在 System.Xml.XPath 中启用 XPath2 查询（XPathException：无效令牌）

Question

Microsoft 的 System.Xml.XPath nuget-package，可用于 .NET 4.6，声称支持 XPath 1.0 和 2.0。 De documentation 说描述了命名空间：

System.Xml.XPath 命名空间包含定义用于导航和编辑 XML 信息项的游标模型的类，作为 XQuery 1.0 和 XPath 2.0 数据模型的实例。

在升级 Visual Studio、升级和我的所有项目到框架版本 4.6 之后，我仍然无法让最简单的 XPath-2.0 for-expression 工作。根据specification，它们应该可以工作。

我无法想象微软声称支持它实际上不支持的东西，所以很明显我做错了什么。 如何正确使用 XPath2 查询？

[TestMethod]
public void TestXPath2()
{
    // The System.Xml.XPath namespace contains the classes that define a cursor model for navigating and editing XML information items as instances of the 
    // XQuery 1.0 and XPath 2.0 Data Model.

    var expression = "for $x in /Root/Foo/Bar return $x";
    var compiledExpression = System.Xml.XPath.XPathExpression.Compile(expression); 
    // throws XPathException: "for ... has an invalid token"
}

附： 我真正想要的是让这样的事情起作用：

    [TestMethod]
    public void TestLibraryForCustomer1()
    {
        string xmlFromMessage = @"<Library>
            <Writer ID=""writer1""><Name>Shakespeare</Name></Writer>
            <Writer ID=""writer2""><Name>Tolkien</Name></Writer>
            <Book><WriterRef REFID=""writer1"" /><Title>King Lear</Title></Book>
            <Book><WriterRef REFID=""writer2"" /><Title>The Hobbit</Title></Book>
            <Book><WriterRef REFID=""writer2"" /><Title>Lord of the Rings</Title></Book>
             </Library>"; 

        var titleXPathFromConfigurationFile = "./Title"; 
        var writerXPathFromConfigurationFile = "for $curr in . return /Library/Writer[@ID=$curr/WriterRef/@REFID]/Name";

        var library = ExtractBooks(xmlFromMessage, titleXPathFromConfigurationFile, writerXPathFromConfigurationFile).ToDictionary(b => b.Key, b => b.Value);

        Assert.AreEqual("Shakespeare", library["King Lear"]);
        Assert.AreEqual("Tolkien", library["The Hobbit"]);
        Assert.AreEqual("Tolkien", library["Lord of the Rings"]);
    }

    [TestMethod]
    public void TestLibraryForCustomer2()
    {
        string xmlFromMessage = @"<Library>
                <Writer ID=""writer1"">
                    <Name>Shakespeare</Name>
                    <Book><Title>Sonnet 18</Title></Book>
                </Writer>
                <Writer ID=""writer2"">
                    <Name>Tolkien</Name>
                    <Book><Title>The Hobbit</Title></Book>
                    <Book><Title>Lord of the Rings</Title></Book>
                </Writer>
            </Library>";

        var titleXPathFromConfigurationFile = "./Title";
        var writerXPathFromConfigurationFile = "../Name";

        var library = ExtractBooks(xmlFromMessage, titleXPathFromConfigurationFile, writerXPathFromConfigurationFile).ToDictionary(b => b.Key, b => b.Value);

        Assert.AreEqual("Shakespeare", library["Sonnet 18"]);
        Assert.AreEqual("Tolkien", library["The Hobbit"]);
        Assert.AreEqual("Tolkien", library["Lord of the Rings"]);
    }


    public IEnumerable<KeyValuePair<string,string>> ExtractBooks(string xml, string titleXPath,  string writerXPath)
    {
        var library = XDocument.Parse(xml);
        foreach(var book in library.Descendants().Where(d => d.Name == "Book"))
        {
            var title = book.XPathSelectElement(titleXPath).Value;
            var writer = book.XPathSelectElement(writerXPath).Value;
            yield return new KeyValuePair<string, string>(title, writer);
        }
    }

Answer 1

Tomalek 指出正确：

• .NET（在 System.Xml.XPath 中）不支持 XPath 2.0，句号
• 数据模型和查询语言是分开的。

所以我通过使用第三方 XPath 2 库 XPath2 nuget package 解决了这个问题。这允许像

这样的表达式

for $c in . return ../Writer[@ID=$c/WriterRef/@REFID]/Name

请注意，我需要使用从书到作者的相对路径。这不起作用：

# does not work due to the absolute path
for $c in . return /Library/Writer[@ID=$c/WriterRef/@REFID]/Name

供将来参考：此代码在安装 nuget 包后有效：

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;
using Wmhelp.XPath2;

namespace My.Library
{
    [TestClass]
    public class WmhelpTests
    {
        [TestMethod]
        public void LibraryTest()
        {
            string xmlFromMessage = @"<Library>
                <Writer ID=""writer1""><Name>Shakespeare</Name></Writer>
                <Writer ID=""writer2""><Name>Tolkien</Name></Writer>
                <Book><WriterRef REFID=""writer1"" /><Title>King Lear</Title></Book>
                <Book><WriterRef REFID=""writer2"" /><Title>The Hobbit</Title></Book>
                <Book><WriterRef REFID=""writer2"" /><Title>Lord of the Rings</Title></Book>
            </Library>";

            var titleXPathFromConfigurationFile = "./Title";
            var writerXPathFromConfigurationFile = "for $curr in . return ../Writer[@ID=$curr/WriterRef/@REFID]/Name";

            var library = ExtractBooks(xmlFromMessage, titleXPathFromConfigurationFile, writerXPathFromConfigurationFile).ToDictionary(b => b.Key, b => b.Value);

            Assert.AreEqual("Shakespeare", library["King Lear"]);
            Assert.AreEqual("Tolkien", library["The Hobbit"]);
            Assert.AreEqual("Tolkien", library["Lord of the Rings"]);
        }


        [TestMethod]
        public void TestLibraryForCustomer2()
        {
            string xmlFromMessage = @"<Library>
                <Writer ID=""writer1"">
                    <Name>Shakespeare</Name>
                    <Book><Title>Sonnet 18</Title></Book>
                </Writer>
                <Writer ID=""writer2"">
                    <Name>Tolkien</Name>
                    <Book><Title>The Hobbit</Title></Book>
                    <Book><Title>Lord of the Rings</Title></Book>
                </Writer>
            </Library>";

            var titleXPathFromConfigurationFile = "./Title";
            var writerXPathFromConfigurationFile = "../Name";

            var library = ExtractBooks(xmlFromMessage, titleXPathFromConfigurationFile, writerXPathFromConfigurationFile).ToDictionary(b => b.Key, b => b.Value);

            Assert.AreEqual("Shakespeare", library["Sonnet 18"]);
            Assert.AreEqual("Tolkien", library["The Hobbit"]);
            Assert.AreEqual("Tolkien", library["Lord of the Rings"]);
        }


        public IEnumerable<KeyValuePair<string, string>> ExtractBooks(string xml, string titleXPath, string writerXPath)
        {
            var library = XDocument.Parse(xml);
            foreach (var book in library.Descendants().Where(d => d.Name == "Book"))
            {
                var title = book.XPath2SelectElement(titleXPath).Value;
                var writer = book.XPath2SelectElement(writerXPath).Value;
                yield return new KeyValuePair<string, string>(title, writer);
            }
        }
    }
}

Answer 2

我怀疑是否真的支持 XPath 2.0 查询语言。

但是对于像您的要求这样简单的事情，XPath 1.0 单行就足够了。

string xml = @"<Root>
    <Foo ID=""foo1"">One</Foo>
    <Foo ID=""foo2"">Two</Foo>
    <Bar><FooRef REFID=""foo2"" /></Bar>
    </Root>";
var doc = XDocument.Parse(xml);
var matchingFoo = doc.XPathSelectElement("/Root/Foo[@ID = //Bar/FooRef/@REFID]");

Assert.AreEqual("Two", matchingFoo.value);

对于超出 XPath 1.0 功能的查询，请尝试使用 LINQ。

在您的扩展示例中，您希望我们将书籍与其作者联系起来。这很容易在 LINQ 连接中完成，如下所示：

var xmlFromMessage = @"<Library>
    <Writer ID=""writer1""><Name>Shakespeare</Name></Writer>
    <Writer ID=""writer2""><Name>Tolkien</Name></Writer>
    <Book><WriterRef REFID=""writer1"" /><Title>King Lear</Title></Book>
    <Book><WriterRef REFID=""writer2"" /><Title>The Hobbit</Title></Book>
    <Book><WriterRef REFID=""writer2"" /><Title>Lord of the Rings</Title></Book>
     </Library>"; 

var libraryDoc = XDocument.Parse(xmlFromMessage);

var library = from title in libraryDoc.Descendants("Title")
    join writer in libraryDoc.Descendants("Writer")
    on title.Parent.Element("WriterRef").Attribute("REFID").Value equals writer.Attribute("ID").Value
    select new KeyValuePair<string, string>(title.Value, writer.Value);

现在library 是一个IEnumerable<KeyValuePair<string,string>>，具有标题/作者配对。