Как написать метод оси LINQ to XML (LINQ to XML)

Статья
07.04.2023

Метод оси в XML извлекает коллекцию XML-элементов из XML-документа или предка. Можно написать свои собственные методы оси для получения коллекций из XML-дерева. Один из лучших способов выполнения этого состоит в написании метода расширения, возвращающего коллекцию элементов или атрибутов. Метод расширения можно написать для возврата указанных поднаборов элементов или атрибутов с учетом требований приложения. В этой статье приведен пример для C# и Visual Basic.

Пример

В следующем примере используются два метода расширения. Первый метод расширения, GetXPath, действует применительно к XObject и возвращает выражение XPath, которое после его вычисления возвращает узел или атрибут. Второй метод расширения, Find, применяется к XElement. Он возвращает коллекцию объектов XAttribute и XElement, содержащих некоторый указанный текст.

В этом примере используется XML-файл XML-документа Sample XML: несколько заказов на покупку.

public static class MyExtensions
{
    private static string GetQName(XElement xe)
    {
        string prefix = xe.GetPrefixOfNamespace(xe.Name.Namespace);
        if (xe.Name.Namespace == XNamespace.None || prefix == null)
            return xe.Name.LocalName.ToString();
        else
            return prefix + ":" + xe.Name.LocalName.ToString();
    }

    private static string GetQName(XAttribute xa)
    {
        string prefix =
            xa.Parent.GetPrefixOfNamespace(xa.Name.Namespace);
        if (xa.Name.Namespace == XNamespace.None || prefix == null)
            return xa.Name.ToString();
        else
            return prefix + ":" + xa.Name.LocalName;
    }

    private static string NameWithPredicate(XElement el)
    {
        if (el.Parent != null && el.Parent.Elements(el.Name).Count() != 1)
            return GetQName(el) + "[" +
                (el.ElementsBeforeSelf(el.Name).Count() + 1) + "]";
        else
            return GetQName(el);
    }

    public static string StrCat<T>(this IEnumerable<T> source,
        string separator)
    {
        return source.Aggregate(new StringBuilder(),
                   (sb, i) => sb
                       .Append(i.ToString())
                       .Append(separator),
                   s => s.ToString());
    }

    public static string GetXPath(this XObject xobj)
    {
        if (xobj.Parent == null)
        {
            XDocument doc = xobj as XDocument;
            if (doc != null)
                return ".";
            XElement el = xobj as XElement;
            if (el != null)
                return "/" + NameWithPredicate(el);
            // The XPath data model doesn't include white space text nodes
            // that are children of a document, so this method returns null.
            XText xt = xobj as XText;
            if (xt != null)
                return null;
            XComment com = xobj as XComment;
            if (com != null)
                return
                    "/" +
                    (
                        com
                        .Document
                        .Nodes()
                        .OfType<XComment>()
                        .Count() != 1 ?
                        "comment()[" +
                        (com
                        .NodesBeforeSelf()
                        .OfType<XComment>()
                        .Count() + 1) +
                        "]" :
                        "comment()"
                    );
            XProcessingInstruction pi = xobj as XProcessingInstruction;
            if (pi != null)
                return
                    "/" +
                    (
                        pi.Document.Nodes()
                        .OfType<XProcessingInstruction>()
                        .Count() != 1 ?
                        "processing-instruction()[" +
                        (pi
                        .NodesBeforeSelf()
                        .OfType<XProcessingInstruction>()
                        .Count() + 1) +
                        "]" :
                        "processing-instruction()"
                    );
            return null;
        }
        else
        {
            XElement el = xobj as XElement;
            if (el != null)
            {
                return
                    "/" +
                    el
                    .Ancestors()
                    .InDocumentOrder()
                    .Select(e => NameWithPredicate(e))
                    .StrCat("/") +
                    NameWithPredicate(el);
            }
            XAttribute at = xobj as XAttribute;
            if (at != null)
                return
                    "/" +
                    at
                    .Parent
                    .AncestorsAndSelf()
                    .InDocumentOrder()
                    .Select(e => NameWithPredicate(e))
                    .StrCat("/") +
                    "@" + GetQName(at);
            XComment com = xobj as XComment;
            if (com != null)
                return
                    "/" +
                    com
                    .Parent
                    .AncestorsAndSelf()
                    .InDocumentOrder()
                    .Select(e => NameWithPredicate(e))
                    .StrCat("/") +
                    (
                        com
                        .Parent
                        .Nodes()
                        .OfType<XComment>()
                        .Count() != 1 ?
                        "comment()[" +
                        (com
                        .NodesBeforeSelf()
                        .OfType<XComment>()
                        .Count() + 1) + "]" :
                        "comment()"
                    );
            XCData cd = xobj as XCData;
            if (cd != null)
                return
                    "/" +
                    cd
                    .Parent
                    .AncestorsAndSelf()
                    .InDocumentOrder()
                    .Select(e => NameWithPredicate(e))
                    .StrCat("/") +
                    (
                        cd
                        .Parent
                        .Nodes()
                        .OfType<XText>()
                        .Count() != 1 ?
                        "text()[" +
                        (cd
                        .NodesBeforeSelf()
                        .OfType<XText>()
                        .Count() + 1) + "]" :
                        "text()"
                    );
            XText tx = xobj as XText;
            if (tx != null)
                return
                    "/" +
                    tx
                    .Parent
                    .AncestorsAndSelf()
                    .InDocumentOrder()
                    .Select(e => NameWithPredicate(e))
                    .StrCat("/") +
                    (
                        tx
                        .Parent
                        .Nodes()
                        .OfType<XText>()
                        .Count() != 1 ?
                        "text()[" +
                        (tx
                        .NodesBeforeSelf()
                        .OfType<XText>()
                        .Count() + 1) + "]" :
                        "text()"
                    );
            XProcessingInstruction pi = xobj as XProcessingInstruction;
            if (pi != null)
                return
                    "/" +
                    pi
                    .Parent
                    .AncestorsAndSelf()
                    .InDocumentOrder()
                    .Select(e => NameWithPredicate(e))
                    .StrCat("/") +
                    (
                        pi
                        .Parent
                        .Nodes()
                        .OfType<XProcessingInstruction>()
                        .Count() != 1 ?
                        "processing-instruction()[" +
                        (pi
                        .NodesBeforeSelf()
                        .OfType<XProcessingInstruction>()
                        .Count() + 1) + "]" :
                        "processing-instruction()"
                    );
            return null;
        }
    }

    public static IEnumerable<XObject> Find(this XElement source, string value)
    {
        if (source.Attributes().Any())
        {
            foreach (XAttribute att in source.Attributes())
            {
                string contents = (string)att;
                if (contents.Contains(value))
                    yield return att;
            }
        }
        if (source.Elements().Any())
        {
            foreach (XElement child in source.Elements())
                foreach (XObject s in child.Find(value))
                    yield return s;
        }
        else
        {
            string contents = (string)source;
            if (contents.Contains(value))
                yield return source;
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        XElement purchaseOrders = XElement.Load("PurchaseOrders.xml");

        IEnumerable<XObject> subset =
            from xobj in purchaseOrders.Find("1999")
            select xobj;

        foreach (XObject obj in subset)
        {
            Console.WriteLine(obj.GetXPath());
            if (obj.GetType() == typeof(XElement))
                Console.WriteLine(((XElement)obj).Value);
            else if (obj.GetType() == typeof(XAttribute))
                Console.WriteLine(((XAttribute)obj).Value);
        }
    }
}

Imports System.Runtime.CompilerServices
Imports System.Text

Module Module1
    Sub Main()
        Dim purchaseOrders = XElement.Load("..\..\PurchaseOrders.xml")

        Dim subset = From xobj In purchaseOrders.Find("1999")

        For Each obj In subset
            Console.WriteLine(obj.GetXPath())
            If obj.GetType() = GetType(XElement) Then
                Console.WriteLine(CType(obj, XElement).Value)
            ElseIf obj.GetType() = GetType(XAttribute) Then
                Console.WriteLine(CType(obj, XAttribute).Value)
            End If
        Next
    End Sub
End Module

Public Module MyExtensions

    Private Function GetQName(ByVal xe As XElement) As String
        Dim prefix = xe.GetPrefixOfNamespace(xe.Name.Namespace)
        If xe.Name.Namespace = XNamespace.None OrElse prefix Is Nothing Then
            Return xe.Name.LocalName
        Else
            Return prefix & ":" & xe.Name.LocalName
        End If
    End Function

    Private Function GetQName(ByVal xa As XAttribute) As String
        Dim prefix = xa.Parent.GetPrefixOfNamespace(xa.Name.Namespace)
        If xa.Name.Namespace = XNamespace.None OrElse prefix Is Nothing Then
            Return xa.Name.LocalName
        Else
            Return prefix & ":" & xa.Name.LocalName
        End If
    End Function

    Private Function NameWithPredicate(ByVal el As XElement) As String
        If el.Parent IsNot Nothing AndAlso
           el.Parent.Elements(el.Name).Count() <> 1 Then
            Return GetQName(el) & "[" &
                (el.ElementsBeforeSelf(el.Name).Count() + 1) & "]"
        Else
            Return GetQName(el)
        End If
    End Function

    <Extension()>
    Public Function StrCat(Of T)(ByVal source As IEnumerable(Of T),
                                 ByVal separator As String) As String
        Return source.Aggregate(New StringBuilder,
                                Function(sb, i) sb.
                                    Append(i.ToString()).
                                    Append(separator),
                                    Function(s) s.ToString())
    End Function

    <Extension()>
    Public Function GetXPath(ByVal xobj As XObject) As String

        If xobj.Parent Is Nothing Then
            Dim doc = TryCast(xobj, XDocument)
            If doc IsNot Nothing Then Return "."

            Dim el = TryCast(xobj, XElement)
            If el IsNot Nothing Then Return "/" + NameWithPredicate(el)

            ' The XPath data model doesn't include white space text nodes
            ' that are children of a document, so this method returns null.

            Dim xt = TryCast(xobj, XText)
            If xt IsNot Nothing Then Return Nothing

            Dim com = TryCast(xobj, XComment)
            If com IsNot Nothing Then
                Return "/" &
                    If(com.Document.Nodes().OfType(Of XComment)().Count() <> 1,
                       "comment()[" & (com.NodesBeforeSelf().OfType(Of XComment)().Count() + 1) & "]",
                       "comment()")
            End If

            Dim pi = TryCast(xobj, XProcessingInstruction)
            If pi IsNot Nothing Then
                Return "/" &
                    If(pi.Document.Nodes().OfType(Of XProcessingInstruction)().Count() <> 1,
                       "processing-instruction()[" &
                           (pi.NodesBeforeSelf().OfType(Of XProcessingInstruction)().Count() + 1) & "]",
                       "processing-instruction()")
            End If
            Return Nothing
        Else
            Dim el = TryCast(xobj, XElement)
            If el IsNot Nothing Then
                Return "/" &
                    el.Ancestors().
                    InDocumentOrder().
                    Select(Function(e) NameWithPredicate(e)).StrCat("/") & NameWithPredicate(el)
            End If
            Dim at = TryCast(xobj, XAttribute)
            If at IsNot Nothing Then
                Return "/" &
                    at.Parent.
                    AncestorsAndSelf().
                    InDocumentOrder().
                    Select(Function(e) NameWithPredicate(e)).StrCat("/") & "@" & GetQName(at)
            End If
            Dim com = TryCast(xobj, XComment)
            If com IsNot Nothing Then
                Return "/" &
                    com.Parent.
                    AncestorsAndSelf().
                    InDocumentOrder().
                    Select(Function(e) NameWithPredicate(e)).StrCat("/") &
                        If(com.Parent.Nodes().OfType(Of XComment)().Count() <> 1,
                           "comment()[" & (com.NodesBeforeSelf().OfType(Of XComment)().Count() + 1) & "]",
                           "comment()")
            End If

            Dim cd = TryCast(xobj, XCData)
            If cd IsNot Nothing Then
                Return "/" &
                    cd.Parent.
                    AncestorsAndSelf().
                    InDocumentOrder().
                    Select(Function(e) NameWithPredicate(e)).StrCat("/") &
                        If(cd.Parent.Nodes().OfType(Of XText)().Count() <> 1,
                           "text()[" & (cd.NodesBeforeSelf().OfType(Of XText)().Count() + 1) & "]",
                           "text()")
            End If
            Dim tx = TryCast(xobj, XText)
            If tx IsNot Nothing Then
                Return "/" &
                    tx.Parent.
                    AncestorsAndSelf().
                    InDocumentOrder().
                    Select(Function(e) NameWithPredicate(e)).StrCat("/") &
                        If(tx.Parent.Nodes().OfType(Of XText)().Count() <> 1,
                           "text()[" & (tx.NodesBeforeSelf().OfType(Of XText)().Count() + 1) & "]",
                           "text()")
            End If
            Dim pi As XProcessingInstruction = TryCast(xobj, XProcessingInstruction)
            If pi IsNot Nothing Then
                Return "/" &
                    pi.Parent.
                    AncestorsAndSelf().
                    InDocumentOrder().
                    Select(Function(e) NameWithPredicate(e)).StrCat("/") &
                        If(pi.Parent.Nodes().OfType(Of XProcessingInstruction)().Count() <> 1,
                           "processing-instruction()[" &
                               (pi.NodesBeforeSelf().OfType(Of XProcessingInstruction)().Count() + 1) & "]",
                           "processing-instruction()")
            End If
            Return Nothing
        End If
    End Function

    <Extension()>
    Public Function Find(ByVal source As XElement, ByVal value As String) As IEnumerable(Of XObject)
        Dim results = From att In source.Attributes()
                      Where att.Value.Contains(value)
                      Let a As XObject = att
                      Select a

        If source.Elements().Any Then
            For Each result In From child In source.Elements() Select Find(child, value)
                results = If(results Is Nothing, result, results.Union(result))
            Next
        Else
            If source.Value.Contains(value) Then
                results = If(results Is Nothing,
                             New List(Of XObject) From {source},
                             results.Union(New List(Of XObject) From {source}))
            End If
        End If

        Return results
    End Function

End Module

В примере получается следующий вывод.

Выходные данные

/PurchaseOrders/PurchaseOrder[1]/@OrderDate
1999-10-20
/PurchaseOrders/PurchaseOrder[1]/Items/Item[2]/ShipDate
1999-05-21
/PurchaseOrders/PurchaseOrder[2]/@OrderDate
1999-10-22
/PurchaseOrders/PurchaseOrder[3]/@OrderDate
1999-10-22

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Как написать метод оси LINQ to XML (LINQ to XML)

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