LINQ to XML 軸メソッドを記述する方法 (LINQ to XML)

[アーティクル]
04/07/2023

XML 軸メソッドによって、XML ドキュメントまたは先祖要素から XML 要素のコレクションが取得されます。 XML ツリーからコレクションを取得する独自の軸メソッドを記述できます。これを行うための最適な方法の 1 つは、要素または属性のコレクションを返す拡張メソッドを記述することです。アプリケーションの要件に基づいて、要素または属性の特定のサブセットを返す拡張メソッドを記述できます。この記事では、C# と Visual Basic の例を示します。

例

次の例では、2 つの拡張メソッドを使用します。最初の拡張メソッドである GetXPath は、XObject を処理して、評価時にノードまたは属性を返す XPath 式を返します。 2 番目の拡張メソッドである Find は、XElement を処理して、指定されたテキストを含む XAttribute オブジェクトと XElement オブジェクトのコレクションを返します。

この例では、XML ドキュメント「サンプル XML ファイル: 複数の購買発注書」を使用します。

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

この例を実行すると、次の出力が生成されます。

/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

次の方法で共有

LINQ to XML 軸メソッドを記述する方法 (LINQ to XML)

例

その他のリソース