使用 C# XmlDocument 选择相对 XPath 节点 [英] Relative XPath node selection with C# XmlDocument

问题描述

想象以下 XML 文档:

Imagine the following XML document:

<root>
    <person_data>
        <person>
            <name>John</name>
            <age>35</age>
        </person>
        <person>
            <name>Jim</name>
            <age>50</age>
        </person>
    </person_data>
    <locations>
        <location>
            <name>John</name>
            <country>USA</country>
        </location>
        <location>
            <name>Jim</name>
            <country>Japan</country>
        </location>
    </locations>
</root>

然后我为 Jim 选择人员节点:

I then select the person node for Jim:

XmlNode personNode = doc.SelectSingleNode("//person[name = 'Jim']");

现在从这个带有单个 XPath 选择的节点中，我想检索 Jim 的位置节点.类似的东西:

And now from this node with a single XPath select I would like to retrieve Jim's location node. Something like:

XmlNode locationNode = personNode.SelectSingleNode("//location[name = {reference to personNode}/name]");

因为我是根据 personNode 进行选择的，所以如果我可以在选择中引用它会很方便.这可能吗?...有联系吗?

Since I am selecting based on the personNode it would be handy if I could reference it in the select. Is this possible?.. is the connection there?

当然，我可以添加一些额外的代码行并将名称放入一个变量中，然后在 XPath 字符串中使用它，但这不是我要问的.

Sure I could put in a few extra lines of code and put the name into a variable and use this in the XPath string but that is not what I am asking.

推荐答案

这不是很有效，但应该有效.文件越大，速度就越慢.

This is not very efficient, but it should work. The larger the file gets, the slower will this be.

string xpath = "//location[name = //person[name='Jim']/name]";
XmlNode locationNode = doc.SelectSingleNode(xpath);

这就是效率低下的原因:

Here is why this is inefficient:

• //"速记会导致对所有节点进行文档范围的扫描.
• []"谓词在循环中运行，对于与//person"匹配的每个 一次.
• 第二个//"导致再次进行文档范围的扫描，这次对每个进行一次.

• The "//" shorthand causes a document-wide scan of all nodes.
• The "[]" predicate runs in a loop, once for each <person> matched by "//person".
• The second "//" causes a causes a document-wide scan again, this time once for each <person>.

这意味着你会得到二次 O(n²) 最坏情况的性能，这很糟糕.如果您的文档中有 n 个 和 n 个 ，则会发生 n x n 次文档范围扫描.完全来自一个看起来很无辜的 XPath 表达式.

This means you get quadratic O(n²) worst-case performance, which is bad. If there are n <person>s and n <location>s in your document, n x n document wide scans happen. All out of one innocent looking XPath expression.

我建议不要使用这种方法.两步选择(先找到人，然后是位置)效果会更好.

I'd recommend against that approach. A two-step selection (first, find the person, then the location) will perform better.

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