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Understanding Xpath

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Kirk Allen Evans, author of "XML and ASP.NET", presents the fundamentals of XPath—one of the ancilliary technologies that is the true power of XML.
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What is XPath?

XML is simply markup for data. That's it. XML is not a magic wand; it does not specify how data is transmitted over the wire, it does not specify how data is stored. XML simply determines the format of the data: what you do with the data is up to you. That said, the real power behind XML is not solely its ability to represent data: XML's real power lies in ancillary technologies that, when combined with XML, provide robust solutions, and XPath is one of those ancillary technologies.

Version 1.0 of the XML Path Language became a World Wide Web Consortium (W3C) recommendation on November 16th, 1999. You can view the W3C recommendation for XPath 1.0 at http://www.w3.org/TR/xpath. This document shows all information relating to XPath including an overview of XPath and a description of its components.

XPath grew out of efforts to share a common syntax between XSL Transformations (XSLT) and XPointer. It allows for the search and retrieval of information within an XML document structure. XPath is not an XML syntax: rather, it uses a syntax that relates to the logical structure of an XML document.

An Analogy to SQL

Consider a relational database. Is the real power of a database the ability to simply store data, index the data, and specify relations between tables of data? After all, a database is supposed to hold data, so is the capability of persisting data the real advantage behind a relational database? If so, a simple file would suffice for this. It is easy to see that the real power of a database is the ability to use Structured Query Language (SQL) statements to retrieve subsets of data. To take this example one step further, the fact that SQL is an ANSI standard makes your knowledge of SQL applicable to different databases running on different platforms.

Using this same logic, XML would simply be a format for data storage without a prescribed way of retrieving that data. This is exactly what XPath is: XPath is the query language for XML documents. XPath is the common name used for XML Path Language. Using XPath statements, you can retrieve complex subsets of data from XML documents using a syntax that is universal across implementations. The same XPath statements that work within the System.Xml and System.Xml.XPath namespaces should work exactly the same as XPath statements in the MSXML Parser, and both should work exactly the same as other parsers that implement the W3C XPath recommendation.

An Analogy to a File Path

Computers are built around files and the organization of those files. To access files, you need to be able to navigate to different portions of the file system. One way to navigate a file system is to use the Uniform Naming Convention (UNC) for specifying the location of resources on a local area network (LAN). UNC separates folders and files using a backslash (\) character. In the good ol' DOS days before point-and-click, file systems were navigated using command-line syntax.

Go to the Start button on your computer; choose Run, and type cmd in the text box to bring up a DOS command shell window. You will see the following text:

Microsoft Windows 2000 [Version 5.00.2195]
(C) Copyright 1985-2000 Microsoft Corp.
C:\>

At the command prompt, change directories from the C: root to the Program Files\Microsoft Visual studio directory.

C:\>cd Program Files\Microsoft Visual Studio

To change directories, you specified a path for the file system to navigate. More to the point, you specified a series of location steps used to navigate to a new folder based on the current folder. XPath uses a very similar syntax. Imagine your file system as an XML document.

<?xml version="1.0" encoding="utf-8" ?>
<C>
   <INETPUB>
     <WWWROOT>
        <ASPNET_CLIENT/>
     </WWWROOT>
   </INETPUB>
<C>

We could easily represent this as an XPath statement:

C/INETPUB/WWWROOT/ASPNET_CLIENT

If we are currently positioned at the very beginning of the document there are four location steps made. But what if we were currently positioned on the WWWROOT element and wanted to reposition to the ASPNET_CLIENT element? We would specify the following XPath statement:

./ASPNET_CLIENT

The period (.) at the beginning of the XPath statement represents the expression "the context node", meaning the node that we originally started from. Instead of specifying that we are navigating based on the context node, we can also use a short form of XPath that specifies a path relative to the context node:

ASPNET_CLIENT

A location path is composed of three parts: the axis, the node-test, and zero or more predicates.

XPath Axis

The axis component of an XPath query determines the direction of the node selection in relation to the context node. An axis can be thought of as a directional query. The axes listed in Table 1 are provided in XPath.

Table 1 XPath Axes

Axis

Description

ancestor

The context node's parent, the parent's parent, and so on.

ancestor-or-self

The context node as well as its ancestors.

attribute

The attributes of the context node.

child

All children of the context element (attributes cannot have children).

descendant

All descendants of the context: children, children's children, and so on.

descendant-or-self

All descendants as well as the context node.

following

All nodes in the same document as the context node that are after the context node. This does not include descendants, attribute nodes, or namespace nodes.

following-sibling

All the following siblings of the context node. A sibling is an element occurring at the same level in the tree.

namespace

The namespace nodes of the context node.

parent

The parent of the context node.

preceding

All nodes in the same document as the context node that are immediately before the context node.

preceding-sibling

Contains the preceding siblings. If the context node is either an attribute or a name-space node, the preceding-sibling axis is empty


The examples so far have used forward axes: that is, we have only navigated to nodes that are descendants of the context node. Let's look at some examples of XPath statements using reverse axes, or axes that navigate up the document hierarchy. Consider the following representation of a file system, with drives A, C, and D, and D has a backup copy of the contents of the C drive. The context node is highlighted. This document is represented in Listing 1. Note that the line numbers are represented only for explanation and are not actually part of the XML document.

Listing 1—An XML Representation of a File System

1 <?xml version="1.0" encoding="utf-8" ?>
2 <FILESYSTEM>
3    <DRIVE LETTER="A"/>
4    <DRIVE LETTER="C">
5     <FOLDER NAME="INETPUB">
6        <FOLDER NAME="WWWROOT">
7          <FOLDER NAME="ASPNET_CLIENT" />
8        </FOLDER>
9     </FOLDER>
10     <FOLDER NAME="Program Files">
11        <FOLDER NAME="Microsoft Visual Studio .NET">
12          <FOLDER NAME="Framework SDK">
13             <FOLDER NAME="BIN"></FOLDER>
14          </FOLDER>
15        </FOLDER>
16     </FOLDER>
17   </DRIVE>
18   <DRIVE LETTER="D">
19     <FOLDER NAME="INETPUB">
20        <FOLDER NAME="WWWROOT">
21          <FOLDER NAME="ASPNET_CLIENT" />
22        </FOLDER>
23     </FOLDER>
24     <FOLDER NAME="Program Files">
25        <FOLDER NAME="Microsoft Visual Studio .NET"/>
26     </FOLDER>
27   </DRIVE>
28 </FILESYSTEM>

Working with the preceding XML structure, we introduce the following XPath statement:

parent::*

This XPath query translates to "retrieve all parent nodes of the context node", which would return the element "FOLDER" on line 10.

ancestor-or-self::*

This query would return a more complex structure, which is depicted in Listing 2. The returned nodes are highlighted.

Listing 2—An XML Representation of a File System

<?xml version="1.0" encoding="utf-8" ?>
<FILESYSTEM>
   <DRIVE LETTER="A"/>
   <DRIVE LETTER="C">
     <FOLDER NAME="INETPUB">
        <FOLDER NAME="WWWROOT">
          <FOLDER NAME="ASPNET_CLIENT" />
        </FOLDER>
     </FOLDER>
     <FOLDER NAME="Program Files">
        <FOLDER NAME="Microsoft Visual Studio .NET">
          <FOLDER NAME="Framework SDK">
             <FOLDER NAME="BIN"></FOLDER>
          </FOLDER>
        </FOLDER>
     </FOLDER>
   </DRIVE>
   <DRIVE LETTER="D">
     <FOLDER NAME="INETPUB">
        <FOLDER NAME="WWWROOT">
          <FOLDER NAME="ASPNET_CLIENT" />
        </FOLDER>
     </FOLDER>
     <FOLDER NAME="Program Files">
        <FOLDER NAME="Microsoft Visual Studio .NET"/>
     </FOLDER>
   </DRIVE>
</FILESYSTEM>

As you can see in Listing 2, a path is depicted from the context node directly to the root node.

Our examples of axes used an axis with an accompanying asterisk. The asterisk is considered a wildcard that translates to "all nodes within the specified path". This identifier is known as the node-test.

Location paths can be relative or absolute. Relative location paths consist of one or more location paths separated by backslashes. Absolute location paths consist of a backslash optionally followed by a relative location path. In other words, relative location paths navigate relative to the context node. Absolute paths specify the absolute position within the document. An absolute location path would then be:

/FILESYSTEM/DRIVE[@LETTER='C']/FOLDER[@NAME='Program Files']

Using an absolute location path, the current context node is ignored when evaluating the XPath query, except for the fact that the path being searched exists in the same document.

XPath Node Test

The XPath node test does just what its name implies: it tests nodes to determine if they meet a condition. We already used one test, the asterisk character, which specified all nodes should be returned. We can limit the nodes that are returned by specifying names. Using the document in Listing 1 again, we want to retrieve all ancestor elements that are named "DRIVE".

ancestor::DRIVE

By specifying the node name in the node test component of the XPath statement, we limit the results so that only a single node is returned, the DRIVE element on line four.

Besides using names for node-tests, we can also use node types. In Table 1, we saw that one of the axes is an attribute axis, which retrieves an attribute based on the specified node test. Again, using the document in Listing 1, the following node test would return the attribute NAME for the context node (highlighted in Listing 1):

attribute::NAME

If we wanted to select all attributes for the context node, we could also issue a wildcard node test:

attribute::*

So, the type of node returned depends partially on the axes specified. Attributes are not children of elements, so using the following XPath statement would not return any nodes:

child::NAME

This is because there is no child element of the context node that is named "NAME". We can also use XPath functions as node tests to return certain nodes. The available node tests are listed in Table 2.

Table 2 Available XPath Function Node Tests

Axis

Description

comment()

Returns True if the matched node is a comment node.

node()

Returns True for any matched node, or False if no match was found.

processing-instruction()

Returns True if the matched node is a processing-instruction.

text()

Returns True if the matched node is a text node.


Considering the analogy of an XPath statement to a SQL statement, we have looked at the equivalent in XPath to a SQL SELECT statement. Now, let's look at the equivalent to a SQL WHERE clause in XPath.

XPath Predicates

Predicates filter the resulting node sets of an XPath query, producing a new node set. A predicate can be evaluated as a Boolean or a number. When evaluated as a number, nodes matching the positional number are returned, where the index of nodes is 1-based. Listing 3 shows the same document as in Listing 1, but highlights a new context node on line 18.

Listing 3—An XML Representation of a File System

1 <?xml version="1.0" encoding="utf-8" ?>
2 <FILESYSTEM>
3    <DRIVE LETTER="A"/>
4    <DRIVE LETTER="C">
5     <FOLDER NAME="INETPUB">
6        <FOLDER NAME="WWWROOT">
7          <FOLDER NAME="ASPNET_CLIENT" />
8        </FOLDER>
9     </FOLDER>
10     <FOLDER NAME="Program Files">
11        <FOLDER NAME="Microsoft Visual Studio .NET">
12          <FOLDER NAME="Framework SDK">
13             <FOLDER NAME="BIN"></FOLDER>
14          </FOLDER>
15        </FOLDER>
16     </FOLDER>
17   </DRIVE>
18   <DRIVE LETTER="D">
19     <FOLDER NAME="INETPUB">
20        <FOLDER NAME="WWWROOT">
21          <FOLDER NAME="ASPNET_CLIENT" />
22        </FOLDER>
23     </FOLDER>
24     <FOLDER NAME="Program Files">
25        <FOLDER NAME="Microsoft Visual Studio .NET"/>
26     </FOLDER>
27   </DRIVE>
28 </FILESYSTEM>

Position and Predicates

Using a predicate, we can return the FOLDER element on line 19 using the following XPath:

child::*[position() = 1]

We can also use an abbreviated syntax to specify the same result:

child[1]

Besides using numeric position related to the context node, we can also use compound predicates to express complex Boolean results. The modulus operator is a common mechanism to test a value to see if it is even or odd. We can retrieve the even numbered child elements of the context node:

child::*[position() mod 2 = 0]

If we wanted to return only the last node, we can use the XPath function last() to test if the position of a node is the same as the position of the last node, returning the last node:

child::*[position()=last()]

Besides complex predicates, we can also specify complex location steps using axes, node tests, and predicates. If we wanted to find out all the drives on the current machine using the document in Listing 3, we could issue the following:

parent::FILESYSTEM/child::DRIVE

Abbreviated Location Path Syntax

Because XPath statements can become quite verbose, there also exists an abbreviated version of XPath statements. Using abbreviated syntax, the preceding XPath query is equivalent to:

parent::FILESYSTEM/DRIVE

Another abbreviation uses the backslash character to notate the root node of the document containing the context node. This was previously explained as an absolute location path. As an example, this XPath statement returns the FOLDER element on line 19.

/FILESYSTEM/DRIVE[@LETTER='D']/FOLDER[@NAME='INETPUB']

Using two backslashes successively indicates that the entire document should be searched recursively. This is a common misconception for developers used to UNC notation for working with directory paths. While useful in certain situations where an element pattern may occur anywhere in the current document, it is rarely used in this context.

//FOLDER[@NAME="INETPUB"]/FOLDER[@NAME="WWWROOT"]/FOLDER

This notation, while seemingly simple, becomes very complex when dissected. We begin by searching the entire document for an element called FOLDER with a child named FOLDER and a grandchild named FOLDER. We further limit the location paths by specifying the value of the NAME attribute for each FOLDER element. Finally, we return all matching grandchild FOLDER elements. This example would return the FOLDER elements on lines 7 and 21. Note that this is not the same as parent::FILESYSTEM/DRIVE, where we limit the search to a specified path and not the entire document.

Attributes and Predicates

We have seen examples of using attributes as predicates, but have not formally addressed attributes. Attributes can be retrieved using the attribute axis or by using the abbreviated syntax, an at(@) symbol. Referring to Listing 3 again, where the context node is represented on line 18, we can retrieve all attributes where the name of the attribute is LETTER:

attribute::*[name()='LETTER']

This syntax can be abbreviated to specify searching only the LETTER attribute and no other attributes:

attribute::LETTER

This syntax can be abbreviated further using the at symbol:

@LETTER

XPath Functions

We have mentioned Boolean expressions in the context of predicates, but let's take a look how we can leverage Boolean expressions in predicates. There are 29 different XPath functions relating to strings, numbers, node-sets, and Booleans. Without listing them all here, we will focus on the Boolean function not(). The not() function returns true if the argument is false, false if the argument is true. Let's take a look at what this really means by looking at an example. Here, we will select ourselves only if we contain an attribute named LETTER.

self::*[@LETTER]

What if we wanted to select ourselves only if we did not contain an attribute named LETTER? One way is to use the XPath function not().

self::*[not(@LETTER)]

This statement can be misleading, so let's think about what is really being queried. It would be easy to misinterpret this statement as " return the context node's children that are not an attribute named LETTER." Recall from Table 1 that the self-axis returns the context node. So, we actually return the DRIVE element if the predicate matches. The not() function tests to see if a LETTER attribute is present. If the LETTER attribute is present, the node-test returns false, and the context node is not selected.

XPath functions cannot be used as statements themselves. For instance, the following XPath statement is not legal:

not(@LETTER)

This is because the statement must evaluate as a node-set. In other words, we omitted two parts of the location step: the axis and the node test, we skipped right to the predicate.

Why Use XPath?

XPath is used in conjunction with XPointer and XSLT for searching documents. XPath statements can also be used individually using a Document Object Model (DOM) representation of an XML document. Chapter 6 of XML and ASP.NET, "Exploring the System.Xml Namespace", delves deeper into XPath in the .NET Framework and looks at how XPath statements can be issued using the System.Xml.XPath namespace. Chapter 5 of XML and ASP.NET, "MSXML Parser", also shows how to use the MSXML Parser to query using XPath statements.

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