XmlNode.SelectSingleNode Method (String) (System.Xml)

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SelectSingleNode Method (String)

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Microsoft Visual Studio 2008/.NET Framework 3.5

Other versions are also available for the following:

.NET Framework Class Library

XmlNode..::.SelectSingleNode Method (String)

Selects the first XmlNode that matches the XPath expression.

Namespace: System.Xml
Assembly: System.Xml (in System.Xml.dll)

Syntax

Visual Basic (Declaration)

Public Function SelectSingleNode ( _
    xpath As String _
) As XmlNode

Visual Basic (Usage)

Dim instance As XmlNode
Dim xpath As String
Dim returnValue As XmlNode

returnValue = instance.SelectSingleNode(xpath)

public XmlNode SelectSingleNode(
    string xpath
)

Visual C++

public:
XmlNode^ SelectSingleNode(
    String^ xpath
)

JScript

public function SelectSingleNode(
    xpath : String
) : XmlNode

Parameters

xpath: Type: System..::.String
The XPath expression.

Return Value

Type: System.Xml..::.XmlNode
The first XmlNode that matches the XPath query or nullNothingnullptra null reference (Nothing in Visual Basic) if no matching node is found. The XmlNode should not be expected to be connected "live" to the XML document. That is, changes that appear in the XML document may not appear in the XmlNode, and vice versa.

Exceptions

Exception	Condition
XPathException	The XPath expression contains a prefix.

Remarks

If the XPath expression requires namespace resolution, you must use the SelectSingleNode overload which takes an XmlNamespaceManager as its argument. The XmlNamespaceManager is used to resolve namespaces.

Note:
If the XPath expression does not include a prefix, it is assumed that the namespace URI is the empty namespace. If your XML includes a default namespace, you must still use the XmlNamespaceManager and add a prefix and namespace URI to it; otherwise, you will not get a selected node. For more information, see Select Nodes Using XPath Navigation.

Note:
A common issue when formulating XPath expressions is how to include a single quote (') or double quote (") in the expression. If you have to search for a value that includes a single quote, you must enclose the string in double quotes. If you need to search for a value that includes a double quote, you must enclose the string in single quotes.

For example, suppose you have the following XML:

 <bookstore>
   <book>
     <title>&apos;Emma&apos;</title>
   </book>
 </bookstore>

The following Visual Basic code selects an element that contains single quotes:

 book = root.SelectSingleNode("descendant::book[title=""'Emma'""]")

This method is a Microsoft extension to the Document Object Model (DOM).

Examples

The following example changes the price of the first Jane Austen book.

Visual Basic

Imports System
Imports System.IO
Imports System.Xml

public class Sample

  public shared sub Main()

    'Create the XmlDocument.
    Dim doc as XmlDocument = new XmlDocument()
    doc.Load("booksort.xml")

    Dim book as XmlNode
    Dim root as XmlNode = doc.DocumentElement

    book=root.SelectSingleNode("descendant::book[author/last-name='Austen']")

    'Change the price on the book.
    book.LastChild.InnerText="15.95"

    Console.WriteLine("Display the modified XML document....")
    doc.Save(Console.Out)

  end sub
end class

using System;
using System.IO;
using System.Xml;

public class Sample {

  public static void Main() {

    XmlDocument doc = new XmlDocument();
    doc.Load("booksort.xml");

    XmlNode book;
    XmlNode root = doc.DocumentElement;

    book=root.SelectSingleNode("descendant::book[author/last-name='Austen']");

    //Change the price on the book.
    book.LastChild.InnerText="15.95";

    Console.WriteLine("Display the modified XML document....");
    doc.Save(Console.Out);    
  }
}

Visual C++

#using <System.Xml.dll>

using namespace System;
using namespace System::IO;
using namespace System::Xml;
int main()
{
   XmlDocument^ doc = gcnew XmlDocument;
   doc->Load( "booksort.xml" );
   XmlNode^ book;
   XmlNode^ root = doc->DocumentElement;
   book = root->SelectSingleNode( "descendant::book[author/last-name='Austen']" );

   //Change the price on the book.
   book->LastChild->InnerText = "15.95";
   Console::WriteLine( "Display the modified XML document...." );
   doc->Save( Console::Out );
}

The example uses the file, booksort.xml, as input.

None

<?xml version="1.0"?>
<!-- A fragment of a book store inventory database -->
<bookstore xmlns:bk="urn:samples">
  <book genre="novel" publicationdate="1997" bk:ISBN="1-861001-57-8">
    <title>Pride And Prejudice</title>
    <author>
      <first-name>Jane</first-name>
      <last-name>Austen</last-name>
    </author>
    <price>24.95</price>
  </book>
  <book genre="novel" publicationdate="1992" bk:ISBN="1-861002-30-1">
    <title>The Handmaid's Tale</title>
    <author>
      <first-name>Margaret</first-name>
      <last-name>Atwood</last-name>
    </author>
    <price>29.95</price>
  </book>
  <book genre="novel" publicationdate="1991" bk:ISBN="1-861001-57-6">
    <title>Emma</title>
    <author>
      <first-name>Jane</first-name>
      <last-name>Austen</last-name>
    </author>
    <price>19.95</price>
  </book>
  <book genre="novel" publicationdate="1982" bk:ISBN="1-861001-45-3">
    <title>Sense and Sensibility</title>
    <author>
      <first-name>Jane</first-name>
      <last-name>Austen</last-name>
    </author>
    <price>19.95</price>
  </book>
</bookstore>

Platforms

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

Version Information

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Reference

XmlNode Class

XmlNode Members

SelectSingleNode Overload

System.Xml Namespace

SelectNodes

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Annotations

SelectSingleNode can be used to update nodes in an XML document.

Sean Kellt - MSFT | Edit | Show History

The guidance under return value is confusing.

The XmlNode should not be expected to be connected "live" to the XML document. That is, changes that appear in the XML document may not appear in the XmlNode, and vice versa.

This makes it seem as though using SelectSingleNode is a read-only operation in that changes made to the resulting node will not be reflected back into the XML document. Consider the following XML

 <books>

   <book>

     <title>The Hitchiker's Guide</title>

   </book>

 </books>

It is perfectly legal and expected that you could use the following code to correct the title.

  XmlDocument xml = new XmlDocument();
 
  xml.Load(@"C:\library.xml");
  
 
  XmlNode node = sessionXml.DocumentElement.SelectSingleNode"/library/book/title");
  if (node != null)
    node.InnerText = "The Hitchhiker's Guide to the Galaxy";
  
 

  
Reading the article titled Select Nodes Using XPath Navigation (http://msdn2.microsoft.com/en-us/library/d271ytdx.aspx) we see a similar but different warning.

  When the underlying document is modified, it is advisable to rerun the select. If the node modified is one that could cause the node to be added to the node list when it was not previously, or would now cause it to be removed from the node list, there is no guarantee that the node list is now accurate.

So what this means is that updates to the node returned by SelectSingleNode will make it back into the XML document. However, if the XML document changes the node may no longer point to the node that would satisfy the XPath query. For example, say we add another book element to the beginning of the xml file like so
 <books>
   <book>
     <title>The Lord of the Rings</title>
   </book>
   <book>
     <title>The Hitchiker's Guide</title>
   </book>
 </books>
 
 
Our node would now point to the second book in the list. If we called SelectSingleNode again as in the sample above, we would get the first book instead of the second.