.NET XPath 从特定元素开始并反向引用它 [英] .NET XPath starting from a specific element and backreferencing to it

问题描述

我有(实际上将有)一个应用程序，我需要在其中使用 XPath 查找特定的 xml 元素(称为元素 A)，然后找到元素 A 引用的另一个元素(元素 B).示例:

I have (actually going to have) an app where I need to find a specific xml element (call it element A) using XPath and then find another element (element B) referenced by eleement A. Example:

public static void Main(string[] args)
    {
        var s = @"
<Root>
    <Start refId = ""5""/>
    <Start refId = ""6""/>
    <Item id = ""5""/>
    <Item id = ""6""/>
</Root>
";

        var doc = new XmlDocument();
        using (var sr = new StringReader(s))
        {
            doc.Load(sr);
        }

        XmlNode nodeA = doc.SelectNodes(@"Root/Start[1]")[0];
        var selected = nodeA.SelectNodes(@"/Root/Item[@id = @refId]");
    }

这里的 nodeA 是我能找到的节点 A.selected 是我找不到的节点 B 的集合.我的问题是在 [@id = @refId] 中，我想返回到我开始的 nodeA 并获取其属性refId"的值，但我找不到方法.这有可能实现吗?

Here nodeA is the node A that I can find. selected is a collection of nodes B that I can't find. My problem is in [@id = @refId] where I want to return to nodeA that I start from and get the value of its attribute "refId", but I can't find a way how. Is this possible to achieve?

推荐答案

我认为完成您在下面评论中描述的内容的唯一方法是使用 LINQ to XML、XPath 变量、或定义您自己的 current() 函数.XPath 变量和当前函数有点麻烦，因为它们需要编写您自己的类来处理它们，但是您可以使用这些相对简单的通用类:

The only way I can see accomplishing what you describe below in the comments is to use LINQ to XML, XPath variables, or define your own current() function. XPath variables and current functions are a bit of a pain because they require writing your own classes to handle them, but you can use these relatively simple generic classes:

public class VariableContext : XsltContext
{
    IXPathNavigable CurrentNode;

    Dictionary<string, object> Values = new Dictionary<string, object>();
    public void SetValue(string name, object value)
    {
        Values[name] = value;
    }
    public void SetCurrentNode(IXPathNavigable currentNode)
    {
        CurrentNode = currentNode;
    }

    public override int CompareDocument(string baseUri, string nextbaseUri)
    {
        throw new NotImplementedException();
    }

    public override bool PreserveWhitespace(System.Xml.XPath.XPathNavigator node)
    {
        return false;
    }

    public override IXsltContextFunction ResolveFunction(string prefix, string name, System.Xml.XPath.XPathResultType[] ArgTypes)
    {
        if (name.Equals("current"))
        {
            return new CurrentFunction(CurrentNode);
        }
        throw new NotImplementedException();
    }

    public override IXsltContextVariable ResolveVariable(string prefix, string name)
    {
        object value;
        if (Values.TryGetValue(name, out value))
        {
            return new ContextVariable(name, value);
        }
        throw new ApplicationException("Unknown variable: " + name);
    }

    public override bool Whitespace
    {
        get { return false; }
    }
}

public class ContextVariable : IXsltContextVariable
{
    private string m_name;
    private object m_parameter;

    public ContextVariable(string name, object parameter)
    {
        m_name = name;
        m_parameter = parameter;

        IXPathNavigable navigable = m_parameter as IXPathNavigable;
        if (navigable != null)
        {
            m_parameter = navigable.CreateNavigator().Select(".");
        }
    }

    #region IXsltContextVariable Members

    public object Evaluate(XsltContext xsltContext)
    {
        return m_parameter;
    }

    public bool IsLocal
    {
        get { return true; }
    }

    public bool IsParam
    {
        get { return true; }
    }

    public XPathResultType VariableType
    {
        get
        {
            return XPathResultType.Any;
        }
    }

    #endregion
}

public class CurrentFunction : IXsltContextFunction
{
    private XPathNodeIterator CurrentNodeIterator;
    internal CurrentFunction(IXPathNavigable currentNode)
    {
        if (currentNode != null)
        {
            CurrentNodeIterator = currentNode.CreateNavigator().Select(".");
        }
    }

    public XPathResultType[] ArgTypes
    {
        get { return new XPathResultType[0]; }
    }

    public object Invoke(XsltContext xsltContext, object[] args, 
                         XPathNavigator docContext)
    {
        if (CurrentNodeIterator == null)
        {
            throw new ApplicationException("Current node is not set.");
        }
        return CurrentNodeIterator;
    }

    public int Maxargs
    {
        get { return 0; }
    }

    public int Minargs
    {
        get { return 0; }
    }

    public XPathResultType ReturnType
    {
        get { return XPathResultType.NodeSet; }
    }
}

一旦你在某处定义了它，你可以执行以下操作:

Once you have that defined somewhere, you can do the following:

VariableContext vc = new VariableContext();
XmlNode nodea = doc.SelectSingleNode("Root/Start[1]");
vc.SetValue("a", nodea);
XmlNodeList selected = doc.SelectNodes("/Root/Item[@id = $a/@refId]", vc);

或者使用自定义的current()函数:

or use a custom-defined current() function:

VariableContext vc2 = new VariableContext();
XmlNode nodea2 = doc.SelectSingleNode("Root/Start[1]");
vc2.SetCurrentNode(nodea2);
XmlNodeList selected2 = doc.SelectNodes("/Root/Item[@id = current()/@refId]", vc2);
PrintNodeList(selected2);

工作ideone:http://ideone.com/7wsQ46

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