如何执行大型 XML 文档的流式转换 (LINQ to XML)
有时，你必须转换任意大的 XML 文件并在编写你的应用程序时可以预测应用程序的内存需求量。 如果您试图用大 XML 文件填充 XML 树，则内存占用量将与文件大小成正比，也就是说会占用过多内存。 因此，您应改用流处理技术。
流处理技术最适合只需处理一次源文档的情况，您可以按文档顺序处理各个元素。 某些标准查询运算符（如 OrderBy）可以循环访问其源、收集所有数据、对数据排序，最后生成序列中的第一项。 请注意，如果使用可在生成第一项之前具体化源的查询运算符，则不会使应用程序保持小的内存需求量。
即使使用如何流处理可访问标头信息的 XML 片段中说明的技术，在试图装配包含转换的文档的 XML 树时，内存占用量也会过大。
主要方法有两种。 一种方法是使用 XStreamingElement 的延迟处理特性。 另一种方法是创建一个 XmlWriter 并使用 LINQ to XML 的功能将元素写入 XmlWriter。 本文演示了这两种方法。
示例：使用 XStreamingElement 的延迟执行功能对输出进行流式处理
以下示例在如何流处理可访问标头信息的 XML 片段中的示例基础上建立。
本示例使用 XStreamingElement 的延迟执行功能对输出进行流式处理。 本示例可在保持很小的内存需求量的同时转换非常大的文档。
请注意，自定义轴 (StreamCustomerItem) 经过专门编写，可以处理具有 Customer、Name 和 Item 元素，并且这些元素将按下面 Source.xml 文档排列的文档。 不过，将会准备一个更可靠的实现以分析无效文档。
下面是源文档 Source.xml：
<?xml version="1.0" encoding="utf-8" ?>
<Root>
<Customer>
<Name>A. Datum Corporation</Name>
<Item>
<Key>0001</Key>
</Item>
<Item>
<Key>0002</Key>
</Item>
<Item>
<Key>0003</Key>
</Item>
<Item>
<Key>0004</Key>
</Item>
</Customer>
<Customer>
<Name>Fabrikam, Inc.</Name>
<Item>
<Key>0005</Key>
</Item>
<Item>
<Key>0006</Key>
</Item>
<Item>
<Key>0007</Key>
</Item>
<Item>
<Key>0008</Key>
</Item>
</Customer>
<Customer>
<Name>Southridge Video</Name>
<Item>
<Key>0009</Key>
</Item>
<Item>
<Key>0010</Key>
</Item>
</Customer>
</Root>
static IEnumerable<XElement> StreamCustomerItem(string uri)
{
using (XmlReader reader = XmlReader.Create(uri))
{
XElement name = null;
XElement item = null;
reader.MoveToContent();
// Parse the file, save header information when encountered, and yield the
// Item XElement objects as they're created.
// Loop through Customer elements.
while (reader.Read())
{
if (reader.NodeType == XmlNodeType.Element
&& reader.Name == "Customer")
{
// move to Name element
while (reader.Read())
{
if (reader.NodeType == XmlNodeType.Element &&
reader.Name == "Name")
{
name = XElement.ReadFrom(reader) as XElement;
break;
}
}
// loop through Item elements
while (reader.Read())
{
if (reader.NodeType == XmlNodeType.EndElement)
break;
if (reader.NodeType == XmlNodeType.Element
&& reader.Name == "Item")
{
item = XElement.ReadFrom(reader) as XElement;
if (item != null)
{
XElement tempRoot = new XElement("Root",
new XElement(name)
);
tempRoot.Add(item);
yield return item;
}
}
}
}
}
}
}
static void Main(string[] args)
{
XStreamingElement root = new XStreamingElement("Root",
from el in StreamCustomerItem("Source.xml")
select new XElement("Item",
new XElement("Customer", (string)el.Parent.Element("Name")),
new XElement(el.Element("Key"))
)
);
root.Save("Test.xml");
Console.WriteLine(File.ReadAllText("Test.xml"));
}
Module Module1
Sub Main()
Dim root = New XStreamingElement("Root",
From el In New StreamCustomerItem("Source.xml")
Select <Item>
<Customer><%= el.Parent.<Name>.Value %></Customer>
<%= el.<Key> %>
</Item>
)
root.Save("Test.xml")
Console.WriteLine(My.Computer.FileSystem.ReadAllText("Test.xml"))
End Sub
End Module
Public Class StreamCustomerItem
Implements IEnumerable(Of XElement)
Private _uri As String
Public Sub New(ByVal uri As String)
_uri = uri
End Sub
Public Function GetEnumerator() As IEnumerator(Of XElement) Implements IEnumerable(Of XElement).GetEnumerator
Return New StreamCustomerItemEnumerator(_uri)
End Function
Public Function GetEnumerator1() As IEnumerator Implements IEnumerable.GetEnumerator
Return Me.GetEnumerator()
End Function
End Class
Public Class StreamCustomerItemEnumerator
Implements IEnumerator(Of XElement)
Private _current As XElement
Private _customerName As String
Private _reader As Xml.XmlReader
Private _uri As String
Public Sub New(ByVal uri As String)
_uri = uri
_reader = Xml.XmlReader.Create(_uri)
_reader.MoveToContent()
End Sub
Public ReadOnly Property Current As XElement Implements IEnumerator(Of XElement).Current
Get
Return _current
End Get
End Property
Public ReadOnly Property Current1 As Object Implements IEnumerator.Current
Get
Return Me.Current
End Get
End Property
Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext
Dim item As XElement
Dim name As XElement
' Parse the file, save header information when encountered, and return the
' current Item XElement.
' loop through Customer elements
While _reader.Read()
If _reader.NodeType = Xml.XmlNodeType.Element Then
Select Case _reader.Name
Case "Customer"
' move to Name element
While _reader.Read()
If _reader.NodeType = Xml.XmlNodeType.Element AndAlso
_reader.Name = "Name" Then
name = TryCast(XElement.ReadFrom(_reader), XElement)
_customerName = If(name IsNot Nothing, name.Value, "")
Exit While
End If
End While
Case "Item"
item = TryCast(XElement.ReadFrom(_reader), XElement)
Dim tempRoot = <Root>
<Name><%= _customerName %></Name>
<%= item %>
</Root>
_current = item
Return True
End Select
End If
End While
Return False
End Function
Public Sub Reset() Implements IEnumerator.Reset
_reader = Xml.XmlReader.Create(_uri)
_reader.MoveToContent()
End Sub
#Region "IDisposable Support"
Private disposedValue As Boolean ' To detect redundant calls
' IDisposable
Protected Overridable Sub Dispose(ByVal disposing As Boolean)
If Not Me.disposedValue Then
If disposing Then
_reader.Close()
End If
End If
Me.disposedValue = True
End Sub
Public Sub Dispose() Implements IDisposable.Dispose
Dispose(True)
GC.SuppressFinalize(Me)
End Sub
#End Region
End Class
该示例产生下面的输出：
<?xml version="1.0" encoding="utf-8"?>
<Root>
<Item>
<Customer>A. Datum Corporation</Customer>
<Key>0001</Key>
</Item>
<Item>
<Customer>A. Datum Corporation</Customer>
<Key>0002</Key>
</Item>
<Item>
<Customer>A. Datum Corporation</Customer>
<Key>0003</Key>
</Item>
<Item>
<Customer>A. Datum Corporation</Customer>
<Key>0004</Key>
</Item>
<Item>
<Customer>Fabrikam, Inc.</Customer>
<Key>0005</Key>
</Item>
<Item>
<Customer>Fabrikam, Inc.</Customer>
<Key>0006</Key>
</Item>
<Item>
<Customer>Fabrikam, Inc.</Customer>
<Key>0007</Key>
</Item>
<Item>
<Customer>Fabrikam, Inc.</Customer>
<Key>0008</Key>
</Item>
<Item>
<Customer>Southridge Video</Customer>
<Key>0009</Key>
</Item>
<Item>
<Customer>Southridge Video</Customer>
<Key>0010</Key>
</Item>
</Root>
示例：使用 LINQ to XML 将元素写入 XmlWriter
以下示例同样在如何流处理可访问标头信息的 XML 片段中的示例基础上建立。
本示例使用 LINQ to XML 的功能将元素写入 XmlWriter。 本示例可在保持很小的内存需求量的同时转换非常大的文档。
请注意，自定义轴 (StreamCustomerItem) 经过专门编写，可以处理具有 Customer、Name 和 Item 元素，并且这些元素将按下面 Source.xml 文档排列的文档。 不过，更可靠的实现将会使用 XSD 验证源文档或将会准备一个更可靠的实现以分析无效文档。
本示例与前一示例使用同一个源文档 Source.xml。 它也生成完全相同的输出。
使用 XStreamingElement 对输出 XML 进行流式处理胜于写入到 XmlWriter。
static IEnumerable<XElement> StreamCustomerItem(string uri)
{
using (XmlReader reader = XmlReader.Create(uri))
{
XElement name = null;
XElement item = null;
reader.MoveToContent();
// Parse the file, save header information when encountered, and yield the
// Item XElement objects as they're created.
// Loop through Customer elements.
while (reader.Read())
{
if (reader.NodeType == XmlNodeType.Element
&& reader.Name == "Customer")
{
// move to Name element
while (reader.Read())
{
if (reader.NodeType == XmlNodeType.Element &&
reader.Name == "Name")
{
name = XElement.ReadFrom(reader) as XElement;
break;
}
}
// Loop through Item elements
while (reader.Read())
{
if (reader.NodeType == XmlNodeType.EndElement)
break;
if (reader.NodeType == XmlNodeType.Element
&& reader.Name == "Item")
{
item = XElement.ReadFrom(reader) as XElement;
if (item != null) {
XElement tempRoot = new XElement("Root",
new XElement(name)
);
tempRoot.Add(item);
yield return item;
}
}
}
}
}
}
}
static void Main(string[] args)
{
IEnumerable<XElement> srcTree =
from el in StreamCustomerItem("Source.xml")
select new XElement("Item",
new XElement("Customer", (string)el.Parent.Element("Name")),
new XElement(el.Element("Key"))
);
XmlWriterSettings xws = new XmlWriterSettings();
xws.OmitXmlDeclaration = true;
xws.Indent = true;
using (XmlWriter xw = XmlWriter.Create("Output.xml", xws)) {
xw.WriteStartElement("Root");
foreach (XElement el in srcTree)
el.WriteTo(xw);
xw.WriteEndElement();
}
string str = File.ReadAllText("Output.xml");
Console.WriteLine(str);
}
Module Module1
Sub Main()
Dim srcTree =
From el In New StreamCustomerItem("Source.xml")
Select <Item>
<Customer><%= el.Parent.<Name>.Value %></Customer>
<%= el.<Key> %>
</Item>
Dim xws = New Xml.XmlWriterSettings()
xws.OmitXmlDeclaration = True
xws.Indent = True
Using xw = Xml.XmlWriter.Create("Output.xml", xws)
xw.WriteStartElement("Root")
For Each el In srcTree
el.WriteTo(xw)
Next
xw.WriteEndElement()
End Using
Dim s = My.Computer.FileSystem.ReadAllText("Output.xml")
Console.WriteLine(s)
End Sub
End Module
Public Class StreamCustomerItem
Implements IEnumerable(Of XElement)
Private _uri As String
Public Sub New(ByVal uri As String)
_uri = uri
End Sub
Public Function GetEnumerator() As IEnumerator(Of XElement) Implements IEnumerable(Of XElement).GetEnumerator
Return New StreamCustomerItemEnumerator(_uri)
End Function
Public Function GetEnumerator1() As IEnumerator Implements IEnumerable.GetEnumerator
Return Me.GetEnumerator()
End Function
End Class
Public Class StreamCustomerItemEnumerator
Implements IEnumerator(Of XElement)
Private _current As XElement
Private _customerName As String
Private _reader As Xml.XmlReader
Private _uri As String
Public Sub New(ByVal uri As String)
_uri = uri
_reader = Xml.XmlReader.Create(_uri)
_reader.MoveToContent()
End Sub
Public ReadOnly Property Current As XElement Implements IEnumerator(Of XElement).Current
Get
Return _current
End Get
End Property
Public ReadOnly Property Current1 As Object Implements IEnumerator.Current
Get
Return Me.Current
End Get
End Property
Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext
Dim item As XElement
Dim name As XElement
' Parse the file, save header information when encountered, and return the
' current Item XElement.
' loop through Customer elements
While _reader.Read()
If _reader.NodeType = Xml.XmlNodeType.Element Then
Select Case _reader.Name
Case "Customer"
' move to Name element
While _reader.Read()
If _reader.NodeType = Xml.XmlNodeType.Element AndAlso
_reader.Name = "Name" Then
name = TryCast(XElement.ReadFrom(_reader), XElement)
_customerName = If(name IsNot Nothing, name.Value, "")
Exit While
End If
End While
Case "Item"
item = TryCast(XElement.ReadFrom(_reader), XElement)
Dim tempRoot = <Root>
<Name><%= _customerName %></Name>
<%= item %>
</Root>
_current = item
Return True
End Select
End If
End While
Return False
End Function
Public Sub Reset() Implements IEnumerator.Reset
_reader = Xml.XmlReader.Create(_uri)
_reader.MoveToContent()
End Sub
#Region "IDisposable Support"
Private disposedValue As Boolean ' To detect redundant calls
' IDisposable
Protected Overridable Sub Dispose(ByVal disposing As Boolean)
If Not Me.disposedValue Then
If disposing Then
_reader.Close()
End If
End If
Me.disposedValue = True
End Sub
Public Sub Dispose() Implements IDisposable.Dispose
Dispose(True)
GC.SuppressFinalize(Me)
End Sub
#End Region
End Class
该示例产生下面的输出：
<Root>
<Item>
<Customer>A. Datum Corporation</Customer>
<Key>0001</Key>
</Item>
<Item>
<Customer>A. Datum Corporation</Customer>
<Key>0002</Key>
</Item>
<Item>
<Customer>A. Datum Corporation</Customer>
<Key>0003</Key>
</Item>
<Item>
<Customer>A. Datum Corporation</Customer>
<Key>0004</Key>
</Item>
<Item>
<Customer>Fabrikam, Inc.</Customer>
<Key>0005</Key>
</Item>
<Item>
<Customer>Fabrikam, Inc.</Customer>
<Key>0006</Key>
</Item>
<Item>
<Customer>Fabrikam, Inc.</Customer>
<Key>0007</Key>
</Item>
<Item>
<Customer>Fabrikam, Inc.</Customer>
<Key>0008</Key>
</Item>
<Item>
<Customer>Southridge Video</Customer>
<Key>0009</Key>
</Item>
<Item>
<Customer>Southridge Video</Customer>
<Key>0010</Key>
</Item>
</Root>
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