如何使用xPath从命名空间xml中检索特定元素

Question

我有一个关于从命名空间xml中检索节点的问题。只要我在查询中指定xml的单个元素，我就可以检索所有匹配的元素，并且可以遍历他们的子元素。我尝试通过指定更详细的查询来检索单个唯一元素时，出现问题。

XML文件，我使用测试

<?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> 

考虑下面的代码：

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，只要查询比单个节点名称我没有得到任何结果的更多。我无法弄清楚如何指定更详细的查询。

Answer 1

它应该使用//ns:Samples/ns:Position

AFAIK XPath中，你必须指定每个分部命名空间前缀。这也适用于属性！

所以尝试：

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']" 

如果你不想考虑命名空间，你也可以使用@*[local-name()='Samples']语法。这将忽略所有的名称空间，只检查节点名称本身。