Visual Basic 1002005 [A Developers Notebook] [Electronic resources] نسخه متنی

5.10. Use the New XPathDocument and XPathNavigator

.NET provides a range of options for
dealing with XML in the System.Xml
namespaces. One common choice is XmlDocument, which lets
you navigate in-memory XML as a collection of node objects. For more
efficient performance, the XmlWriter and
XmlReader classes offer a streamlined way to read
and write a stream of XML. Unfortunately, neither solution is
perfect. The XmlDocument consumes too much memory,
and navigating its structure requires too much code. Furthermore,
because the XmlDocument is based on a third-party
standard (the XML DOM, or document
object model), it's difficult to improve
it without breaking compatibility. On the other hand, the
XmlWriter and XmlReader are too
restrictive, forcing you to access information linearly from start to
finish. They also make it prohibitively difficult for a developer to
provide an XML interface to non-XML data.

Note: Talk about an improvement! The revamped XPathDocument sets
a new standard for XML parsing in . NET.

.NET 2.0 proposes a solution with the
System.Xml.XPath.XPathDocument. The
XPathDocument is a cursor-based XML reader that
aims to become the only XML interface you need to use. It gives you
the freedom to move to any position in a document, and it provides
blistering speed when used with other XML standards such as XQuery,
XPath, XSLT, and XML Schema validation.

5.10.1. How do I do that?

To use an XPathDocument, you begin by loading the
document from a stream, XmlReader, or URI (which
can include a file path or an Internet address). To load the content,
you can use the Load( ) method or a constructor
argumentthey both work in the same way. In this example, the
XPathDocument is filled with the content from a
local file:

Dim Doc As New XPathDocument("c:\MyDocument.xml")

To actually move around an XPathDocument, you need
to create an
XPathNavigator by calling the
CreateNavigator( ) method.

Dim Navigator As XPathNavigator = Doc.CreateNavigator( )

The XPathNavigator includes a generous group of
methods for navigating the structure of the XML document. Some of the
methods include:

MoveToRoot( )

Jumps to the root, or document element
that contains all the other elements.

MoveToID( )

Moves to an element that has a specific
ID, as identified with the ID attribute.

MoveToNext( )

Moves to the next node at the same level
(technically called a sibling).

MoveToPrevious( )

Moves to the previous node at the same
level (technically called a sibling).

MoveToFirstChild( )

Moves down a level to the first node
contained by the current node.

MoveToParent( )

Moves up a level to the parent that
contains the current node.

Once you're positioned on an element, you can read
the element name from the Name property. You can
retrieve the contained text content from the Value
property.

Now that you've learned this much,
it's worth trying a basic example. In it,
we'll use an XML document that contains a product
catalog based on Microsoft's ASP.NET Commerce
Starter Kit. This XML file (which is available with the downloadable
content for this chapter) has the structure shown in Example 5-10.

Example 5-10. Sample XML for a product catalog

<?xml version="1.0" standalone="yes"?>
<Products>
<Product>
<ProductID>356</ProductID>
<ModelName>Edible Tape</ModelName>
<ModelNumber>STKY1</ModelNumber>
<UnitCost>3.99</UnitCost>
<CategoryName>General</CategoryName>
</Product>
<Product>
<ProductID>357</ProductID>
<ModelName>Escape Vehicle (Air)</ModelName>
<ModelNumber>P38</ModelNumber>
<UnitCost>2.99</UnitCost>
<CategoryName>Travel</CategoryName>
</Product>
...
</Products>

Example 5-11 loads this document, creates an
XPathNavigator, and moves through the nodes,
looking for the <ModelName> element for each
<Product>. When that element is found, its
value is displayed.

Example 5-11. Navigating an XML document with XPathNavigator

Imports System.Xml.XPath
Imports System.Xml
Module XPathNavigatorTest
Sub Main( )
' Load the document.
Dim Doc As New XPathDocument( _
My.Computer.FileSystem.CurrentDirectory & _
"\ProductList.xml")
' Navigate the document with an XPathNavigator.
Dim Navigator As XPathNavigator = Doc.CreateNavigator( )
' Move to the root <Products> element.
Navigator.MoveToFirstChild( )
' Move to the first contained <Product> element.
Navigator.MoveToFirstChild( )
' Loop through all the <Product> elements.
Do
' Search for the <ModelName> element inside <Product>
' and display its value.
Navigator.MoveToFirstChild( )
Do
If Navigator.Name = "ModelName" Then
Console.WriteLine(Navigator.Value)
End If
Loop While Navigator.MoveToNext( )
' Move back to the <Product> element.
Navigator.MoveToParent( )
Loop While Navigator.MoveToNext( )
End Sub
End Module

When you run this code, you'll see a display with a
list of model names for all the products.

Interestingly, the XPathNavigator also provides
strong typing for data values. Instead of
retrieving the current value as a string using the
Value property, you can use one of the properties
that automatically converts the value to another data type. Supported
properties include:

ValueAsBooleanValueAsDateTimeValueAsDoubleValueAsIntValueAsLong

To try this out, you can rewrite the loop in Example 5-11 so that it converts the price to a double
value and then displays a total with added sales tax:

Do
If Navigator.Name = "ModelName" Then
Console.WriteLine(Navigator.Value)
ElseIf Navigator.Name = "UnitCost" Then
Dim Price As Double = Navigator.ValueAsDouble * 1.15
Console.WriteLine(vbTab & "Total with tax: " & Math.Round(Price, 2))
End If
Loop While Navigator.MoveToNext( )

5.10.2. What about...

...other ways to search an XML document with the
XPathNavigator? To simplify life, you can select a
portion of the XML document to work with in an
XPathNavigator. To select this portion, you use
the Select( ) or
SelectSingleNode( )
methods of the XPathNavigator class. Both of these
methods require an XPath expression that identifies the nodes you
want to retrieve. (For more information about the XPath standard, see
the "Introducing XPath" sidebar.)

For example, the following code selects the
<ModelName> element for every product
that's in the Tools category:

' Use an XPath expression to get just the nodes that interest you
' (in this case, all product names in the Tools category).
Dim XPathIterator As XPathNodeIterator
XPathIterator = Navigator.Select ( _
"/Products/Product/ModelName[../CategoryName='Tools']")
Do While (XPathIterator.MoveNext( ))
' XPathIterator.Current is an XPathNavigator object pointed at the
' current node.
Console.WriteLine(XPathIterator.Current.Value)
Loop

Tip: The examples in this lab use an XML document with no namespace.
However, namespaces are often used in programming scenarios to allow
your program to uniquely identify the type of document it references.
If your document uses namespaces, you need to use the
XmlNamespaceManager
class and rewrite your XPath expressions to use a namespace prefix.
If you'd like an example of this technique, refer to
the downloadable samples for this lab, which demonstrate an example
with a product catalog that uses XML namespaces.

5.10.3. Where can I learn more?

The XPathNavigator class is too detailed to cover
completely in this lab. For more information, refer to both classes
in the MSDN Help. Additionally, you can learn about XML standards
like XPath, XQuery, and XML Schema from the excellent online
tutorials


at http://www.w3schools.com.

In addition, you'll find one more lab that can help
you extend your XPathDocument skills:
"Edit an XML Document with
XPathDocument," which explains the editing features
of the XPathDocument.

Warning: The editable XPathNavigator has undergone extensive changes, and the
features demonstrated in the next lab (Section 5.11) weren't
working in the last build we tested. Although it's
expected to return, features are sometimes cut even at this late
stage. If the coding model changes, you'll find
updated code in the downloadable examples for the book.