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

Microsoft 的 System.Xml.XPath nuget-package 可用于 .NET 4.6，声称支持 XPath 1.0 和 2.0。德文档 https://msdn.microsoft.com/en-us/library/system.xml.xpath(v=vs.110).aspx说描述了命名空间：

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

升级 Visual Studio、将我的所有项目升级到框架版本 4.6 后，我仍然无法让最简单的 XPath-2.0 for 表达式工作。根据规格 https://www.w3.org/TR/xpath20/#id-for-expressions，他们应该工作。

我无法想象微软声称支持它实际上并不支持的东西，所以显然我做错了。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发挥作用：

    [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);
        }
    }

托马利克正确地指出：

• .NET（在 System.Xml.XPath 中）不支持 XPath 2.0，期间
• 数据模型和查询语言是不同的东西。

所以我通过使用第三方 XPath 2 库解决了这个问题XPath2 nuget 包 https://www.nuget.org/packages/XPath2/。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。这确实not work:

# 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);
            }
        }
    }
}