Use Visual C# to navigate XML documents with the XPathNavigator class

This article describes how to navigate XML documents with an XPathNavigator object that is created from an XPathDocument object.

Original product version: Visual C#
Original KB number: 308343

Summary

This sample loads an XPathDocument object with XML data, creates an XPathNavigator object as a view onto the data, and displays the XML by walking through the document.

This article refers to the following Microsoft .NET Framework Class Library namespaces:

System.Xml
System.Xml.XPath

For a Microsoft Visual Basic .NET version of this article, see How to navigate XML with the XPathNavigator class by using Visual Basic.

Requirements

This article assumes that you're familiar with the following topics:

Visual C#
XML terminology
Creating and reading an XML file
XML Path Language (XPath) syntax

How to use the XPathNavigator class to navigate XML

Create a new Visual C# Console Application in Visual Studio.

Note

This example uses a file named Books.xml. You can create your own Books.xml file, or you can use the sample that is included with the .NET Software Development Kit (SDK) Quickstarts. If you do not have the Quickstarts installed and don't want to install them, see the References section for the Books.xml download location. If you have the Quickstarts installed, Books.xml is located in the following folder:
\Program Files\Microsoft.NET\FrameworkSDK\Samples\Quickstart\Howto\Samples\Xml\Transformxml\VB

You can copy Books.xml to the \Bin\Debug folder that is located under the folder in which you created this project.
Make sure that the project references the System.Xml namespace.
Use the using statement on the Xml and XPath namespaces so that you aren't required to qualify declarations in those namespaces later in your code. You can use the using statement before any other declarations, as follows:
```
using System.Xml;
using System.Xml.XPath;
```
Declare the appropriate variables. Declare an XPathDocument object to hold the XML document and an XPathNavigator object to evaluate XPath expressions and move through the document. Declare a String object to hold the XPath expression. Add the declaration code in the Main procedure in Module1.
```
XPathNavigator nav;
XPathDocument docNav;
```
Load an XPathDocument object with the sample file Books.xml. The XPathDocument class uses Extensible Stylesheet Language Transformations (XSLT) to provide a fast and performance-oriented cache for XML document processing. It's similar to the XML Document Object Model (DOM) but is highly optimized for XSLT processing and the XPath data model.
```
// Open the XML.
docNav = new XPathDocument(@"c:\books.xml");
```
Create an XPathNavigator object from the document. XPathNavigator enables you to move through both the attributes nodes and the namespace nodes in an XML document.
```
// Create a navigator to query with XPath.
nav = docNav.CreateNavigator();
```
Move to the root of the document with the MoveToRoot method. MoveToRoot sets the navigator to the document node that contains the entire tree of nodes.
```
//Initial XPathNavigator to start at the root.
nav.MoveToRoot();
```
Use the MoveToFirstChild method to move to the children of the XML document. The MoveToFirstChild method moves to the first child of the current node. If there's the Books.xml source, you're moving away from the root document into the children, the Comment section, and the Bookstore node.
```
//Move to the first child node (comment field).
nav.MoveToFirstChild();
```
Use the MoveToNext method to iterate through nodes at the sibling level. The MoveToNext method moves to the next sibling of the current node.
```
//Loop through all of the root nodes.
do
{
} while (nav.MoveToNext());
```

Use the NodeType property to make sure that you're only processing element nodes, and use the Value property to display the text representation of the element.

do
{
   //Find the first element.
   if (nav.NodeType == XPathNodeType.Element)
   {
        //Determine whether children exist.
        if (nav.HasChildren == true)
        {
            //Move to the first child.
            nav.MoveToFirstChild();

            //Loop through all the children.
            do
            {
                //Display the data.
                Console.Write("The XML string for this child ");
                Console.WriteLine("is '{0}'", nav.Value);
            } while (nav.MoveToNext());
        }
    }
} while (nav.MoveToNext());

Use the HasAttributes property to determine whether a node has any attributes. You can also use other methods, such as MoveToNextAttribute, to move to an attribute and inspect its value.

Note

This code segment only walks through the descendents of the root node and not the entire tree.

do
{
   //Find the first element.
   if (nav.NodeType == XPathNodeType.Element)
   {
        //if children exist
        if (nav.HasChildren == true)
        {
            //Move to the first child.
            nav.MoveToFirstChild();
            //Loop through all the children.
            do
            {
                //Display the data.
                Console.Write("The XML string for this child ");
                Console.WriteLine("is '{0}'", nav.Value);

                //Check for attributes.
                if (nav.HasAttributes == true)
                {
                    Console.WriteLine("This node has attributes");
                }
            } while (nav.MoveToNext());
        }
    }
} while (nav.MoveToNext());

Use the ReadLine method of the Console object to add a pause at the end of the console display to more readily display the above results.
```
//Pause.
Console.ReadLine();
```
Build and run the Visual C# project.

Complete code listing

using System;
using System.Xml;
using System.Xml.XPath;

namespace q308343
{
    class Class1
    {
        static void Main(string[] args)
        {
            XPathNavigator nav;
            XPathDocument docNav;

            docNav = new XPathDocument(@"c:\books.xml");
            nav = docNav.CreateNavigator();
            nav.MoveToRoot();

            //Move to the first child node (comment field).
            nav.MoveToFirstChild();

            do
            {
                //Find the first element.
                if (nav.NodeType == XPathNodeType.Element)
                {
                    //Determine whether children exist.
                    if (nav.HasChildren == true)
                    {
                        //Move to the first child.
                        nav.MoveToFirstChild();
                        //Loop through all of the children.
                        do
                        {
                            //Display the data.
                            Console.Write("The XML string for this child ");
                            Console.WriteLine("is '{0}'", nav.Value);
                            //Check for attributes.
                            if (nav.HasAttributes == true)
                            {
                                Console.WriteLine("This node has attributes");
                            }
                        } while (nav.MoveToNext());
                    }
                }
            } while (nav.MoveToNext());
            //Pause.
            Console.ReadLine();
        }
    }
}

Troubleshooting

When you test the code, you may receive the following exception error message:

An unhandled exception of type System.Xml.XmlException occurred in system.xml.dll
Additional information: Unexpected XML declaration. The XML declaration must be the first node in the document, and no white space characters are allowed to appear before it. Line 1, position

The exception error occurs on the following line of code:

docNav = new XPathDocument("c:\\books.xml");

To resolve the error, remove the white-space character(s) that precede the first node in the books.xml document.

References

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Last updated on 2022-05-08