xpath表达式count()异常失败，表达式必须求值为一个节点集

本文关键字：表达式 节点 点集 一个 异常 count 失败 xpath | 更新日期: 2023-09-27 18:11:40

我得到一个异常:System.Xml.XPath.XPathException:表达式必须计算为节点集。

当我运行这段代码时，我期望计算节点的数量，我做错了什么?

var doc = new XPathDocument("contosoBooks.xml");
var nav = doc.CreateNavigator();
var expr = nav.Compile("count(//bookstore/book)");
var iterator = nav.Select(expr);
while (iterator.MoveNext())
{
    if (iterator.Current != null && !iterator.Current.HasChildren) continue;
    if (iterator.Current != null)
    {
        var childIter = iterator.Current.SelectChildren(XPathNodeType.Element);
        while (childIter.MoveNext())
        {
            if (childIter.Current != null) Console.WriteLine(childIter.Current.Value);
        }
    }
}

我的XML文件看起来像这样

<?xml version="1.0" encoding="utf-8" ?>
<bookstore>
  <book genre="autobiography" publicationdate="1981-03-22" ISBN="1-861003-11-0">
    <title>The Autobiography of Benjamin Franklin</title>
    <author>
      <first-name>Benjamin</first-name>
      <last-name>Franklin</last-name>
    </author>
    <price>8.99</price>
  </book>
  <book genre="novel" publicationdate="1967-11-17" ISBN="0-201-63361-2">
    <title>The Confidence Man</title>
    <author>
      <first-name>Herman</first-name>
      <last-name>Melville</last-name>
    </author>
    <price>11.99</price>
  </book>
  <book genre="philosophy" publicationdate="1991-02-15" ISBN="1-861001-57-6">
    <title>The Gorgias</title>
    <author>
      <name>Plato</name>
    </author>
    <price>9.99</price>
  </book>
</bookstore>

注意:谢谢你对我的帮助。我想到了这个解决方案:

var doc = new XPathDocument("contosoBooks.xml");
var nav = doc.CreateNavigator();
const string query = "count(//bookstore/book)";            
var expr = nav.Compile(query);
switch (expr.ReturnType)
{        
    case XPathResultType.Number:
        Console.WriteLine(nav.Evaluate(query));
        break;
    case XPathResultType.String:
    // case XPathResultType.Navigator:
        Console.WriteLine(nav.Evaluate(query));
        break;
    case XPathResultType.Boolean:
        Console.WriteLine(nav.Evaluate(query));
        break;
    case XPathResultType.NodeSet:
        var nodes = nav.Select(expr);
        while (nodes.MoveNext())
        {                      
            if (nodes.Current != null && !nodes.Current.HasChildren) continue;
            if (nodes.Current == null) continue;
            var childIter = nodes.Current.SelectChildren(XPathNodeType.Element);
            while (childIter.MoveNext())
            {
                if (childIter.Current != null) Console.WriteLine(childIter.Current.Value);
            }
        }
        break;
    case XPathResultType.Any:
        Console.WriteLine(nav.Evaluate(query));
        break;              
    default:
        Console.WriteLine(nav.Evaluate(query));                    
        break;
}

xpath表达式count()异常失败，表达式必须求值为一个节点集

使用Evaluate，而不是Select:

这一行:

var iterator = nav.Select(expr);

expr必须是返回节点集的XPath表达式。XPath表达式返回一个标量值，即图书节点的计数。

改成:

var expr = nav.Compile("//bookstore/book");

var expr = nav.Compile("count(//bookstore/book)");
Console.WriteLine(nav.Evaluate(expr));
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