XPathSelectElement Giving NullReferenceException

本文关键字：NullReferenceException Giving XPathSelectElement | 更新日期: 2023-09-27 18:27:17

我在正确使用XPathSelectElement时遇到问题。我正试图通过XPathSelectElement在XML文件中查找元素。

this.xdocument = XDocument.Parse (this.loadDialogues ());
XElement element = this.xdocument.XPathSelectElement ("/dialogues/npc[@npcid='1']/conversation[@id='1']/message[@mid='1']/options/option[@oid='1']/text()");

它给了我这个错误：

NullReferenceException：对象引用未设置为对象的实例

但是当我运行ex.这个查询：

XElement element = this.xdocument.XPathSelectElement ("/dialogues/npc");

一切都好。有人能告诉我怎么了吗？在那种情况下，我找不到关于这个错误的任何信息。

我的XML示例文件：

<dialogues>
<!-- Greetings messages -->
<greetings>
    <friendly>
        <text id="1">Witaj przybyszu! W czym mogę Ci pomóc?</text>
        <text id="2">Aniele, co Cię do mnie sprowadza?</text>
        <text id="3">Witaj przybyszu! Czy szukasz czegoś konkretnego?</text>
    </friendly>
    <agressive>
        <text id="1">Czego chcesz? Nie mam ochoty rozmawiać więc się streszczaj.</text>
        <text id="2">Możesz mnie zostawić? To wszystko mnie przerasta...</text>
    </agressive>
</greetings>
<!-- Farewells -->
<farewells>
    <friendly>
        <text id="1">Żegnaj</text>
        <text id="2">Dziękuję, żegnaj</text>
        <text id="3">Do zobaczenia</text>
    </friendly>
    <agressive>
        <text id="1">***** z moich oczu, przybłędo</text>
        <text id="2">Odejdź stąd natychmiast, nie masz tu czego szukać</text>
    </agressive>
</farewells>
<!-- Monologs -->
<monologs>
    <monolog id="1">
        <text id="1">
            Monolog się rozpoczyna...
        </text>
        <text id="2">
            Monolog, część środkowa...
        </text>
        <text id="3">
            Monolog się kończy...
        </text>
    </monolog>
</monologs>
<!-- NPC -->
<npc npcid="1"> 
    <conversation id="1">
        <message mid="1" trigger="greetingRandom(friendly)">                
            <options>
                <option oid="1" nextmid="2">Tak, możesz mi powiedzieć gdzie znajdę Kaplicę?</option>
                <option trigger="endConversationRandom(friendly)"></option>
            </options>
        </message>
        <message mid="2">
            <text>Musisz pójść prosto, u dołu tych schodów będzie strażnik. Powiedz mu, żę Cię przysyłam, wtedy nie powinien robić problemów.</text>
            <options>
                <option oid="1" trigger="endConversation">Dziękuję, żegnaj.</option>
            </options>
        </message>
    </conversation>
</npc>

//编辑：完整错误，包含更多信息

NullReferenceException: Object reference not set to an instance of an object
System.Xml.Linq.XNodeNavigator.get_NodeType ()  
System.Xml.XPath.NodeTypeTest.Match (IXmlNamespaceResolver nsm, System.Xml.XPath.XPathNavigator nav)  
System.Xml.XPath.AxisIterator.MoveNextCore ()  
System.Xml.XPath.BaseIterator.MoveNext ()  
System.Xml.XPath.SimpleSlashIterator.MoveNextCore ()  
System.Xml.XPath.BaseIterator.MoveNext ()  
System.Xml.XPath.SlashIterator.MoveNextCore ()  
System.Xml.XPath.BaseIterator.MoveNext ()  
System.Xml.XPath.SortedIterator..ctor (System.Xml.XPath.BaseIterator iter)  
System.Xml.XPath.ExprSLASH2.Evaluate (System.Xml.XPath.BaseIterator iter)  
System.Xml.XPath.Expression.EvaluateNodeSet (System.Xml.XPath.BaseIterator iter)  
System.Xml.XPath.CompiledExpression.EvaluateNodeSet (System.Xml.XPath.BaseIterator iter)  
System.Xml.XPath.XPathNavigator.Select (System.Xml.XPath.XPathExpression expr, IXmlNamespaceResolver ctx)  
System.Xml.XPath.XPathNavigator.Select (System.Xml.XPath.XPathExpression expr)  
System.Xml.XPath.XPathNavigator.SelectSingleNode (System.Xml.XPath.XPathExpression expression)  
System.Xml.XPath.XPathNavigator.SelectSingleNode (System.String xpath, IXmlNamespaceResolver nsResolver)  
System.Xml.XPath.Extensions.XPathSelectElement (System.Xml.Linq.XNode node, System.String xpath, IXmlNamespaceResolver nsResolver)  
System.Xml.XPath.Extensions.XPathSelectElement (System.Xml.Linq.XNode node, System.String xpath)  
DialogueParser.getConversation (Int32 npcID, Int32 conversationID) (at Assets/Dedicated Assets/Scripts/Dialogue/DialogueParser.cs:38)  
DialogueManager.Awake () (at Assets/Dedicated Assets/Scripts/Dialogue/DialogueManager.cs:33)

XPathSelectElement Giving NullReferenceException

它被称为XPathSelectElement，因为它允许您选择element节点。以text()结尾的路径尝试选择text节点，因此您将始终获得null XElement引用，因为您的路径没有选择任何元素节点。因此，使用路径选择元素节点，而不是文本节点。

最后我使用XMLDocument而不是XDocument解决了这个问题。

在Unity3D中，System.Xml.XXPath.有问题

public class DialogueParser {
    /// <summary>
    /// XMLDocument Instance
    /// </summary>
    private XmlDocument xmldocument;
    public DialogueParser()
    {
        //Populate XMLDocument Instance
        this.xmldocument = new XmlDocument ();
        this.xmldocument.LoadXml (this.loadDialogues());
    }
    /// <summary>
    /// Loads the dialogues from file into TextStream.
    /// </summary>
    /// <returns>dialogues.xml content as text for parsing</returns>
    public string loadDialogues()
    {
        TextAsset dialogues = (TextAsset) Resources.Load<TextAsset> ("dialogues");
        return dialogues.text;
    }
    public void getConversation(int npcID, int conversationID)
    {
        XmlNode el = this.xmldocument.SelectSingleNode (".//dialogues/npc[@npcid='1']/conversation[@id='1']/message[@mid='2']/text/text()");
        Debug.Log (el.Value);
    }
}

有了XMLDocument，一切都很好，所以我建议使用它而不是XDocument来读取XML文件。

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