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

5.11. Edit an XML Document with XPathNavigator

The XPathNavigator is
the XML interface of choice for Visual Basic 2005 applications. And
in .NET 2.0, it doesn't just work as a view onto
read-only XML datait also allows you to change XML documents,
such as by modifying text content, inserting new elements, or
removing a branch of nodes.

Note: The best feature of the XPathNavigator is its new support
for editing and inserting content.

5.11.1. How do I do that?

In the previous lab, Section 5.10,
you learned how to load XML data into an
XPathDocument, and then browse and search through
it using an XPathNavigator. If you want to make
changes, you still start with the same
XPathDocument. The secret is that you also use a
couple of additional methods in the
XPathNavigator:

SetValue( )

This method inserts a new value in the
current element, replacing the existing value.

DeleteCurrent( )

This method removes the current node from
the document.

Remember, you have two basic
choices for creating an XPathNavigator:

Use the XPathDocument.CreateNavigator( ) method

This method returns an
XPathNavigator for the whole document. You can
then move to the portion of the document you want to change.

Use the XPathDocument.Select( ) method with an XPath expression

This returns an
XPathNodeIterator that allows you to move through
your results, retrieving an XPathNavigator for
each selected node.

Example 5-12 modifies the XML document shown in Example 5-10. It increases all the prices by 10% and then
deletes nodes that don't fall into the Tools
category. Finally, it displays the altered XML document.

Example 5-12. Modifying 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")
' Use the XPathNavigator to make updates.
Dim XPathIterator As XPathNodeIterator = Doc.Select("//UnitCost")
' Increase the price by 10%.
For Each Editor As XPathNavigator In XPathIterator
Editor.SetValue((1.1 * Editor.ValueAsDouble).ToString( ))
Next
' Delete nodes that aren't in the Tools category.
XPathIterator = Doc.Select("/Products/Product[CategoryName!='Tools']")
For Each Editor As XPathNavigator In XPathIterator
Editor.DeleteCurrent( )
Next
' Show changes.
XPathEditor.MoveToRoot( )
Console.WriteLine(XPathEditor.OuterXml)
End Sub
End Module

When you run this application, the XML for the changed document is
displayed in the console window. You can also open the
ProductList_new.xml file where the changes are
saved.

In many cases, you won't just want to change a
valueyou'll need a way to insert new elements
or entire sections. The XPathNavigator includes a
handful of methods for inserting new elements and attributes in one
shot. However, the easiest way to add a block of XML is to use an
XmlWriter. If
you've worked with XML and .NET before, you probably
recognize the XmlWriter. The
XmlWriter was commonly used to write XML content
directly to a file in .NET 1.x applications. The difference in .NET
2.0 is that the XPathEditor allows you
to use the XmlWriter to write directly to your
in-memory XPathDocument.

All you need to do is start by calling one of the
XPathEditor methods that returns an
XmlWriter. These include the following, which
differ on where each places the inserted XML:

AppendChild( )

Adds a new element inside the current
element, after all existing child elements.

PrependChild( )

Adds a new element inside the current
element, before any existing child elements.

InsertAfter( )

Adds a new element after the current
element (and at the same level).

InsertBefore( )

Adds a new element just before the current
element (and at the same level).

Example 5-13 uses the AppendChild() method to add a new product to the product list XML
document.

Example 5-13. Using the AppendChild( ) method to add a new element to an XML document

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")
' Create a new product.
Dim XPathEditor As XPathNavigator = Doc.CreateEditor( )
XPathEditor.MoveToRoot( )
XPathEditor.MoveToFirstChild( )
' Use the XmlWriter to add a new <Product> complete with
' all child elements.
Dim Writer As XmlWriter = XPathEditor.AppendChild
' Insert the opening <Product> tag.
Writer.WriteStartElement("Product", _
"http://www.ibuyspy.com/ProductCatalog")
' The WriteElementString( ) method inserts a whole element at once.
Writer.WriteElementString("ProductID", "999")
Writer.WriteElementString("ModelName", "Rubber Pants")
Writer.WriteElementString("ModelNumber", "NOZ999")
Writer.WriteElementString("UnitCost", "12.99")
Writer.WriteElementString("CategoryName", "Clothing")
' Insert the closing </Product> tag and close the writer.
Writer.WriteEndElement( )
Writer.Close( )
' Show changes.
XPathEditor.MoveToRoot( )
Console.WriteLine(XPathEditor.OuterXml)
End Sub
End Module

Running Example 5-13 generates the following XML,
which is displayed in the console window and saved to the newly
generated XML file:

...
<Product>
<ProductID>999</ProductID>
<ModelName>Rubber Pants</ModelName>
<ModelNumber>NOZ999</ModelNumber>
<UnitCost>12.99</UnitCost>
<CategoryName>Clothing</CategoryName>
</Product>
...

Note: You can create multiple navigator and editor objects to
work with the same XPathDocument. However, the editors
don't perform any locking, so you
can't edit an XPathDocument on multiple threads at
the same time unless you take your own safeguards.

5.11.2. What about...

...validating
your XML? The XPathNavigator and the
XmlWriter both force you to write valid XML.
However, it's also important to check XML documents
to make sure they match specific rules. The best tool for this task
is an
XML
schema document that defines the elements,
structure, data types, and constraints for a document.

The actual schema standard is beyond the scope of this chapter. (For
a good introduction, refer to the tutorial at http://www.w3schools.com/schema.) However,
assuming you have a schema for your XML, you can validate your
document at any time by calling XPathNavigator.CheckValidity(). This method returns true if the
document conforms to the schema. Here's how to do
it:

' Load the document.
Dim Doc As New XPathDocument("c:\ProductList.xml")
' (Make updates).
' Load the schema.
' Technically, you can load a collection of schemas,
' one for each namespace in the document that you want to validate.
Dim Schemas As New XmlSchemaSet( )
Schemas.Add("http://www.ibuyspy.com/ProductCatalog", "c:\ProductListSchema.xsd")
Schemas.Compile( )
' Validate with the schema.
' Instead of submitting a null reference (Nothing), you can supply
' a delegate that points to a callback method that will be triggered
' every time an error is found when the validation check is performed.
Dim Valid As Boolean
Valid = Doc.CreateNavigator( ).CheckValidity(Schemas, Nothing)

5.11.3. Where can I learn more?

For more information about editing the
XPathDocument, look up the
"XPathNavigator class" index entry
in the MSDN Help. If you've used earlier betas of
.NET 2.0, which included the same features in a different class
( XPathEditableNavigator),
you may want to refer to http://blogs.msdn.com/dareobasanjo/archive/2004/09/03/225070.aspx
for some explanation straight from Microsoft bloggers.