在System.Xml.XPath中啓用XPath2查詢（XPathException：無效令牌）

Question

Microsofts System.Xml.XPath nuget-package，可用於.NET 4.6，聲稱支持XPath 1.0和2.0。 De documentation說descibes命名空間：

System.Xml.XPath命名空間包含定義用於瀏覽和編輯XML信息項的遊標模型的類作爲XQuery 1.0和XPath 2.0數據模型的實例。

升級Visual Studio後，升級和我的所有項目到框架版本4.6我仍然無法獲得最簡單的XPath-2.0 for-expression工作。根據specification，他們應該工作。

我無法想象微軟聲稱支持它實際上不支持的東西，所以顯然我做錯了什麼。 如何正確使用XPath2查詢？

[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" 
} 

P.S. 其實我是想什麼，是爲了得到這樣的工作：

[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

我懷疑的XPath 2.0查詢語言真正支持。

但是對於像您的要求那樣簡單的事情，XPath 1.0單行就足夠了。

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); 

對於超出XPath 1.0功能的查詢，請嘗試使用LINQ。

在您的擴展示例中，您想要將我們鏈接到書籍的作者。這是很容易在LINQ做加盟，這樣的：

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); 

現在library是IEnumerable<KeyValuePair<string,string>>與標題/寫器配對。

Answer 2

Tomalek指出正確：

• 的XPath 2.0未在.NET支持（在System.Xml.XPath），週期
• 的數據模型和查詢語言是分開的東西。

所以我解決了它通過使用第三方XPath 2庫，XPath2 nuget package。這允許表達式如

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

請注意，我需要使用從書到作家的相對路徑。這確實不工作：

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

以供將來參考：安裝的NuGet pacage後，此代碼的工作：

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); 
      } 
     } 
    } 
}