我在 C# 中使用 XPath 有什么问题？

Question

我正在尝试在 C# 应用程序中进行一些抓取。

我正在尝试访问以下页面上的 4 条信息： https://smstestbed.nist.gov/vds/current

• 创建时间
• 可用性
• 线性 X 和 Y 坐标

以下功能是我从远程加工工具轮询实时数据馈送的地方。 我遇到的问题是，虽然我能够将“CreationTime”打印到终端，但我的 XPath 使用非常笨拙，而且就This Link 而言，我似乎应该能够在两行中做我正在做的事情在我的评论之后

"//这应该是访问数据的更好方法，但由于某种原因第二行失败"

不幸的是，我得到 AvailabilityNode 为 Null。

public static void PollNIST()
    {
        string NISTSourceURL = "https://smstestbed.nist.gov/vds/current";  // Gives us a human friendly reference to the HTM
        //-------------------------------- Current (mostly) Working Version---------------------------------------------------------------------------------
        // Retrieve raw HTML
        var NISTTargetURL = NISTSourceURL;
        var NISTHttpClient = new HttpClient();
        var NISTXMLRaw = NISTHttpClient.GetStringAsync(NISTTargetURL);  // We now have all of the HTML / XML Data as a raw string
                                                                        //Console.WriteLine(MazXMLRaw.Result);                   // Prints the resulting HTML to a terminal as a debug tool    (Works)   
        XmlDocument CurNISTXML = new XmlDocument();               // Generate Blank XML Doc
        CurNISTXML.LoadXml(NISTXMLRaw.Result);                     // This (".result") passes the actual string?, should then be loaded into new XML file

        var elementHeader = CurNISTXML.GetElementsByTagName("Header");
        var curNISTHeader = elementHeader.Item(0);
        var creationTime = curNISTHeader.Attributes[0];  // We actually have the creationTime            
        string CurNISTTime = creationTime.InnerText; ; //      //*[@id="mtconnect content"]/ul/li[1]

        //This should be a far better way of accessing the data but for some reason the second line fails
        XmlNode AvailabilityNode = CurNISTXML.SelectSingleNode("/table[1]/tbody/tr[1]");  //*[@id="mtconnect content"]/table[1]/tbody/tr[1]/td[7] // Xpath Availability
        var CurNISTStatus = AvailabilityNode.InnerText; //      //*[@id="mtconnect content"]/ul/li[1]


        string CurNistX = ""; //      //*[@id="mtconnect content"]/table[5]/tbody/tr/td[7]
        string CurNistY = ""; //      //*[@id="mtconnect content"]/table[6]/tbody/tr/td[7]

        Console.WriteLine("-------BEGIN NIST DATA PACKET-------");
        Console.WriteLine("NIST Time  : " + creationTime.InnerText);
        Console.WriteLine("NIST Status: " + CurNISTStatus);    
        Console.WriteLine("NIST X Pos.: " + CurNistX);
        Console.WriteLine("NIST Y Pos.: " + CurNistY);
        Console.WriteLine("--------END NIST DATA PACKET--------");

        //var currentNIST = new NISTDataSet()// Create new instance ofNISTdata object
    }

有什么想法吗？

Answer 1

XPath 表达式

/table[1]/tbody/tr[1]

仅当文档的最外层元素是table 元素时才会成功，这似乎不太可能。我没有试图理解页面或代码的逻辑，但这看起来肯定是错误的。路径表达式开头的“/”从树的根中选择。

Answer 2

所以事实证明，我提取 XML 的方式没有任何问题，只有我的路径。

public static void PollNIST()
        {
            string NISTSourceURL = "https://smstestbed.nist.gov/vds/current";  // Gives us a human friendly reference to the HTMl
            // string NistXmlUrl = // Someone on stackexchange is claiming that there is another url for the XML but viewsource says otherwise 
            //-------------------------------- Current (mostly) Working Version---------------------------------------------------------------------------------
            var NISTHttpClient = new HttpClient();
            var NISTXMLRaw = NISTHttpClient.GetStringAsync(NISTSourceURL);  // We now have all of the HTML / XML Data as a raw string
                                                                            //Console.WriteLine(MazXMLRaw.Result);                   // Prints the resulting HTML to a terminal as a debug tool    (Works)   
            XmlDocument CurNISTXML = new XmlDocument();               // Generate Blank XML Doc
            CurNISTXML.LoadXml(NISTXMLRaw.Result);                     // This (".result") passes the actual string?, should then be loaded into new XML file

            // Get CreationTime (WORKING!)
            XmlNodeList elementHeader = CurNISTXML.GetElementsByTagName("Header");
            XmlNode curNISTHeader = elementHeader.Item(0);
            XmlAttribute creationTime = curNISTHeader.Attributes[0];  // We now have the creationTime element          
            string CurNISTTime = creationTime.InnerText;  //      //*[@id="mtconnect content"]/ul/li[1]

            // Get availability (WORKING!)
            XmlNodeList nodeAvailability = CurNISTXML.GetElementsByTagName("Availability");
            XmlNode availability = nodeAvailability.Item(0); // I think this is maybe a bit of a hackish / improper way to do this?
            string curNISTStatus = availability.InnerText;

            //Get linear tool X Coord.
            XmlNodeList deviceStream = CurNISTXML.GetElementsByTagName("ComponentStream");
            XmlNode linearCompXStream = deviceStream.Item(4);
            string curNISTX = linearCompXStream.InnerText; //  We do not need to break down the nodes any further as the value is the only text within

            //Get Linear tool y Coord.            
            XmlNode linearCompYStream = deviceStream.Item(5);
            string curNISTY = linearCompYStream.InnerText; //  We do not need to break down the nodes any further as the value is the only text within


            Console.WriteLine("-------BEGIN NIST DATA PACKET-------");
            Console.WriteLine("NIST Time  : " + creationTime.InnerText);
            Console.WriteLine("NIST Status: " + curNISTStatus);    
            Console.WriteLine("NIST X Pos.: " + curNISTX);
            Console.WriteLine("NIST Y Pos.: " + curNISTY);
            Console.WriteLine("--------END NIST DATA PACKET--------");

            //var currentNIST = new NISTDataSet()// Create new instance ofNISTdata object
        }

效果很好。