System.Xml.XPath.XPathException：执行 SelectSingleNode(“//(artist|author)”) 时，表达式的计算结果必须为节点集

发布于 2024-07-15 00:23:14 字数 357 浏览 5 评论 0原文

有人可以解释一下为什么这不起作用吗？

我正在执行

XmlNode xmlNode = xmlDocument.SelectSingleNode("//(artist|author)");

并且我得到了，

System.Xml.XPath.XPathException: Expression must evaluate to a node-set.

但是即使有很多艺术家节点，这也有效并且不会引发异常

XmlNode xmlNode = xmlDocument.SelectSingleNode("//artist");

原文

Can somebody explain me why is this not working?

I'm executing

XmlNode xmlNode = xmlDocument.SelectSingleNode("//(artist|author)");

and I get

System.Xml.XPath.XPathException: Expression must evaluate to a node-set.

but this works and does not raise the exception even when there are many artist nodes

XmlNode xmlNode = xmlDocument.SelectSingleNode("//artist");

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情魔剑神 2024-07-22 00:23:14

据我所知，您可以使用“|” 就在 XPath 查询的顶层，因此请尝试查询再见，

    "//artist|//author"

递归搜索 (//) 的方式不是很快，因此请确保您的 dom 文档很小。

更新：

我在规范中查找了它：

3.3 节点集
位置路径可以用作
表达。表达式返回
由路径选择的节点集。
| 运算符计算并集
它的操作数，必须是节点集。

这意味着您在“|”左右写的任何内容需要单独用作 xpath 查询，即“|” 然后从中创建联合。

具体来说，您不能说“递归搜索（称为作者的东西或称为艺术家的东西）”，因为“称为作者的东西”不会评估 xpath 查询（节点集）的结果。

To my knowledge you can use '|' just at the top level of an XPath Query, so try the query

    "//artist|//author"

Bye the way doing recursive searches (//) isn't very fast, so make sure your dom document is small.

Update:

I looked it up in the specification:

3.3 Node-sets
A location path can be used as an
expression. The expression returns the
set of nodes selected by the path.
The | operator computes the union of
its operands, which must be node-sets.

That means whatever you write left and right of "|" needs to be usable as an xpath query on its own, the "|" then just creates the union from it.

Specifically you can not say "search recursively for (something called author OR something called artist)" because "something called author" does not evaluate to the result of an xpath-query (a node set).

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