如何：执行文本到 XML 的流式转换
处理文本文件的一种方法是编写使用 yield return 构造一次流式处理一行文本文件的扩展方法。然后可以编写以迟缓延迟方式处理文本文件的 LINQ 查询。如果之后使用 XStreamingElement 对输出进行流式处理，则可以创建占用最少量内存的从文本文件到 XML 的转换，而不管源文本文件大小如何。
关于流式转换存在一些告诫。流式转换最适用于可以一次性处理整个文件并且可以按照源文档中的行顺序处理各行的情况。如果必须多次处理文件或者必须在处理行之前对行进行排序，则将失去使用流式技术所具有的许多好处。
示例
下面的文本文件 People.txt 是本示例的源文件。
#This is a comment
1,Tai,Yee,Writer
2,Nikolay,Grachev,Programmer
3,David,Wright,Inventor
下面的代码包含以延迟方式流式处理文本文件中各行的扩展方法。
备注
下面的示例使用 C# 的 yield return 构造。在 Visual Basic 中使用实现 IEnumerable(Of XElement) 接口的类来提供等效代码。有关在 Visual Basic 中实现 IEnumerable(Of T) 的示例，请参见演练：在 Visual Basic 中实现 IEnumerable(Of T)。
public static class StreamReaderSequence
{
public static IEnumerable<string> Lines(this StreamReader source)
{
String line;
if (source == null)
throw new ArgumentNullException("source");
while ((line = source.ReadLine()) != null)
{
yield return line;
}
}
}
class Program
{
static void Main(string[] args)
{
StreamReader sr = new StreamReader("People.txt");
XStreamingElement xmlTree = new XStreamingElement("Root",
from line in sr.Lines()
let items = line.Split(',')
where !line.StartsWith("#")
select new XElement("Person",
new XAttribute("ID", items[0]),
new XElement("First", items[1]),
new XElement("Last", items[2]),
new XElement("Occupation", items[3])
)
);
Console.WriteLine(xmlTree);
sr.Close();
}
}
Module Module1
Sub Main()
Dim sr = New IO.StreamReader("..\..\People.txt")
Dim xmlTree = New XStreamingElement("Root",
From line In sr.Lines()
Let items = Split(line, ",")
Where Not line.StartsWith("#")
Select <Person ID=<%= items(0) %>>
<First><%= items(1) %></First>
<Last><%= items(2) %></Last>
<Occupation><%= items(3) %></Occupation>
</Person>
)
Console.WriteLine(xmlTree)
sr.Close()
End Sub
End Module
Module StreamReaderSequence
<System.Runtime.CompilerServices.Extension()>
Public Function Lines(ByRef source As IO.StreamReader) As IEnumerable(Of String)
If source Is Nothing Then Throw New ArgumentNullException("source")
Return New StreamReaderEnumerable(source)
End Function
End Module
Public Class StreamReaderEnumerable
Implements IEnumerable(Of String)
Private _source As IO.StreamReader
Public Sub New(ByVal source As IO.StreamReader)
_source = source
End Sub
Public Function GetEnumerator() As Generic.IEnumerator(Of String) Implements IEnumerable(Of String).GetEnumerator
Return New StreamReaderEnumerator(_source)
End Function
Public Function GetEnumerator1() As IEnumerator Implements IEnumerable.GetEnumerator
Return Me.GetEnumerator()
End Function
End Class
Public Class StreamReaderEnumerator
Implements IEnumerator(Of String)
Private _current As String
Private _source As IO.StreamReader
Public Sub New(ByVal source As IO.StreamReader)
_source = source
End Sub
Public ReadOnly Property Current As String Implements Generic.IEnumerator(Of String).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
_current = _source.ReadLine()
Return If(_current IsNot Nothing, True, False)
End Function
Public Sub Reset() Implements IEnumerator.Reset
_current = Nothing
_source.DiscardBufferedData()
_source.BaseStream.Seek(0, IO.SeekOrigin.Begin)
End Sub
Public Sub Dispose() Implements IDisposable.Dispose
End Sub
End Class
此示例产生以下输出：
<Root>
<Person ID="1">
<First>Tai</First>
<Last>Yee</Last>
<Occupation>Writer</Occupation>
</Person>
<Person ID="2">
<First>Nikolay</First>
<Last>Grachev</Last>
<Occupation>Programmer</Occupation>
</Person>
<Person ID="3">
<First>David</First>
<Last>Wright</Last>
<Occupation>Inventor</Occupation>
</Person>
</Root>