Use Visual C# to query XML with an XPath expression

This article introduces how to query an XPathDocument object with an XML Path Language (XPath) expression by using the XPathNavigator class.

Original product version:   Visual Studio, .NET Framework
Original KB number:   308333


XPath is used programmatically to evaluate expressions and select specific nodes in a document.

This article refers to the Microsoft .NET Framework Class Library namespace System.Xml.XPath.

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

  • Visual C#
  • XML terminology
  • Creating and reading an XML file
  • XPath syntax

Query XML with an XPath expression

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


    • 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 don't 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, the file can be found in Program Files\Microsoft.NET\FrameworkSDK\Samples\Quickstart\Howto\Samples\Xml\Transforxml\VB folder. You can copy the file to the \Bin\Debug folder, which is located under the folder in which you created this project.
  2. Make sure that the project references the System.Xml namespace.

  3. 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;
  4. Declare the appropriate variables. Declare an XPathDocument object to hold the XML document, an XpathNavigator object to evaluate XPath expressions, and an XPathNodeIterator object to iterate through selected nodes. Declare a String object to hold the XPath expressions. Add the declaration code in the Main function in Class1.

    XPathNavigator nav;
    XPathDocument docNav;
    XPathNodeIterator NodeIter;
    String strExpression;
  5. Load an XPathDocument 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");
  6. Create an XPathNavigator from the document. The XPathNavigator object is used for read-only XPath queries. The XPath queries may return a resulting value or many nodes.

    // Create a navigator to query with XPath.
    nav = docNav.CreateNavigator();
  7. Create an XPath expression to find the average cost of a book. This XPath expression returns a single value. For full details on XPath syntax, see XPath Syntax in the References section.

    // Find the average cost of a book.
    // This expression uses standard XPath syntax.
    strExpression = "sum(/bookstore/book/price) div count(/bookstore/book/price)";
  8. Use the Evaluate method of the XPathNavigator object to evaluate the XPath expression. The Evaluate method returns the results of the expression.

    // Use the Evaluate method to return the evaluated expression.
    Console.WriteLine("The average cost of the books are {0}", nav.Evaluate(strExpression));
  9. Create an XPath expression to find all of the books that cost more than 10 dollars. This XPath expression returns only Title nodes from the XML source.

    // Find the title of the books that are greater then $10.00.
    strExpression = "/bookstore/book/title[../price>10.00]";
  10. Create an XPathNodeIterator for the nodes that are selected with the Select method of the XPathNavigator. The XPathNodeIterator represents an XPath nodeset, and supports operations on this nodeset.

    // Select the node and place the results in an iterator.
    NodeIter = nav.Select(strExpression);
  11. Use the XPathNodeIterator, which was returned from the Select method of XPathNavigator, to move through the selected nodes. In this case, you can use the MoveNext method of the XPathNodeIterator to iterate through all of the selected nodes.

    Console.WriteLine("List of expensive books:");
    //Iterate through the results showing the element value.
    while (NodeIter.MoveNext())
        Console.WriteLine("Book Title: {0}", NodeIter.Current.Value);
  12. Use the ReadLine method to add a pause at the end of the console display to more readily display the above results.

    // Pause
  13. Build and run your project.


    The results are displayed in the console window.


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: System error.

The exception error occurs on the following line of code:

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

The exception error is caused by an invalid processing instruction. For example, the processing instruction may contain extraneous spaces. The following example is an invalid processing instruction:

<?xml version='1.0' ?>

To resolve the exception, use one of the following resolutions:

  • Correct the invalid processing instruction. The following example is a valid processing instruction:

    <?xml version='1.0'?>
  • Remove the XML processing instruction from the Books.xml file.