.net 在检查节点是否存在时，如何解决错误“表达式必须评估为节点集”？
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How do I resolve the error "Expression must evaluate to a node-set" when checking for the existence of a node?
提问by Alex Angas
I'm attempting to check for the existence of a node using the following .NET code:
我正在尝试使用以下 .NET 代码检查节点是否存在：
xmlDocument.SelectSingleNode(
String.Format("//ErrorTable/ProjectName/text()='{0}'", projectName));
This always raises:
这总是引发：
XPathException: Expression must evaluate to a node-set.
XPathException：表达式必须计算为节点集。
Why am I getting this error and how can I resolve it? Thank you.
为什么我会收到此错误，我该如何解决？谢谢你。
回答by TToni
The expression given evaluates to a boolean, not a node-set. I assume you want to check whether the ProjectName equals the parametrized text. In this case you need to write
给定的表达式计算为布尔值，而不是节点集。我假设您想检查 ProjectName 是否等于参数化文本。在这种情况下，您需要编写
//ErrorTable/ProjectName[text()='{0}']
This gives you a list of all nodes (a nodeset) matching the given condition. This list may be empty, in which case the C#-Expression in your sample will return null.
这为您提供了与给定条件匹配的所有节点（节点集）的列表。此列表可能为空，在这种情况下，示例中的 C#-Expression 将返回 null。
As an afterthought: You canuse the original xpath expression, but not with SelectSingleNode, but with Evaluate, like this:
作为事后的想法：您可以使用原始 xpath 表达式，但不能使用 SelectSingleNode，而是使用 Evaluate，如下所示：
(bool)xmlDocument.CreateNavigator().Evaluate(String.Format("//ErrorTable/ProjectName/text()='{0}'", projectName));
回答by rjohnston
Try:
尝试：
Node node = xmlDocument.SelectSingleNode(String.Format("//ErrorTable/ProjectName = '{0}'", projectName));
if (node != null) {
// and so on
}
Edit: silly error
编辑：愚蠢的错误
回答by Alex Angas
The XPath expression contained a subtle error. It should have been:
XPath 表达式包含一个微妙的错误。本来应该是：
xmlDocument.SelectSingleNode(String.Format("//ErrorTable/ProjectName[text()='{0}']", projectName));
The previous expression was evaluating to a boolean, which explains the exception error. Thanks for the help!
前面的表达式计算结果为布尔值，这解释了异常错误。谢谢您的帮助！
