[英]XPath query to find (lookup) another element by ID
我正在編寫使用XPath查詢來解析XML文件的類。 XML可能看起來像這樣:
<?xml version="1.0" encoding="UTF-8"?>
<Doc>
<Name id="aa">Alice</Name>
<Name id="bb">Bob</Name>
<Name id="cc">Candice</Name>
<Person nameid="aa"></Person>
<Person nameid="bb"></Person>
<Person nameid="aa"></Person>
</Doc>
所需的輸出是:
Alice
Bob
Alice
我正在使用C#來解析人員:
// these are dynanically defined elsewhere.
const string personXPath = "/Doc/Person";
const string nameXPath = "/Doc/Name[@id=current()/@nameid]"; // <== modify this line
void ParseXDocument(XDocument doc)
{
foreach (var personElement in doc.XPathSelectElements(personXPath))
{
var nameElement = personElement.XPathSelectElement(nameXPath);
Console.WriteLine(nameElement.Value);
}
}
僅通過修改nameXPath變量就能做到嗎? (我的軟件不應該“知道” XML結構,唯一將XML映射到我自己的類的是x-path,它們是可配置的。)
另一個例子:
[TestMethod]
public void TestLibrary()
{
string xmlFromMessage = @"<Library>
<Writer ID=""writer1""><Name>Shakespeare</Name></Writer>
<Writer ID=""writer2""><Name>Tolkien</Name></Writer>
<Book><WriterRef REFID=""writer1"" /><Title>Sonnet 18</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 = "??? what to put here ???";
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);
}
}
您應該將從第一個XPath獲得的值放在第二個表達式中。
const string personXPath = "/Doc/Person";
const string nameXPath = "/Doc/Name[@id='{0}']";
foreach (var personElement in doc.XPathSelectElements(personXPath))
{
var nameid = personElement.Attribute("nameid").Value;
var nameElement = doc.XPathSelectElement(string.Format(nameXPath, nameid));
Console.WriteLine(nameElement.Value);
}
Sombody正確指出:
因此,我通過使用第三方XPath 2庫XPath2 nuget包來解決了這個問題。 這允許像這樣的表達式
for $c in . return ../Writer[@ID=$c/WriterRef/@REFID]/Name
請注意,我需要使用從書本到作家的相對路徑。 這不起作用 :
# does not work due to the absolute path
for $c in . return /Library/Writer[@ID=$c/WriterRef/@REFID]/Name
供以后參考:此代碼在安裝nuget pacage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]
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"]);
}
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);
}
}
}
}
我使用了一個骯臟的修復程序:在我的xpath中,將“ current()”替換為實際值。 這樣,當前功能的行為類似於xslt-standard 。
class MyClass
{
// these are dynanically defined elsewhere.
const string personXPath = "/Doc/Person";
const string nameXPath = "/Doc/Name[@id=current()/@nameid]";
XElement _node;
void ParseXDocument(XDocument doc)
{
foreach (var personElement in doc.XPathSelectElements(personXPath))
{
_node = personElement; // my actual code is a bit cleaner
var nameElement = personElement.XPathSelectElement(PreParse(nameXPath));
Console.WriteLine(nameElement.Value);
}
}
/// <summary>
/// Pre-evaluates calls to current()
/// </summary>
/// <param name="xpath"></param>
/// <returns></returns>
private string PreParse(string xpath)
{
var sb = new StringBuilder();
foreach (var part in Tokenize(xpath))
{
if (part.Trim().StartsWith("current()"))
{
var query = part.Replace("current()", ".");
sb.Append("'")
.Append(EvaluateXPath(query))
.Append("'");
}
else
{
sb.Append(part);
}
}
return sb.ToString();
}
private IEnumerable<string> Tokenize(string path)
{
var begin = 0;
for (var i = 0; i < path.Length; i++)
{
if ("[=]".Contains(path[i]))
{
yield return path.Substring(begin, i - begin);
yield return path[i].ToString();
begin = i + 1;
}
}
yield return path.Substring(begin);
}
private string EvaluateXPath(string xpath)
{
var result = _node.XPathEvaluate(xpath);
if (result is IEnumerable)
foreach (var node in (IEnumerable)result)
return (node as XElement)?.Value ?? (node as XAttribute).Value;
return string.Format(CultureInfo.InvariantCulture, "{0}", result);
}
}
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