C# SelectSingleNode 使用 XPath 为已知好的 xml 节点路径返回 null
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原文地址: http://stackoverflow.com/questions/1089196/
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SelectSingleNode returning null for known good xml node path using XPath
提问by p.campbell
Consider this simple XML document. The serialized XML shown here is the result of an XmlSerializer from a complex POCO object whose schema I have no control over.
考虑这个简单的 XML 文档。此处显示的序列化 XML 是来自复杂 POCO 对象的 XmlSerializer 的结果，我无法控制其架构。
<My_RootNode xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="">
<id root="2.16.840.1.113883.3.51.1.1.1" extension="someIdentifier" xmlns="urn:hl7-org:v3" />
<creationTime xsi:nil="true" xmlns="urn:hl7-org:v3" />
</My_RootNode>
The goal is to extract the value of the extension attribute on the id node. In this case, we are using the SelectSingleNode method, and given an XPath expression as such:
目标是在 id 节点上提取扩展属性的值。在这种情况下，我们使用 SelectSingleNode 方法，并给出一个 XPath 表达式，如下所示：
XmlNode idNode = myXmlDoc.SelectSingleNode("/My_RootNode/id");
//idNode is evaluated to null at this point in the debugger!
string msgID = idNode.Attributes.GetNamedItem("extension").Value;
The problem is that the SelectSingleNodemethod returns null for the given XPath expression.
问题是该SelectSingleNode方法为给定的 XPath 表达式返回 null。
Question:any ideas on this XPath query's correctness, or why this method call + XPath expression would return a null value? Perhaps the namespaces are part of the problem?
问题：关于这个 XPath 查询的正确性的任何想法，或者为什么这个方法调用 + XPath 表达式会返回一个空值？也许命名空间是问题的一部分？
采纳答案by Jon Skeet
I strongly suspect the problem is to do with namespaces. Try getting rid of the namespace and you'll be fine - but obviously that won't help in your real case, where I'd assume the document is fixed.
我强烈怀疑这个问题与命名空间有关。尝试摆脱命名空间，你会没事的 - 但显然这在你的真实情况下无济于事，我假设文档是固定的。
I can't remember offhand how to specify a namespace in an XPath expression, but I'm sure that's the problem.
我不记得如何在 XPath 表达式中指定命名空间，但我确定这就是问题所在。
EDIT: Okay, I've remembered how to do it now. It's not terribly pleasant though - you need to create an XmlNamespaceManagerfor it. Here's some sample code that works with your sample document:
编辑：好的，我现在记得怎么做。不过，这并不是非常令人愉快 - 您需要XmlNamespaceManager为它创建一个。以下是一些适用于您的示例文档的示例代码：
using System;
using System.Xml;
public class Test
{
static void Main()
{
XmlDocument doc = new XmlDocument();
XmlNamespaceManager namespaces = new XmlNamespaceManager(doc.NameTable);
namespaces.AddNamespace("ns", "urn:hl7-org:v3");
doc.Load("test.xml");
XmlNode idNode = doc.SelectSingleNode("/My_RootNode/ns:id", namespaces);
string msgID = idNode.Attributes["extension"].Value;
Console.WriteLine(msgID);
}
}
回答by John Saunders
Sorry, you forgot the namespace. You need:
抱歉，您忘记了命名空间。你需要：
XmlNamespaceManager ns = new XmlNamespaceManager(myXmlDoc.NameTable);
ns.AddNamespace("hl7","urn:hl7-org:v3");
XmlNode idNode = myXmlDoc.SelectSingleNode("/My_RootNode/hl7:id", ns);
In fact, whether here or in web services, getting null back from an XPath operation or anything that depends on XPath usually indicates a problem with XML namespaces.
事实上，无论是在这里还是在 Web 服务中，从 XPath 操作或任何依赖于 XPath 的操作返回 null 通常都表明 XML 名称空间存在问题。
回答by Roisgoen
Well... I had the same issue and it was a headache. Since I didn't care much about the namespace or the xml schema, I just deleted this data from my xml and it solved all my issues. May not be the best answer? Probably, but if you don't want to deal with all of this and you ONLY care about the data (and won't be using the xml for some other task) deleting the namespace may solve your problems.
嗯...我有同样的问题，这是一个头痛。由于我不太关心命名空间或 xml 模式，我只是从我的 xml 中删除了这些数据，它解决了我所有的问题。可能不是最好的答案？可能，但是如果您不想处理所有这些并且您只关心数据（并且不会将 xml 用于其他任务），则删除命名空间可能会解决您的问题。
XmlDocument vinDoc = new XmlDocument();
string vinInfo = "your xml string";
vinDoc.LoadXml(vinInfo);
vinDoc.InnerXml = vinDoc.InnerXml.Replace("xmlns=\"http://tempuri.org\/\", "");
回答by mrzli
If you want to ignore namespaces completely, you can use this:
如果你想完全忽略命名空间，你可以使用这个：
static void Main(string[] args)
{
string xml =
"<My_RootNode xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" xmlns=\"\">\n" +
" <id root=\"2.16.840.1.113883.3.51.1.1.1\" extension=\"someIdentifier\" xmlns=\"urn:hl7-org:v3\" />\n" +
" <creationTime xsi:nil=\"true\" xmlns=\"urn:hl7-org:v3\" />\n" +
"</My_RootNode>";
XmlDocument doc = new XmlDocument();
doc.LoadXml(xml);
XmlNode idNode = doc.SelectSingleNode("/*[local-name()='My_RootNode']/*[local-name()='id']");
}
回答by tandrasz
This should work in your case without removing namespaces:
这应该适用于您的情况，而无需删除名称空间：
XmlNode idNode = myXmlDoc.GetElementsByTagName("id")[0];
回答by Mad Doc's Hairy Belly
Just to build upon solving the namespace issues, in my case I've been running into documents with multiple namespaces and needed to handle namespaces properly. I wrote the function below to get a namespace manager to deal with any namespace in the document:
只是为了解决命名空间问题，就我而言，我遇到了具有多个命名空间的文档，需要正确处理命名空间。我编写了下面的函数来获得一个命名空间管理器来处理文档中的任何命名空间：
private XmlNamespaceManager GetNameSpaceManager(XmlDocument xDoc)
{
XmlNamespaceManager nsm = new XmlNamespaceManager(xDoc.NameTable);
XPathNavigator RootNode = xDoc.CreateNavigator();
RootNode.MoveToFollowing(XPathNodeType.Element);
IDictionary<string, string> NameSpaces = RootNode.GetNamespacesInScope(XmlNamespaceScope.All);
foreach (KeyValuePair<string, string> kvp in NameSpaces)
{
nsm.AddNamespace(kvp.Key, kvp.Value);
}
return nsm;
}
回答by dong
just use //id instead of /id. It works fine in my code
只需使用 //id 而不是 /id。它在我的代码中运行良好
回答by Erica Ackerman
The rule to keep in mind is: if your document specifies a namespace, you MUST use an XmlNamespaceManagerin your call to SelectNodes()or SelectSingleNode(). That's a good thing.
要记住的规则是：如果您的文档指定了namespace，则您必须XmlNamespaceManager在对SelectNodes()或的调用中使用 an SelectSingleNode()。这是好事。
See the article Advantages of namespaces. Jon Skeet does a great job in his answer showing how to use XmlNamespaceManager. (This answer should really just be a comment on that answer, but I don't quite have enough Rep Points to comment.)
请参阅文章名称空间的优势。Jon Skeet 在他的回答中做得很好，展示了如何使用XmlNamespaceManager. （这个答案实际上应该只是对该答案的评论，但我没有足够的代表点数来评论。）
回答by David
Roisgoen's answer worked for me, but to make it more general, you can use a RegEx:
Roisgoen 的回答对我有用，但为了使它更通用，您可以使用 RegEx：
//Substitute "My_RootNode" for whatever your root node is
string strRegex = @"<My_RootNode(?<xmlns>\s+xmlns([\s]|[^>])*)>";
var myMatch = new Regex(strRegex, RegexOptions.None).Match(myXmlDoc.InnerXml);
if (myMatch.Success)
{
var grp = myMatch.Groups["xmlns"];
if (grp.Success)
{
myXmlDoc.InnerXml = myXmlDoc.InnerXml.Replace(grp.Value, "");
}
}
I fully admit that this is not a best-practice answer, but but it's an easy fix and sometimes that's all we need.
我完全承认这不是最佳实践答案，但它很容易解决，有时这就是我们所需要的。
