XML Navigation in VB.NET

As you've seen, XmlNode provides a way to navigate DOM tree with the help of its FirstChild, ChildNodes, LastChild, PreviousNode, NextSibling, and PreviousSibling methods.

Besides XmlNode, the XML.NET has two more classes, which help you navigate XML documents. These classes are XPathDocument and XPathNavigator. The System.Xml.Xpath namespace defines both these classes.

The XPath namespace contains classes to provide read-only, fast access to documents. Before using these classes, you must add a reference of the System.Xml.Xpath namespace to your application.

XPathNodeIterator, XPathExpression, and XPathException are other classes defined in this namespace. The XPathNodeIterator class provides iteration capabilities to a node. XPathExpression provides selection criteria to select a set of nodes from a document based on those criteria, and the XPathExection class is an exception class. The XPathDocument class provides a fast cache for XML document processing using XSLT and XPath.

You use the XPathDocument constructor to create an instance of XmlPathDocument. It has many overloaded constructors. You can pass an XmlReader, TextReader, or even direct XML filenames.

The XPathNavigator class

The XPathNavigator class implements the functionality to navigate through a document. It has easy-to-use and self-explanatory methods. You create an XPathNavigator instance by calling XPpathDocument's CreateNavigator method.

You can also create a XPathNavigator object by calling XmlDocument's CreateNavigator method. For example, the following code calls XmlDocument's CreateNavigator method to create a XPathNavigator object:

        ' Load books.xml document
        Dim xmlDoc As New XmlDocument()
        xmlDoc.Load("c:\books.xml")
        ' Create XPathNavigator object by calling create Navigator of XmlDocument
        Dim nav As XPathNavigator = xmlDoc.CreateNavigator()


Note: Don't forget to add a reference of the System.Xml .XPath to your project before using any of its classes.

XPathNavigator contain methods and properties to move to the first, next, child, parent, and root nodes to the document.

X path Navigator move methods

Table 6-8 describes the XPathNavigator class's move methods. Some of these methods are MoveToFirst, moveToNext , MoveToroot, MoveToFirstAttribute, MoveToFirstChild, MoveToId, MoveToNamespace, MoveToPrevious, MoveToParent and so on.

Table 6-8. XPathNavigator Memebers

So, with the help of these methods, you can move through a document as a DOM tree. Listing 6-31 uses the MoveToRoot and MoveToFirstChild methods to move to the root node and first child of the root node. Once you have a root, you can display corresponding information such as name, value, node type, and so on.

Listing 6-31. Moving to root and first child nodes using XpathNavigator

       ' Load books.xml document
        Dim xmlDoc As New XmlDocument()
        xmlDoc.Load("c:\books.xml")
 
        ' Create XPathNavigator object by calling CreateNavigator of XmlDocument
        Dim nav As XPathNavigator = xmlDoc.CreateNavigator()

        'Move to root node
        nav.MoveToRoot()
        Dim name As String = nav.Name
        Console.WriteLine("Root node info: ")
        Console.WriteLine("Base URI" & nav.BaseURI.ToString())
        Console.WriteLine("Name:" & nav.Name.ToString())
        Console.WriteLine("Node Type: " & nav.NodeType.ToString())
        Console.WriteLine("Node Value: " & nav.Value.ToString())

        If nav.HasChildren Then
            nav.MoveToFirstChild()
        End If

Now, using the MoveToNext and MoveToParent methods, you can move through the entire document. Listing 6-32 Moves though an entire document and displays the data on the console. The GetNodeInfo method displays a node's information, and you call it recursively.

Listing 6-32. Reading a document using XpathNavigator

    Private Shared Sub Main(ByVal args As String())
        ' Load books.xml document
        Dim xmlDoc As New XmlDocument()
        xmlDoc.Load("c:\books.xml")

        ' Create XPathNavigator object by calling CreateNavigator of XmlDocument
        Dim nav As XPathNavigator = xmlDoc.CreateNavigator()

        ' move to root node
        nav.MoveToRoot()
        Dim name As String = nav.Name
        Console.WriteLine("Root node info: ")
        Console.WriteLine("Base URI" & nav.BaseURI.ToString())
        Console.WriteLine("Name: " & nav.NodeType.ToString())
        Console.WriteLine("Node Type: " & nav.NodeType.ToString())
        Console.WriteLine("Node Value: " & nav.Value.ToString())

        If nav.HasChildren Then
            nav.MoveToFirstChild()
            GetNodeInfo(nav)
        End If
    End Sub
 
    Private Shared Sub GetNodeInfo(ByVal nav1 As XPathNavigator)
        Console.WriteLine("Name: " & nav1.Name.ToString())
        Console.WriteLine("Node Type: " & nav1.NodeType.ToString())
        Console.WriteLine("Node value: " & nav1.Value.ToString())

        ' If node has children, move to first child.
        If nav1.HasChildren Then
            nav1.MoveToFirstChild()
            While nav1.MoveToNext()
                GetNodeInfo(nav1)
                nav1.MoveToParent()
            End While
        Else
 
            ' Else move to next sibling
            nav1.MoveToNext()
            GetNodeInfo(nav1)
        End If
    End Sub

Searching Using XPath Navigator

Select, SelectChildren, SelectAncestors, and SelectDescendents are other useful methods. Specifically, these methods are useful when you need to select a document's items based on an XPath expression. For example, you could use one when selecting nodes for the author tag only and so on. Now, say you want to search and display all <first- name> tag nodes in the books.xml document.

In listing 6-33, you use XPathNavigator's Select method to apply a criteria (all elements with the author-name tag) to read and display all nodes.

Listing 6-33. Use of XPathIterator and Select

        ' Load books.xml document
        Dim xmlDoc As New XmlDocument()
        xmlDoc.Load("c:\books.xml")

        ' Create XPathNavigator object by calling CreateNavigator of XmlDocument
        Dim nav As XPathNavigator = xmlDoc.CreateNavigator()

        ' Look for author's first name
        Console.WriteLine("Author First Name")
        Dim itrator As XPathNodeIterator = nav.[Select]("descendant : : first-name")

        While itrator.MoveNext()
            Console.WriteLine(itrator.Current.Value.ToString())
        End While

Conclusion

Hope this article would have helped you in understanding Navigation in XML. See other articles on the website also for further reference.