如何在C＃中選擇具有給定屬性值的節點？

Question

我只想選擇具有「name」屬性值='syslog'的目標元素。無論如何，我總是得到一個NullReferenceException。任何人都可以幫我弄清楚是什麼問題？

----------------我的代碼---------------------------- ---------------

XmlNode root = _configFile.DocumentElement; // root is not none, and is correct. 
XmlNode syslogNode = root.SelectSingleNode("descendant::targets/target[@name='syslog']"); // the syslogNode is null 

-----------------我的XML文件-------- ------------------------------

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" 
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> 

    <extensions> 
    <add assembly="NLog.Targets.Syslog" /> 
    </extensions> 

    <targets> 
    <target name="syslog" xsi:type="Syslog" syslogserver="127.0.0.1" port="514" facility="Local7" /> 
    <target name="file" xsi:type="File" layout="${level} | ${longdate} | ${callsite:className=true:fileName=false:includeSourcePath=false:methodName=true} | ${message} ${exception:format=tostring}" 
      fileName="${specialfolder:folder=LocalApplicationData}/Televic Conference/CoCon/Log/server_log_${shortdate}.txt" 
      archiveFileName="${specialfolder:folder=LocalApplicationData}/Televic Conference/CoCon/Log/Archives/server_log.{#}.txt" 
      archiveEvery="Day" archiveNumbering="Rolling" maxArchiveFiles="20" concurrentWrites="true" keepFileOpen="false"/> 
    <target name="console" xsi:type="ColoredConsole" layout="${longdate}: ${message}"/> 
    <target name="debug" xsi:type="OutputDebugString" layout="${longdate}: ${message}"/> 
    </targets> 

    <rules> 
    <logger name="*" minLevel="Trace" appendTo="syslog"/> 
    <logger name="*" minlevel="Trace" writeTo="file" /> 
    </rules> 
</nlog> 

Answer 1

使用LINQ2XML..Its 酷

XElement doc = XElement.Load("yourStream.xml"); 
XNamespace g = "http://www.nlog-project.org/schemas/NLog.xsd";//global namespace g 

foreach (var itm in doc.Descendants(g + "targets").Where(x=>x.Atrribute("name").Value=="syslog")) 
{ 
itm;//your required node 
} 

Answer 2

您的文檔有一個默認命名空間 - 您需要將它在您的XPath表達式中考慮或忽略它。在這裏看到：

XPath select node with namespace