C++ Programmer's Cookbook

Solution

Use the XmlDocument.GetElementsByTagName method, which searches an entire document and returns a System.Xml.XmlNodeList containing any matches

This code demonstrates how you could use GetElementsByTagName to calculate the total price of items in a catalog by retrieving all elements with the name "productPrice":

using System;
using System.Xml;

public class FindNodesByName {

    private static void Main() {

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

        // Retrieve all prices.
        XmlNodeList prices = doc.GetElementsByTagName("productPrice");

        decimal totalPrice = 0;
        foreach (XmlNode price in prices) {

            // Get the inner text of each matching element.
            totalPrice += Decimal.Parse(price.ChildNodes[0].Value);
        }

        Console.WriteLine("Total catalog value: " + totalPrice.ToString());
        Console.ReadLine();
    }
}

You can also search portions of an XML document by using the XmlElement.GetElementsByTagName method. It searches all the descendant nodes looking for matches. To use this method, first retrieve an XmlNode that corresponds to an element. Then cast this object to an XmlElement. The following example demonstrates how to find the price node under the first product element.

// Retrieve a reference to the first product.
XmlNode product = doc.GetElementsByTagName("products")[0];

// Find the price under this product.
XmlNode price = ((XmlElement)product).GetElementsByTagName("productPrice")[0];
Console.WriteLine("Price is " + price.InnerText);

If your elements include an attribute of type ID, you can also use a method called GetElementById to retrieve an element that has a matching ID value.

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Get XML Nodes in a Specific XML Namespace

As an example, consider the following compound XML document that includes order and client information, in two different namespaces (http://mycompany/OrderML and http://mycompany/ClientML).

<?xml version="1.0" ?>
<ord:order xmlns:ord="http://mycompany/OrderML"
 xmlns:cli="http://mycompany/ClientML">

  <cli:client>
    <cli:firstName>Sally</cli:firstName>
    <cli:lastName>Sergeyeva</cli:lastName>
  </cli:client>

  <ord:orderItem itemNumber="3211"/>
  <ord:orderItem itemNumber="1155"/>

</ord:order>

Here's a simple console application that selects all the tags in the http://mycompany/OrderML namespace:

using System;
using System.Xml;

public class SelectNodesByNamespace {

    private static void Main() {

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

        // Retrieve all order tags.
        XmlNodeList matches = doc.GetElementsByTagName("*",
          "http://mycompany/OrderML");

        // Display all the information.
        Console.WriteLine("Element \tAttributes");
        Console.WriteLine("******* \t**********");

        foreach (XmlNode node in matches) {

            Console.Write(node.Name + "\t");
            foreach (XmlAttribute attribute in node.Attributes) {
                Console.Write(attribute.Value + "  ");
            }
            Console.WriteLine();
        }
 
        Console.ReadLine();
    }
}

The output of this program is as follows:

Element         Attributes
*******         **********
ord:order       http://mycompany/OrderML  http://mycompany/ClientML
ord:orderItem   3211
ord:orderItem   1155

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Find Elements with an XPath Search

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

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 an Items 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.