How To Specify Fully Qualified Element Names in XPath Queries by Using Visual C# .NET (313188)

SUMMARY

The step-by-step article shows you how to specify fully qualified element names in the NamespacePrefix:ElementName format to select nodes in an XmlDocument object.

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Create the XML File

1. On the Windows Start menu, point to Run, type notepad.exe, and then click OK to open Notepad.
2. Copy and paste the following XML code into Notepad:

   <?xml version="1.0"?>
      <bk:Books xmlns:bk='http://myserver/myschemas/Books'>
        <bk:Book>
            <bk:Title>Just XML</bk:Title>
        </bk:Book>
        <bk:Book>
            <bk:Title>Professional XML</bk:Title>
        </bk:Book>
        <bk:Book>
            <bk:Title>XML Step by Step</bk:Title>
        </bk:Book>
        <bk:Book>
            <bk:Title>XML By Example</bk:Title>
        </bk:Book>
      </bk:Books>
   					

3. On the File menu, click Save.
4. In the Save As dialog box, in the Save as type text box, click All Files. In the File name text box, type Books.xml, and then click OK.

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Create the Visual C# .NET Project

The following code sample uses the following objects and classes:

• The XPathNavigator class: XPathNavigator is based on the XML Path Language (XPath) data model and provides the methods that are required to implement XPath queries over any data store.
• The XPathExpression class: This class encapsulates a compiled XPath expression and is returned when you call Compile. The Select, Evaluate, and Matches methods use this class.
• The XmlNamespaceManager class: XmlNamespaceManager resolves, adds, and removes namespaces to a collection. XmlNamespaceManager also provides scope management for these namespaces. Because Books.xml uses the "bk" namespace in the code to follow, you must use XmlNamespaceManager.
• The XPathNodeIterator class: This object provides an iterator over a set of selected nodes.

To create and run the Visual C# .NET project, follow these steps:

1. Create a new Windows Application project in Visual C# .NET. Form1 is added to the project by default.
2. Place a Button control and a TextBox control on Form1.
3. Set the MultiLine property of the TextBox control to True.
4. Click to expand the TextBox control so that you can view four or five lines of data.
5. Add the following code to the top of the Code window:

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

6. To load the Books.xml file into an XmlDocument object, add the following code to the Click event of the Button control:

   XmlDocument oxmldoc = new XmlDocument();
   oxmldoc.Load(@"c:\Books.xml");
   					

7. Make sure that the Books.xml path in the preceding code points to the correct path on your computer.
8. Use the CreateNavigator method of the XmlDocument object to create the XPathNavigator object so that you can run the XPath query:

   XPathNavigator oXPathNav;
   oXPathNav = oxmldoc.CreateNavigator();
   					

9. Use the Compile method of XPathNavigator to create an XPathExpression class, and then pass the XPath query as the parameter:

   XPathExpression Expr;
   Expr = oXPathNav.Compile("//bk:Book[position()>=2]");
   					

10. Use the the AddNamespace method to add the "bk" namespace to the XmlNamespaceManager object:

    XmlNamespaceManager oxmlNSManager = new XmlNamespaceManager(oXPathNav.NameTable);
    oxmlNSManager.AddNamespace("bk", "http://myserver/myschemas/Books");
    					

11. Use the SetContext method of XPathExpression to set the XPathExpression context to the XmlNamespaceManager:

    Expr.SetContext(oxmlNSManager);
    					

12. To run the XPath query and return the selected nodes, pass the expression to the Select method of the XPathNodeIterator:

    XPathNodeIterator iterator = oXPathNav.Select(Expr);
    while (iterator.MoveNext())				
    {
    	this.textBox1.Text = this.textBox1.Text + "\r\n"+ iterator.Current.Value ;
    }
    					

13. The code in Button1_Click event should appear as follows:

    XmlDocument oxmldoc = new XmlDocument();
    try	
    { 
    oxmldoc.Load("c:\\Books.xml");
    
    XPathNavigator oXPathNav;
    oXPathNav = oxmldoc.CreateNavigator();
    
    XPathExpression Expr;
    Expr = oXPathNav.Compile("//bk:Book[position()>=2]");
    
    XmlNamespaceManager oxmlNSManager = new XmlNamespaceManager(oXPathNav.NameTable);
    oxmlNSManager.AddNamespace("bk", "http://myserver/myschemas/Books");
    Expr.SetContext(oxmlNSManager);
    
    XPathNodeIterator iterator = oXPathNav.Select(Expr);
    				 
    while (iterator.MoveNext())				
    {
    	this.textBox1.Text = this.textBox1.Text + "\r\n"+ iterator.Current.Value ;
    }
                
    oxmlNSManager = null;
    oXPathNav = null;
    oxmldoc = null;
    }
    catch (Exception exc)
    {
            MessageBox.Show(exc.Message);
    }
    					

14. Build and run the project.
15. Click Button1. Notice that a list of books whose position is greater than or equal to 2 appears in the textbox.

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