Find Elements with an XPath Search

The XmlNode class defines two methods that perform XPath searches: SelectNodes and SelectSingleNode. These methods operate on all contained child nodes. Because the XmlDocument inherits from XmlNode, you can call XmlDocument.SelectNodes to search an entire document.

For example, consider the following XML document, which represents an order for two items. This document includes text and numeric data, nested elements, and attributes, and so is a good way to test simple XPath expressions.

<?xml version="1.0"?>
<Order id="2004-01-30.195496">
  <Client id="ROS-930252034">
    <Name>Remarkable Office Supplies</Name>
  </Client>

  <Items>
    <Item id="1001">
      <Name>Electronic Protractor</Name>
      <Price>42.99</Price>
    </Item>
    <Item id="1002">
      <Name>Invisible Ink</Name>
      <Price>200.25</Price>
    </Item>
  </Items>
</Order>

Basic XPath syntax uses a path-like notation. For example, the path /Order/Items/Item indicates an <Item> element that is nested inside an <Items> element, which, in turn, in nested in a root <Order> element. This is an absolute path. The following example uses an XPath absolute path to find the name of every item in an order.

using System;
using System.Xml;

public class XPathSelectNodes {

    private static void Main() {

        // Load the document.
        XmlDocument doc = new XmlDocument();
        doc.Load("orders.xml");

        // Retrieve the name of every item.
        // This could not be accomplished as easily with the
        // GetElementsByTagName() method, because Name elements are
        // used in Item elements and Client elements, and so
        // both types would be returned.
        XmlNodeList nodes = doc.SelectNodes("/Order/Items/Item/Name");
            
        foreach (XmlNode node in nodes) {
            Console.WriteLine(node.InnerText);
        }
     
        Console.ReadLine();
    }
}

The output of this program is as follows:

Electronic Protractor
Invisible Ink

XPath provides a rich and powerful search syntax, and it's impossible to explain all the variations you can use in a short example. However, Table 5.1 outlines some of the key ingredients in more advanced XPath expressions and includes examples that show how they would work with the order document. For a more detailed reference, refer to the W3C XPath recommendation at http://www.w3.org/TR/xpath.

Table 5.1: XPath Expression Syntax

Expression	Description
/	Starts an absolute path that selects from the root node. /Order/Items/Item selects all Item elements that are children of anItems element, which is itself a child of the root Order element.
//	Starts a relative path that selects nodes anywhere. //Item/Name selects all the Name elements that are children of an Item element, regardless of where they appear in the document.
@	Selects an attribute of a node. /Order/@id selects the attribute named id from the root Order element.
*	Selects any element in the path. /Order/* selects both Items and Client nodes because both are contained by a root Order element.
|	Combines multiple paths. /Order/Items/Item/Name|Order/Client/Name selects the Name nodes used to describe a Client and the Name nodes used to describe an Item.
.	Indicates the current (default) node. If the current node is an Order, the expression ./Items refers to the related items for that order.
..	Indicates the parent node. //Name/.. selects any element that is parent to a Name, which includes the Client and Item elements.
[ ]	Define selection criteria that can test a contained node or attribute value. /Order[@id="2004-01-30.195496"] selects the Order elements with the indicated attribute value. /Order/Items/Item[Price > 50] selects products above $50 in price. /Order/Items/Item[Price > 50 and Name="Laser Printer"] selects products that match two criteria.
starts-with	This function retrieves elements based on what text a contained element starts with. /Order/Items/Item[starts-with(Name, "C")] finds all Item elements that have a Name element that starts with the letter C.
position	This function retrieves elements based on position. /Order/Items/Item[position ()=2] selects the second Item element.
count	This function counts elements. You specify the name of the child element to count or an asterisk (*) for all children. /Order/Items/Item[count(Price) = 1] retrieves Item elements that have exactly one nested Price element.

Note

XPath expressions and all element and attribute names that you use inside them are always case sensitive, as XML itself is case sensitive.

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