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How to: Stream XML Fragments with Access to Header Information

Sometimes you have to read arbitrarily large XML files, and write your application so that the memory footprint of the application is predictable. If you attempt to populate an XML tree with a large XML file, your memory usage will be proportional to the size of the file—that is, excessive. Therefore, you should use a streaming technique instead.

One option is to write your application using XmlReader. However, you might want to use LINQ to query the XML tree. If this is the case, you can write your own custom axis method. For more information, see How to: Write a LINQ to XML Axis Method.

To write your own axis method, you write a small method that uses the XmlReader to read nodes until it reaches one of the nodes in which you are interested. The method then calls ReadFrom, which reads from the XmlReader and instantiates an XML fragment. It then yields each fragment through yield return to the method that is enumerating your custom axis method. You can then write LINQ queries on your custom axis method.

Streaming techniques are best applied in situations where you need to process the source document only once, and you can process the elements in document order. Certain standard query operators, such as OrderBy, iterate their source, collect all of the data, sort it, and then finally yield the first item in the sequence. Note that if you use a query operator that materializes its source before yielding the first item, you will not retain a small memory footprint.

Sometimes the problem gets just a little more interesting. In the following XML document, the consumer of your custom axis method also has to know the name of the customer that each item belongs to.

<?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>

The approach that this example takes is to also watch for this header information, save the header information, and then build a small XML tree that contains both the header information and the detail that you are enumerating. The axis method then yields this new, small XML tree. The query then has access to the header information as well as the detail information.

This approach has a small memory footprint. As each detail XML fragment is yielded, no references are kept to the previous fragment, and it is available for garbage collection. Note that this technique creates many short lived objects on the heap.

The following example shows how to implement and use a custom axis method that streams XML fragments from the file specified by the URI. This custom axis is specifically written such that it expects a document that has Customer, Name, and Item elements, and that those elements will be arranged as in the above Source.xml document. It is a simplistic implementation. A more robust implementation would be prepared to parse an invalid document.

NoteNote

The following example uses the yield return construct of C#. Equivalent code is provided in Visual Basic using a class that implements the IEnumerable(Of XElement) interface. For an example of implement IEnumerable(Of T) in Visual Basic, see Walkthrough: Implementing IEnumerable(Of T) in Visual Basic.

Module Module1

    Sub Main()
        Dim xmlTree = <Root>
                          <%=
                              From el In New StreamCustomerItem("Source.xml")
                              Let itemKey = CInt(el.<Key>.Value)
                              Where itemKey >= 3 AndAlso itemKey <= 7
                              Select <Item>
                                         <Customer><%= el.Parent.<Name>.Value %></Customer>
                                         <%= el.<Key> %>
                                     </Item>
                          %>
                      </Root>

        Console.WriteLine(xmlTree)
    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

This code produces the following output:

<Root>
  <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>
</Root>
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