C# System.Xml.XPath.XPathException：执行 SelectSingleNode("//(artist|author)") 时，表达式必须计算为节点集
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System.Xml.XPath.XPathException: Expression must evaluate to a node-set when executing SelectSingleNode("//(artist|author)")
提问by knoopx
Can somebody explain me why is this not working?
有人可以解释我为什么这不起作用吗？
I'm executing
我正在执行
XmlNode xmlNode = xmlDocument.SelectSingleNode("//(artist|author)");
and I get
我得到
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
但这有效并且即使有许多艺术家节点也不会引发异常
XmlNode xmlNode = xmlDocument.SelectSingleNode("//artist");
采纳答案by froh42
To my knowledge you can use '|' just at the top level of an XPath Query, so try the query
据我所知，您可以使用“|” 只是在 XPath 查询的顶层，所以尝试查询
"//artist|//author"
Bye the way doing recursive searches (//) isn't very fast, so make sure your dom document is small.
再见了，递归搜索 (//) 的速度不是很快，因此请确保您的 dom 文档很小。
Update:
更新：
I looked it up in the specification:
我在规范中查了一下：
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.
3.3 节点集
位置路径可以用作表达式。该表达式返回路径选择的节点集。
的| 运算符计算其操作数的并集，它必须是节点集。
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.
这意味着无论你在“|”的左边和右边写什么 需要单独用作 xpath 查询，“|” 然后从它创建联合。
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).
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-query（节点集）的结果。
回答by Ergil
//artist|//authorworks with XPATH 1.0 and 2.0//(artist|author)works with XPATH 2.0
//artist|//author适用于 XPATH 1.0 和 2.0//(artist|author)适用于 XPATH 2.0
Microsoft is a lazy corporation. Their framework support only XPATH 1.0
微软是一个懒惰的公司。他们的框架仅支持 XPATH 1.0
