如何：使用批注转换 XSLT 样式中的 LINQ to XML 树

项目
08/22/2008

更新：November 2007

使用批注可帮助进行 XML 树的转换。

有些 XML 文档“以文档为中心兼有混合内容”。对于这样的文档，您不必知道元素的子节点的形状。例如，包含文本的节点可能具有像下面这样的外观：

<text>A phrase with <b>bold</b> and <i>italic</i> text.</text>

任何给定的文本节点都可以具有任意数量的子 <b> 和 <i> 元素。此方法可扩展到很多其他情况：即页面可以包含各种子元素，如规则段落、带项目符号的段落和位图。表中的单元格可以包含文本，下拉列表或位图。以文档为中心的 XML 的一个主要特性是您不必知道任一特定元素将具有哪些子元素。

如果在转换树中的元素时不必知道有关要转换元素的子级的太多信息，则这种方法（使用批注）就是一种有效的方法。

这种方法摘要如下：

首先，用替换元素批注树中的元素。
然后，循环访问整个树，创建一个新树，并用其批注替换树中的每个元素。本示例在名为 XForm 的函数中实现迭代和创建新树。

具体地说，此方法包括：

执行一个或多个 LINQ to XML 查询，用这些查询返回要从一种形状转换为另一种形状的元素集。对于查询中的每个元素，添加一个新 XElement 对象作为该元素的批注。在转换的新树中会用此新元素替换批注的元素。这是示例中所示的唯一需要编写的代码。
作为批注添加的新元素可以包含新的子节点，它可以形成一个具有任意形状的子树。
有一条特殊规则：如果新元素的子节点位于不同的命名空间，即专门为此建立的命名空间（在本示例中，此命名空间为 https://www.microsoft.com/LinqToXmlTransform/2007），则不会将该子元素复制到新树。而如果命名空间是上面提到的特殊命名空间，并且元素的本地名称为 ApplyTransforms，则会迭代源树中该元素的子节点并将其复制到新树（但批注的子元素本身例外，它们将根据这些规则进行转换）。
这有些类似于 XSL 中的转换规范。用于选择一组节点的查询类似于用于模板的 XPath 表达式。用于创建以批注形式保存的新 XElement 的代码类似于 XSL 中的序列构造函数，ApplyTransforms 元素的功能类似于 XSL 中的 xsl:apply-templates 元素。
采用此方法的优势之一是在用公式表述查询时，您始终是对未修改的源树编写查询。您不必担心对树所做的修改如何影响要编写的查询。

转换一个树

下面的第一个示例将所有 Paragraph 节点重命名为 para。

XNamespace xf = "https://www.microsoft.com/LinqToXmlTransform/2007";
XName at = xf + "ApplyTransforms";

XElement root = XElement.Parse(@"
<Root>
    <Paragraph>This is a sentence with <b>bold</b> and <i>italic</i> text.</Paragraph>
    <Paragraph>More text.</Paragraph>
</Root>");

// replace Paragraph with para
foreach (var el in root.Descendants("Paragraph"))
    el.AddAnnotation(
        new XElement("para",
            // same idea as xsl:apply-templates
            new XElement(xf + "ApplyTransforms")
        )
    );

// The XForm method, shown later in this topic, accomplishes the transform
XElement newRoot = XForm(root);

Console.WriteLine(newRoot);

Imports <xmlns:xf="https://www.microsoft.com/LinqToXmlTransform/2007">

Module Module1
    Dim at As XName = GetXmlNamespace(xf) + "ApplyTransforms"

    Sub Main()
        Dim root As XElement = _
            <Root>
                <Paragraph>This is a sentence with <b>bold</b> and <i>italic</i> text.</Paragraph>
                <Paragraph>More text.</Paragraph>
            </Root>

        ' Replace Paragraph with p.
        For Each el In root...<Paragraph>
            ' same idea as xsl:apply-templates
            el.AddAnnotation( _
                <para>
                    <<%= at %>></>
                </para>)
        Next

        ' The XForm function, shown later in this topic, accomplishes the transform
        Dim newRoot As XElement = XForm(root)
        Console.WriteLine(newRoot)
    End Sub
End Module

本示例生成以下输出：

<Root>
  <para>This is a sentence with <b>bold</b> and <i>italic</i> text.</para>
  <para>More text.</para>
</Root>

更复杂的转换

下面的示例对树进行查询并计算 Data 元素的平均值和总和，并将它们作为新元素添加到树中。

XNamespace xf = "https://www.microsoft.com/LinqToXmlTransform/2007";
XName at = xf + "ApplyTransforms";

XElement data = new XElement("Root",
    new XElement("Data", 20),
    new XElement("Data", 10),
    new XElement("Data", 3)
);

// while adding annotations, you can query the source tree all you want,
// as the tree is not mutated while annotating.
data.AddAnnotation(
    new XElement("Root",
        new XElement(xf + "ApplyTransforms"),
        new XElement("Average",
            String.Format("{0:F4}",
                data
                .Elements("Data")
                .Select(z => (Decimal)z)
                .Average()
            )
        ),
        new XElement("Sum",
            data
            .Elements("Data")
            .Select(z => (int)z)
            .Sum()
        )
    )
);

Console.WriteLine("Before Transform");
Console.WriteLine("----------------");
Console.WriteLine(data);
Console.WriteLine();
Console.WriteLine();

// The XForm method, shown later in this topic, accomplishes the transform
XElement newData = XForm(data);

Console.WriteLine("After Transform");
Console.WriteLine("----------------");
Console.WriteLine(newData);

Imports <xmlns:xf="https://www.microsoft.com/LinqToXmlTransform/2007">

Module Module1
    Dim at As XName = GetXmlNamespace(xf) + "ApplyTransforms"

    Sub Main()
        Dim data As XElement = _
            <Root>
                <Data>20</Data>
                <Data>10</Data>
                <Data>3</Data>
            </Root>

        ' While adding annotations, you can query the source tree all you want,
        ' as the tree is not mutated while annotating.
        data.AddAnnotation( _
            <Root>
                <<%= at %>/>
                <Average>
                    <%= _
                        String.Format("{0:F4}", _
                        data.Elements("Data") _
                        .Select(Function(z) CDec(z)).Average()) _
                    %>
                </Average>
                <Sum>
                    <%= _
                        data.Elements("Data").Select(Function(z) CInt(z)).Sum() _
                    %>
                </Sum>
            </Root> _
        )

        Console.WriteLine("Before Transform")
        Console.WriteLine("----------------")
        Console.WriteLine(data)
        Console.WriteLine(vbNewLine)

        ' The XForm function, shown later in this topic, accomplishes the transform
        Dim newData As XElement = XForm(data)

        Console.WriteLine("After Transform")
        Console.WriteLine("----------------")
        Console.WriteLine(newData)
    End Sub
End Module

本示例生成以下输出：

Before Transform
----------------
<Root>
  <Data>20</Data>
  <Data>10</Data>
  <Data>3</Data>
</Root>


After Transform
----------------
<Root>
  <Data>20</Data>
  <Data>10</Data>
  <Data>3</Data>
  <Average>11.0000</Average>
  <Sum>33</Sum>
</Root>

实施转换

小函数 XForm 可以从原始的、已批注的树创建新的、转换后的树。

该函数的伪代码非常简单：

The function takes an XElement as an argument and returns an XElement. 
If an element has an XElement annotation, then
    Return a new XElement
        The name of the new XElement is the annotation element's name.
        All attributes are copied from the annotation to the new node.
        All child nodes are copied from the annotation, with the
            exception that the special node xf:ApplyTransforms is
            recognized, and the source element's child nodes are
            iterated. If the source child node is not an XElement, it
            is copied to the new tree. If the source child is an
            XElement, then it is transformed by calling this function
            recursively.
If an element is not annotated
    Return a new XElement
        The name of the new XElement is the source element's name
        All attributes are copied from the source element to the
            destination's element.
        All child nodes are copied from the source element.
        If the source child node is not an XElement, it is copied to
            the new tree. If the source child is an XElement, then it
            is transformed by calling this function recursively.

下面是此函数的实现：

// Build a transformed XML tree per the annotations
static XElement XForm(XElement source)
{
    XNamespace xf = "https://www.microsoft.com/LinqToXmlTransform/2007";
    XName at = xf + "ApplyTransforms";

    if (source.Annotation<XElement>() != null)
    {
        XElement anno = source.Annotation<XElement>();
        return new XElement(anno.Name,
            anno.Attributes(),
            anno
            .Nodes()
            .Select(
                (XNode n) =>
                {
                    XElement annoEl = n as XElement;
                    if (annoEl != null)
                    {
                        if (annoEl.Name == at)
                            return (object)(
                                source.Nodes()
                                .Select(
                                    (XNode n2) =>
                                    {
                                        XElement e2 = n2 as XElement;
                                        if (e2 == null)
                                            return n2;
                                        else
                                            return XForm(e2);
                                    }
                                )
                            );
                        else
                            return n;
                    }
                    else
                        return n;
                }
            )
        );
    }
    else
    {
        return new XElement(source.Name,
            source.Attributes(),
            source
                .Nodes()
                .Select(n =>
                {
                    XElement el = n as XElement;
                    if (el == null)
                        return n;
                    else
                        return XForm(el);
                }
                )
        );
    }
}

' Build a transformed XML tree per the annotations.
Function XForm(ByVal source As XElement) As XElement
    If source.Annotation(Of XElement)() IsNot Nothing Then
        Dim anno As XElement = source.Annotation(Of XElement)()
        Return _
            <<%= anno.Name.ToString() %>>
                <%= anno.Attributes() %>
                <%= anno.Nodes().Select(Function(n As XNode) _
                    GetSubNodes(n, source)) %>
            </>
    Else
        Return _
            <<%= source.Name %>>
                <%= source.Attributes() %>
                <%= source.Nodes().Select(Function(n) GetExpandedNodes(n)) %>
            </>
    End If
End Function

Private Function GetSubNodes(ByVal n As XNode, ByVal s As XElement) As Object
    Dim annoEl As XElement = TryCast(n, XElement)
    If annoEl IsNot Nothing Then
        If annoEl.Name = at Then
            Return s.Nodes().Select(Function(n2 As XNode) GetExpandedNodes(n2))
        End If
    End If
    Return n
End Function

Private Function GetExpandedNodes(ByVal n2 As XNode) As XNode
    Dim e2 As XElement = TryCast(n2, XElement)
    If e2 Is Nothing Then
        Return n2
    Else
        Return XForm(e2)
    End If
End Function

完整示例

下面的代码是包括 XForm 函数的完整示例。它包括此类型转换的几种典型用法：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.Xml.Linq;

class Program
{
    static XNamespace xf = "https://www.microsoft.com/LinqToXmlTransform/2007";
    static XName at = xf + "ApplyTransforms";

    // Build a transformed XML tree per the annotations
    static XElement XForm(XElement source)
    {
        if (source.Annotation<XElement>() != null)
        {
            XElement anno = source.Annotation<XElement>();
            return new XElement(anno.Name,
                anno.Attributes(),
                anno
                .Nodes()
                .Select(
                    (XNode n) =>
                    {
                        XElement annoEl = n as XElement;
                        if (annoEl != null)
                        {
                            if (annoEl.Name == at)
                                return (object)(
                                    source.Nodes()
                                    .Select(
                                        (XNode n2) =>
                                        {
                                            XElement e2 = n2 as XElement;
                                            if (e2 == null)
                                                return n2;
                                            else
                                                return XForm(e2);
                                        }
                                    )
                                );
                            else
                                return n;
                        }
                        else
                            return n;
                    }
                )
            );
        }
        else
        {
            return new XElement(source.Name,
                source.Attributes(),
                source
                    .Nodes()
                    .Select(n =>
                    {
                        XElement el = n as XElement;
                        if (el == null)
                            return n;
                        else
                            return XForm(el);
                    }
                    )
            );
        }
    }

    static void Main(string[] args)
    {
        XElement root = new XElement("Root",
            new XComment("A comment"),
            new XAttribute("Att1", 123),
            new XElement("Child", 1),
            new XElement("Child", 2),
            new XElement("Other",
                new XElement("GC", 3),
                new XElement("GC", 4)
            ),
            XElement.Parse(
              "<SomeMixedContent>This is <i>an</i> element that " +
              "<b>has</b> some mixed content</SomeMixedContent>"),
            new XElement("AnUnchangedElement", 42)
        );

        // each of the following serves the same semantic purpose as
        // XSLT templates and sequence constructors

        // replace Child with NewChild
        foreach (var el in root.Elements("Child"))
            el.AddAnnotation(new XElement("NewChild", (string)el));

        // replace first GC with GrandChild, add an attribute
        foreach (var el in root.Descendants("GC").Take(1))
            el.AddAnnotation(
                new XElement("GrandChild",
                    new XAttribute("ANewAttribute", 999),
                    (string)el
                )
            );

        // replace Other with NewOther, add new child elements around original content
        foreach (var el in root.Elements("Other"))
            el.AddAnnotation(
                new XElement("NewOther",
                    new XElement("MyNewChild", 1),
                    // same idea as xsl:apply-templates
                    new XElement(xf + "ApplyTransforms"),
                    new XElement("ChildThatComesAfter")
                )
            );

        // change name of element that has mixed content
        root.Descendants("SomeMixedContent").First().AddAnnotation(
            new XElement("MixedContent",
                new XElement(xf + "ApplyTransforms")
            )
        );

        // replace <b> with <Bold>
        foreach (var el in root.Descendants("b"))
            el.AddAnnotation(
                new XElement("Bold",
                    new XElement(xf + "ApplyTransforms")
                )
            );

        // replace <i> with <Italic>
        foreach (var el in root.Descendants("i"))
            el.AddAnnotation(
                new XElement("Italic",
                    new XElement(xf + "ApplyTransforms")
                )
            );

        Console.WriteLine("Before Transform");
        Console.WriteLine("----------------");
        Console.WriteLine(root);
        Console.WriteLine();
        Console.WriteLine();
        XElement newRoot = XForm(root);

        Console.WriteLine("After Transform");
        Console.WriteLine("----------------");
        Console.WriteLine(newRoot);
    }
}

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports System.Xml
Imports System.Xml.Linq

Imports <xmlns:xf="https://www.microsoft.com/LinqToXmlTransform/2007">

Module Module1
    Dim at As XName = GetXmlNamespace(xf) + "ApplyTransforms"

    ' Build a transformed XML tree per the annotations.
    Function XForm(ByVal source As XElement) As XElement
        If source.Annotation(Of XElement)() IsNot Nothing Then
            Dim anno As XElement = source.Annotation(Of XElement)()
            Return _
                <<%= anno.Name.ToString() %>>
                    <%= anno.Attributes() %>
                    <%= anno.Nodes().Select(Function(n As XNode) _
                        GetSubNodes(n, source)) %>
                </>
        Else
            Return _
                <<%= source.Name %>>
                    <%= source.Attributes() %>
                    <%= source.Nodes().Select(Function(n) GetExpandedNodes(n)) %>
                </>
        End If
    End Function

    Private Function GetSubNodes(ByVal n As XNode, ByVal s As XElement) As Object
        Dim annoEl As XElement = TryCast(n, XElement)
        If annoEl IsNot Nothing Then
            If annoEl.Name = at Then
                Return s.Nodes().Select(Function(n2 As XNode) GetExpandedNodes(n2))
            End If
        End If
        Return n
    End Function

    Private Function GetExpandedNodes(ByVal n2 As XNode) As XNode
        Dim e2 As XElement = TryCast(n2, XElement)
        If e2 Is Nothing Then
            Return n2
        Else
            Return XForm(e2)
        End If
    End Function


    Sub Main()
        Dim root As XElement = _
<Root Att1='123'>
    <!--A comment-->
    <Child>1</Child>
    <Child>2</Child>
    <Other>
        <GC>3</GC>
        <GC>4</GC>
    </Other>
    <SomeMixedContent>This is <i>an</i> element that <b>has</b> some mixed content</SomeMixedContent>
    <AnUnchangedElement>42</AnUnchangedElement>
</Root>

        ' Each of the following serves the same semantic purpose as
        ' XSLT templates and sequence constructors.

        ' Replace Child with NewChild.
        For Each el In root.<Child>
            el.AddAnnotation(<NewChild><%= CStr(el) %></NewChild>)
        Next

        ' Replace first GC with GrandChild, add an attribute.
        For Each el In root...<GC>.Take(1)
            el.AddAnnotation(<GrandChild ANewAttribute='999'><%= CStr(el) %></GrandChild>)
        Next

        ' Replace Other with NewOther, add new child elements around original content.
        For Each el In root.<Other>
            el.AddAnnotation( _
                <NewOther>
                    <MyNewChild>1</MyNewChild>
                    <<%= at %>></>
                    <ChildThatComesAfter/>
                </NewOther>)
        Next

        ' Change name of element that has mixed content.
        root...<SomeMixedContent>(0).AddAnnotation( _
                <MixedContent><<%= at %>></></MixedContent>)

        ' Replace <b> with <Bold>.
        For Each el In root...<b>
            el.AddAnnotation(<Bold><<%= at %>></></Bold>)
        Next

        ' Replace <i> with <Italic>.
        For Each el In root...<i>
            el.AddAnnotation(<Italic><<%= at %>></></Italic>)
        Next

        Console.WriteLine("Before Transform")
        Console.WriteLine("----------------")
        Console.WriteLine(root)
        Console.WriteLine(vbNewLine)
        Dim newRoot As XElement = XForm(root)

        Console.WriteLine("After Transform")
        Console.WriteLine("----------------")
        Console.WriteLine(newRoot)
    End Sub
End Module

本示例生成以下输出：

Before Transform
----------------
<Root Att1="123">
  <!--A comment-->
  <Child>1</Child>
  <Child>2</Child>
  <Other>
    <GC>3</GC>
    <GC>4</GC>
  </Other>
  <SomeMixedContent>This is <i>an</i> element that <b>has</b> some mixed content</SomeMixedContent>
  <AnUnchangedElement>42</AnUnchangedElement>
</Root>


After Transform
----------------
<Root Att1="123">
  <!--A comment-->
  <NewChild>1</NewChild>
  <NewChild>2</NewChild>
  <NewOther>
    <MyNewChild>1</MyNewChild>
    <GrandChild ANewAttribute="999">3</GrandChild>
    <GC>4</GC>
    <ChildThatComesAfter />
  </NewOther>
  <MixedContent>This is <Italic>an</Italic> element that <Bold>has</Bold> some mixed content</MixedContent>
  <AnUnchangedElement>42</AnUnchangedElement>
</Root>

请参见

概念

高级 LINQ to XML 编程

通过

如何：使用批注转换 XSLT 样式中的 LINQ to XML 树

转换一个树

更复杂的转换

实施转换

完整示例

请参见

概念

其他资源