Position[@ns:name='x_pos_act'] 
When sQuery = "//ns:Position[@ns:name='x_pos_act']" 
When sQuery = "//ns:Position/[@ns:name='x_pos_act']" 

如果你不想考慮命名空間，你也可以使用@*[local-name()='Samples']語法。這將忽略所有的名稱空間，只檢查節點名稱本身。