.net – How to resolve the error “Expression must evaluate to a node-set” when checking for the existence of a node

.netxmlxpath

I'm attempting to check for the existence of a node using the following .NET code:

xmlDocument.SelectSingleNode(
        String.Format("//ErrorTable/ProjectName/text()='{0}'", projectName));

This always raises:

XPathException: Expression must evaluate to a node-set.

Why am I getting this error and how can I resolve it? Thank you.

Best Solution

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

//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.

As an afterthought: You can use the original xpath expression, but not with SelectSingleNode, but with Evaluate, like this:

(bool)xmlDocument.CreateNavigator().Evaluate(String.Format("//ErrorTable/ProjectName/text()='{0}'", projectName));
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