Manipulate XML data with XPath and XmlDocument (C#)

Introduction

Based on a section of easy-to-read XML source data, I'll show you how to select and locate XML nodes and navigate through them using XPathNavigator and XPathNodeIterator. I will provide a few straightforward samples about XPath expression with which you could follow without difficulty. In the last part, there is some sample code to update, insert and remove XML nodes.

Some Concepts

XML - Extensible Markup Language, describe data structures in text format and with your own vocabularies, which means it does not use predefined tags and the meaning of these tags are not well understood.

XSL - Extensible Stylesheet Language, is designed for expressing stylesheets for XML documents. XSL is to XML as CSS is to HTML.

XML Transformation - is a user-defined algorithm that transforms a given XML document to another format, such as XML, HTML, XHTML. The algorithm is described by XSL.

XSLT - is designed for use as part of XSL, transforming an XML document into another XML document, or another type of document that is recognized by a browser, like HTML or XHTML. XSLT uses XPath.

XPath - is a set of syntax rules for defining parts of an XML document.

To keep this article simple and clear, I'll break it down into two parts, and put XSL, XSLT to my next article.

Using the code

Here is the source XML data:

<?xml version="1.0" encoding="ISO-8859-1"?>

<catalog>

  <cd country="USA">

    <title>Empire Burlesque</title>

    <artist>Bob Dylan</artist>

    <price>10.90</price>

  </cd>

  <cd country="UK">

    <title>Hide your heart</title>

    <artist>Bonnie Tyler</artist>

    <price>10.0</price>

  </cd>

  <cd country="USA">

    <title>Greatest Hits</title>

    <artist>Dolly Parton</artist>

    <price>9.90</price>

  </cd>

</catalog>

If you want to select all of the price elements, here is the code:

using System.Xml;

using System.Xml.XPath;

....

string fileName = "data.xml";

XPathDocument doc = new XPathDocument(fileName);

XPathNavigator nav = doc.CreateNavigator();

// Compile a standard XPath expression

XPathExpression expr; 

expr = nav.Compile("/catalog/cd/price");

XPathNodeIterator iterator = nav.Select(expr);

// Iterate on the node set

listBox1.Items.Clear();

try

{

  while (iterator.MoveNext())

  {

     XPathNavigator nav2 = iterator.Current.Clone();

     listBox1.Items.Add("price: " + nav2.Value);

  }

}

catch(Exception ex) 

{

   Console.WriteLine(ex.Message);

}

In the above code, we used "/catalog/cd/price" to select all the price elements. If you just want to select all the cd elements with price greater than 10.0, you can use "/catalog/cd[price>10.0]". Here are some more examples of XPath expressions:

/catalog selects the root element

/catalog/cd selects all the cd elements of the catalog element

/catalog/cd/price selects all the price elements of all the cd elements of the catalog element

/catalog/cd[price>10.0] selects all the cd elements with price greater than 10.0

starts with a slash(/) represents an absolute path to an element

starts with two slashes(//) selects all elements that satisfy the criteria

//cd selects all cd elements in the document

/catalog/cd/title | /catalog/cd/artist selects all the title and artist elements of the cd elements of catalog

//title | //artist selects all the title and artist elements in the document

/catalog/cd/* selects all the child elements of all cd elements of the catalog element

/catalog/*/price selects all the price elements that are grandchildren of catalog

/*/*/price selects all price elements which have two ancestors

//* selects all elements in the document

/catalog/cd[1] selects the first cd child of catalog

/catalog/cd[last()] selects the last cd child of catalog

/catalog/cd[price] selects all the cd elements that have price

/catalog/cd[price=10.90] selects cd elements with the price of 10.90

/catalog/cd[price=10.90]/price selects all price elements with the price of 10.90

//@country selects all "country" attributes

//cd[@country] selects cd elements which have a "country" attribute

//cd[@*] selects cd elements which have any attribute

//cd[@country='UK'] selects cd elements with "country" attribute equal to 'UK'

Read more: Codeproject

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