What is XPath expression?

Understanding the increasing need of XML, W3C has developed a new querying language called XPath expression through which we can easily walk into a XML document and select a node. In simple terms, XPath is simple and easily understandable expressions which we can use to select XML node or nodes in an XML document.

To understand this article, we will use the below simple XML.

<?xml version="1.0" encoding="utf-8"?>

<Employees>

  <Employee type="Permanent">

    <ID>100</ID>

    <FirstName>Bala</FirstName>

    <LastName>Murugan</LastName>

    <Dept>Production Support</Dept>

  </Employee>

  <Employee type="Contract">

    <ID>101</ID>

    <FirstName>Peter</FirstName>

    <LastName>Laurence</LastName>

    <Dept>Development</Dept>

  </Employee>

  <Employee type="Permanent">

    <ID>102</ID>

    <FirstName>Rick</FirstName>

    <LastName>Anderson</LastName>

    <Dept>Sales</Dept>

  </Employee>

  <Employee type="Contract">

    <ID>103</ID>

    <FirstName>Ramesh</FirstName>

    <LastName>Kumar</LastName>

    <Dept>HR</Dept>

  </Employee>

  <Employee type="Permanent">

    <ID>104</ID>

    <FirstName>Katie</FirstName>

    <LastName>Yu</LastName>

    <Dept>Recruitment</Dept>

  </Employee>

  <Employee type="Contract">

    <ID>105</ID>

    <FirstName>Suresh</FirstName>

    <LastName>Babu</LastName>

    <Dept>Inventory</Dept>

  </Employee>

</Employees>

XPath expression Syntax

The XPath expression will use / to select the root node of the XML document. For example, to select all the employee nodes in the above XML we have to use the below XPath query,

/Employees/Employee

The above XPath query will return all nodes under every <Employee> nodes. 

The below table list some of the important expressions to build an XPath query.

Moving forward, we will see how these XPath expressions can be used in .net world to access and manipulate XML contents.

In this section, we will see how to read a XML node at a very basic level using XmlDocument class System.Xml namespace.  We can use SelectSingleNode() and SelectNodes() methods of XmlDocument class to query the XML using XPath expression. See below,

To select a single node, for example first name,

XmlDocument xmldoc = new XmlDocument();

xmldoc.Load(Server.MapPath("~/App_Data/Employees.xml"));

Response.Write(xmldoc.SelectSingleNode("/Employees/Employee/FirstName").InnerText);

OUTPUT

Bala

To select nth occurence of a node value, for example to get 3^rd employee’s firstname,

XmlDocument xmldoc = new XmlDocument();

xmldoc.Load(Server.MapPath("~/App_Data/Employees.xml"));

Response.Write(xmldoc.SelectSingleNode("/Employees/Employee[3]/FirstName").InnerText);

OUTPUT

Rick

To select a node conditionally, for example to select firstname of a employee with ID as 101,

XmlDocument xmldoc = new XmlDocument();

xmldoc.Load(Server.MapPath("~/App_Data/Employees.xml"));

Response.Write(xmldoc.SelectSingleNode("/Employees/Employee[ID=101]/FirstName").InnerText);

OUTPUT

Peter

To select a attribute of a XML node, for example to get the type of employee with ID 101,

XmlDocument xmldoc = new XmlDocument();

xmldoc.Load(Server.MapPath("~/App_Data/Employees.xml"));

 Response.Write(xmldoc.SelectSingleNode("/Employees/Employee[ID=101]/@type").InnerText);

OUTPUT

Contract

To get a count of all permanent employees,

XmlDocument xmldoc = new XmlDocument();

xmldoc.Load(Server.MapPath("~/App_Data/Employees.xml"));

Response.Write(xmldoc.SelectNodes("/Employees/Employee[@type='Permanent']").Count.ToString());

OUTPUT

3

To get all employee count,

XmlDocument xmldoc = new XmlDocument();

xmldoc.Load(Server.MapPath("~/App_Data/Employees.xml"));

Response.Write(xmldoc.SelectNodes("//Employee").Count.ToString());

OUTPUT

6

At times, we need to fetch some set of XML nodes to populate an ASP.Net control. The below code will fetch all permanent employees, contract employees, all employees and populate them to a ddlPermanentEmp, ddlContractEmp, ddlAllEmployees DropDownList respectively.

XmlDocument xmldoc = new XmlDocument();

xmldoc.Load(Server.MapPath("~/App_Data/Employees.xml"));

//Get permanent employees

  XmlNodeList nodes = xmldoc.SelectNodes("/Employees/Employee[@type='Permanent']");

        foreach (XmlNode node in nodes)

        {          

            ddlPermanentEmp.Items.Add(new ListItem(node.SelectSingleNode("FirstName").InnerText, node.SelectSingleNode("ID").InnerText));

        }

//Get contract employees

nodes = xmldoc.SelectNodes("/Employees/Employee[@type='Contract']");

        foreach (XmlNode node in nodes)

        {

            ddlContractEmp.Items.Add(new ListItem(node.SelectSingleNode("FirstName").InnerText, node.SelectSingleNode("ID").InnerText));

        }

//Get all employees

        nodes = xmldoc.SelectNodes("//Employee");

        foreach (XmlNode node in nodes)

        {

            ddlAllEmployees.Items.Add(new ListItem(node.SelectSingleNode("FirstName").InnerText, node.SelectSingleNode("ID").InnerText));

        }

Thus, with the above examples we have seen some of basic usages of XPath expressions to query the XML content.

In the coming section, we will see the usages of XPath related classes in System.Xml.XPath namespace in .NetFramework.

XPath support in .NetFramework

The System.Xml.XPath namespace has a class called XPathNavigator which can be used for both forward and backward navigation of XML nodes. Technically, it is an abstract class which defines a cursor model for navigating and editing XML information items as instances of the XPath 2.0 Data Model. Once after selecting the XML nodes using an XPath expression, it uses XPathNodeIterator class to iterate through the XML nodes.

To create XPathNavigator object, we need to call the CreateNavigator() method on XmlDocument or XpathDocument object. The below code will read all permanent employees and load it to a DropDownList ddlEmployees.

        XPathDocument xpathdoc = new XPathDocument(Server.MapPath("~/App_Data/Employees.xml"));

        XPathNavigator navigator = xpathdoc.CreateNavigator();

        XPathExpression expr = navigator.Compile("/Employees/Employee[@type='Permanent']");

        XPathNodeIterator nodes = navigator.Select(expr);

        while (nodes.MoveNext())

        {

            ddlEmployees.Items.Add(new ListItem(nodes.Current.SelectSingleNode("FirstName").Value, nodes.Current.SelectSingleNode("ID").Value));

        }