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XmlNode.SelectSingleNode Method (String)

.NET Framework 4

Selects the first XmlNode that matches the XPath expression.

Namespace:  System.Xml
Assembly:  System.Xml (in System.Xml.dll)
C#
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public XmlNode SelectSingleNode(
	string xpath
)

Parameters

xpath
Type: System.String
The XPath expression.

Return Value

Type: System.Xml.XmlNode
The first XmlNode that matches the XPath query or null 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.
Exception Condition
XPathException

The XPath expression contains a prefix.

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 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 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:

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

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

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

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

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

C#
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VB
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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);    
  }
}

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

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<?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>

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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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Annotations FAQ
The Select* methods return NULL if the namespace uri is not empty
You do need to read the documentation.  The Select* methods will return null if the XML document has a default namespace.  In that case, you need to add the namespace with an XmlNamespace manager, use the namespace prefix in your XPath string, and call the version of the Select* method that takes the XmlNamespaceManager as an argument.  If you are trying to parse XML that you, yourself did not generate, it probably has a namespace.
History
Works fine for me
Not sure why you say it will always return null...  Works fine for me.
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These aren't the classes you're looking for...
This might be the biggest microsoft joke of all. null - that's it! That is all this method will ever return you. Give up now and find a different solution; "move along"
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