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XNode.ReadFrom Method

Creates an XNode from an XmlReader.

Namespace:  System.Xml.Linq
Assembly:  System.Xml.Linq (in System.Xml.Linq.dll)
public static XNode ReadFrom(
	XmlReader reader
)

Parameters

reader
Type: System.Xml.XmlReader

An XmlReader positioned at the node to read into this XNode.

Return Value

Type: System.Xml.Linq.XNode
An XNode that contains the node and its descendant nodes that were read from the reader. The runtime type of the node is determined by the node type (NodeType) of the first node encountered in the reader.
ExceptionCondition
InvalidOperationException

The XmlReader is not positioned on a recognized node type.

XmlException

The underlying XmlReader throws an exception.

You can use this method to write a method that returns a collection of nodes, yielding each node as the node is read from the reader. This method enables you to process arbitrarily large XML files with a very small memory footprint.

The reader that you pass to this method might throw exceptions. ReadFrom does not catch all exceptions thrown by the reader; the unhandled exceptions bubble up to the code that called ReadFrom. In particular, your code should be prepared to handle XmlException.

For an example of how to stream a more complex document, see How to: Stream XML Fragments with Access to Header Information.

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. If you use a query operator that materializes its source before yielding the first item, you will not retain a small memory footprint.

For an example of using LINQ to XML to transform extremely large XML documents while maintaining a small memory footprint, see How to: Perform Streaming Transform of Large XML Documents.

This example uses the following XML file, named Source.xml:

<?xml version="1.0" encoding="utf-8" ?> 
<Root>
  <Child Key="01">
    <GrandChild>aaa</GrandChild>
  </Child>
  <Child Key="02">
    <GrandChild>bbb</GrandChild>
  </Child>
  <Child Key="03">
    <GrandChild>ccc</GrandChild>
  </Child>
</Root>

This example creates a custom axis method that uses ReadFrom. You can query the custom axis by using a LINQ query.

NoteNote

The following example uses the yield return construct of C#. Because there is no equivalent feature in Visual Basic 2008, this example is provided only in C#.

class Program
{
    static IEnumerable<XElement> StreamRootChildDoc(string uri)
    {
        using (XmlReader reader = XmlReader.Create(uri))
        {
            reader.MoveToContent();
            // Parse the file and display each of the nodes.
            while (reader.Read())
            {
                switch (reader.NodeType)
                {
                    case XmlNodeType.Element:
                        if (reader.Name == "Child")
                        {
                            XElement el = XElement.ReadFrom(reader) as XElement;
                            if (el != null)
                                yield return el;
                        }
                        break;
                }
            }
        }
    }
    
    static void Main(string[] args)
    {
        IEnumerable<string> grandChildData =
            from el in StreamRootChildDoc("Source.xml")
            where (int)el.Attribute("Key") > 1
            select (string)el.Element("GrandChild");
    
        foreach (string str in grandChildData)
            Console.WriteLine(str);
    }
}

This example produces the following output.

bbb
ccc

.NET Framework

Supported in: 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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