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Maintenance of XML Elements

 

Locating an Element

 

Introduction

In some cases, you may want to perform an operation on an existing and particular node. For example, you may want to change the value of a node, you may want to add a new child node to an existing node, etc. Before taking any of these actions, you must be able to locate or identify the desired element.

Locating an element consists of looking for a particular node among the nodes. To do this, you must start somewhere. Obviously, the first node you can identify is the root. Once you get the root, you can then get a collection of its children. After getting a collection of the children of the root, you can locate a node in the collection. If the node you are looking for is a child of that first collection, you can then get a collection of the child nodes of that node and proceed.

Fortunately, the System.Xml namespace provides various means of looking for a node in an XML file.

Locating an Element Using its Index

Consider the following XML file named videos.xml:

<?xml version="1.0" encoding="utf-8"?>
<videos>
    <video>
	<title>The Distinguished Gentleman</title>
	<director>Jonathan Lynn</director>
	<length>112 Minutes</length>
	<format>DVD</format>
	<rating>R</rating>
    </video>
    <video>
	<title>Her Alibi</title>
	<director>Bruce Beresford</director>
	<length>94 Minutes</length>
	<format>DVD</format>
	<rating>PG-13</rating>
    </video>
    <video>
	<title>The Day After Tomorrow</title>
	<director>Roland Emmerich</director>
	<length>124 Minutes</length>
	<format>DVD</format>
	<rating>PG-13</rating>
    </video>
    <video>
	<title>Other People's Money</title>
	<director>Alan Brunstein</director>
	<length>114 Minutes</length>
	<format>VHS</format>
	<rating>PG-13</rating>
    </video>
</videos>

In Lesson 7, we saw that the XmlNodeList class was equipped with both a method named Item and an indexed property (named Item). Their syntaxes are:

public abstract XmlNode Item(int index);
public virtual XmlNode this[int i] { get; }

These two members allow you to access an element based on its index. Here are examples:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Xml;

namespace VideoCollection1
{
    public partial class Exercise : Form
    {
        public Exercise()
        {
            InitializeComponent();
        }

        private void btnDocument_Click(object sender, EventArgs e)
        {
            string strFilename = "videos.xml";
            XmlDocument xmlDoc = new XmlDocument();

            if (File.Exists(strFilename))
            {
                xmlDoc.Load(strFilename);

                XmlElement elmRoot = xmlDoc.DocumentElement;
                XmlNodeList lstVideos = elmRoot.ChildNodes;

                MessageBox.Show(lstVideos[1].InnerText);

                MessageBox.Show(lstVideos.Item(3).InnerXml);
            }
        }
    }
}

This would produce:

You can use this characteristic to locate a node. Because XML is very flexible with the names (you can have two child nodes that have the same name) and values (you can have two child nodes that have the same value) of nodes, when creating an XML file, it is your responsibility to create a scheme that would eventually allow you to uniquely identify each element.

Locating an Element Using its Name

To assist you with locating the first child node of a node, the XmlNode class is equipped with an indexed property (named Item) overloaded with two versions. One of the versions is declared as follows:

public virtual XmlElement this[string name] { get; }

This indexed property takes the name of a node as argument. After the property has been called, the parser checks the child nodes of the element on which the property was applied. If the parser finds a node with that name, it returns it as an XmlElement object. Here is an example:

private void btnDocument_Click(object sender, EventArgs e)
{
    string strFilename = "videos.xml";
    XmlDocument xmlDoc = new XmlDocument();

    if (File.Exists(strFilename))
    {
        xmlDoc.Load(strFilename);

        XmlElement elmRoot = xmlDoc.DocumentElement;
        XmlNodeList lstVideos = elmRoot.ChildNodes;

        MessageBox.Show(lstVideos[1]["director"].InnerText);

        MessageBox.Show(lstVideos.Item(3)["format"].InnerXml);
    }
}

Based on the videos.xml file we had earlier, this would produce:

If the node has more than one child with the same name, then it would return the first child with that name. You can use this characteristic to look for, or locate, an element.

Locating an Element Using a Tag Name

To assist you with finding a node, the XmlDocument class is equipped with the GetElementByTagName() method which is overloaded with two versions. One of the syntaxes used is:

public virtual XmlNodeList GetElementsByTagName(string name);

This method takes as argument a string. The string must be the name of a node. If at least one node that holds that name exists in the file, this method returns a collection of the nodes with that name. If there is no node with that name, the collection is returned empty and there is no exception thrown.

Here is an example of calling the method:

private void btnDocument_Click(object sender, EventArgs e)
{
    string strFilename = "videos.xml";
    XmlDocument xmlDoc = new XmlDocument();

    if (File.Exists(strFilename))
    {
        xmlDoc.Load(strFilename);

        // Get a reference to the root node
        XmlElement elmRoot = xmlDoc.DocumentElement;

        // Create a list of nodes whose name is Title
        XmlNodeList lstTitles = xmlDoc.GetElementsByTagName("title");
}

Once you have a list of the nodes of a particular criterion, you can then act as you see fit. For example, For example, you can look for a particular node that holds a text of your choice. Here is an example:

private void btnDocument_Click(object sender, EventArgs e)
{
    string strFilename = "videos.xml";
    XmlDocument xmlDoc = new XmlDocument();

    if (File.Exists(strFilename))
    {
        xmlDoc.Load(strFilename);

        // Get a reference to the root node
        XmlElement elmRoot = xmlDoc.DocumentElement;

        // Create a list of nodes whose name is Title
        XmlNodeList lstTitles = xmlDoc.GetElementsByTagName("title");
	{
            if (node.InnerText == "Her Alibi")
            {
                ;
            }
        }
    }
}

Inserting an Element

 

Inserting an Element as a Last Child

Once again, consider our Videos.xml file. Imagine you want to add a list of actors of the Her Alibi video. The first action to take is to locate the video, which you can do by calling the XmlDocument.GetElementsByTagName() method applied to a collection of nodes whose names are video. From this list of nodes, you can look for the node whose value is "Her Alibi". Once you have found this element, get a reference to its parent. Then add the new node a child its parent. This can be done as follows:

private void btnDocument_Click(object sender, EventArgs e)
{
    string strFilename = "videos.xml";
    XmlDocument xmlDoc = new XmlDocument();

    if (File.Exists(strFilename))
    {
        xmlDoc.Load(strFilename);

        // Get a reference to the root node
        XmlElement elmRoot = xmlDoc.DocumentElement;

        // Create a list of nodes whose name is Title
        XmlNodeList lstTitles = xmlDoc.GetElementsByTagName("title");

        // Now you can check each node of the list
        foreach (XmlNode node in lstTitles)
        {
            if (node.InnerText == "Her Alibi")
            {
                // Create an element named Actors
                XmlElement elmNew = xmlDoc.CreateElement("actors");
                XmlNode elmParent = node.ParentNode;
                // Add a new element named Actors to it
                elmParent.AppendChild(elmNew);
                xmlDoc.Save(strFilename);
            }
        }
    }
}

This would produce:

<?xml version="1.0" encoding="utf-8"?>
<videos>
  . . .
  <video>
    <title>Her Alibi</title>
    <director>Bruce Beresford</director>
    <length>94 Minutes</length>
    <format>DVD</format>
    <rating>PG-13</rating>
    <actors />
  </video>
  . . .
</videos>

This code creates an empty element. If you want to create an element that includes a value, create its text and add that text to the node. Here is an example:

private void btnDocument_Click(object sender, EventArgs e)
{
    string strFilename = "videos.xml";
    XmlDocument xmlDoc = new XmlDocument();

    if (File.Exists(strFilename))
    {
        xmlDoc.Load(strFilename);

        // Get a reference to the root node
        XmlElement elmRoot = xmlDoc.DocumentElement;

        // Create a list of nodes whose name is Title
        XmlNodeList lstTitles = xmlDoc.GetElementsByTagName("title");

        // Now you can check each node of the list
        foreach (XmlNode node in lstTitles)
        {
            // When you get to a node, look for the element's value
            // If you find an element whose value is Her Alibi
            if (node.InnerText == "The Distinguished Gentleman")
            {
                // Create an element named Category
                XmlElement elmNew = xmlDoc.CreateElement("category");
                // Create the text of the new element
                XmlText txtCatetory = xmlDoc.CreateTextNode("Comedy");
                // Get a reference to the parent of the node we have found
                XmlNode elmParent = node.ParentNode;
                // Add the new element to the node we found
                elmParent.AppendChild(elmNew);
                // Specify the text of the new node
                elmParent.LastChild.AppendChild(txtCatetory);
                // Save the file
                xmlDoc.Save(strFilename);
            }
        }
    }
}

This would produce:

<?xml version="1.0" encoding="utf-8"?>
<videos>
  <video>
    <title>The Distinguished Gentleman</title>
    <director>Jonathan Lynn</director>
    <length>112 Minutes</length>
    <format>DVD</format>
    <rating>R</rating>
    <category>Comedy</category>
  </video>
  . . .
</videos>

Using the same approach combined with what we learned about adding an item, you can add a new element that itself has child nodes. Here is an example:

private void btnDocument_Click(object sender, EventArgs e)
{
    string strFilename = "videos.xml";
    XmlDocument xmlDoc = new XmlDocument();

    if (File.Exists(strFilename))
    {
        xmlDoc.Load(strFilename);

        // Get a reference to the root node
        XmlElement elmRoot = xmlDoc.DocumentElement;

        // Create a list of nodes whose name is Title
        XmlNodeList lstTitles = xmlDoc.GetElementsByTagName("title");

        // Now you can check each node of the list
        foreach (XmlNode node in lstTitles)
        {
            // When you get to a node, look for the element's value
            // If you find an element whose value is The Day After Tomorrow
            if (node.InnerText == "The Day After Tomorrow")
            {
                // Create an element named Actors
                XmlElement elmNew = xmlDoc.CreateElement("actors");
                // Get a reference to the parent of the node we have found
                XmlNode elmVideo = node.ParentNode;
                // Add the new element to the node we found
                elmVideo.AppendChild(elmNew);

                // Create an element as a child of the new element
                // Specify its name as Actor
                elmNew = xmlDoc.CreateElement("actor");
                // Create the text of the new element
                XmlText txtActor = xmlDoc.CreateTextNode("Dennis Quaid");
                // Add the new Actor element to the Actors node
                elmVideo.LastChild.AppendChild(elmNew);
                // Specify the text of the new node
                elmVideo.LastChild.LastChild.AppendChild(txtActor);

                // In the same way, add the other Actor nodes
                elmNew = xmlDoc.CreateElement("actor");
                txtActor = xmlDoc.CreateTextNode("Jake Gyllenhaal");
                elmVideo.LastChild.AppendChild(elmNew);
                elmVideo.LastChild.LastChild.AppendChild(txtActor);

                elmNew = xmlDoc.CreateElement("actor");
                txtActor = xmlDoc.CreateTextNode("Emmy Rossum");
                elmVideo.LastChild.AppendChild(elmNew);
                elmVideo.LastChild.LastChild.AppendChild(txtActor);

                elmNew = xmlDoc.CreateElement("actor");
                txtActor = xmlDoc.CreateTextNode("Dash Mihok");
                elmVideo.LastChild.AppendChild(elmNew);
                elmVideo.LastChild.LastChild.AppendChild(txtActor);

                // Save the file
                xmlDoc.Save(strFilename);
            }
        }
    }
}

This would produce:

<?xml version="1.0" encoding="utf-8"?>
<videos>
  . . .
  <video>
    <title>The Day After Tomorrow</title>
    <director>Roland Emmerich</director>
    <length>124 Minutes</length>
    <format>DVD</format>
    <rating>PG-13</rating>
    <actors>
      <actor>Dennis Quaid</actor>
      <actor>Jake Gyllenhaal</actor>
      <actor>Emmy Rossum</actor>
      <actor>Dash Mihok</actor>
    </actors>
  </video>
</videos>

You can also insert one or more elements as children of an existing node after locating that node. Here is an example:

XML File

<?xml version="1.0" encoding="utf-8"?>
<videos>
  <video>
    <title>The Distinguished Gentleman</title>
    <director>Jonathan Lynn</director>
    <length>112 Minutes</length>
    <format>DVD</format>
    <rating>R</rating>
    <category>Comedy</category>
  </video>
  <video>
    <title>Her Alibi</title>
    <director>Bruce Beresford</director>
    <length>94 Mins</length>
    <format>DVD</format>
    <rating>PG-13</rating>
    <actors />
  </video>
</videos>

Code Fragment:

private void btnDocument_Click(object sender, EventArgs e)
{
    string strFilename = "videos.xml";
    XmlDocument xmlDoc = new XmlDocument();

    if (File.Exists(strFilename))
    {
        xmlDoc.Load(strFilename);

        // Get a reference to the root node
        XmlElement elmRoot = xmlDoc.DocumentElement;

        // Create a list of nodes whose name is Title
        XmlNodeList lstTitles = xmlDoc.GetElementsByTagName("title");

        // Now you can check each node of the list
        foreach (XmlNode node in lstTitles)
        {
            // When you get to a node, look for the element's value
            // If you find an element whose value is Her Alibi
            if (node.InnerText == "Her Alibi")
            {
                // Get a reference to the video node that is 
                // the parent of the video titled Her Alibi
                XmlNode elmVideo = node.ParentNode;

                // Create a list of the child nodes of the Her alibi video
                XmlNodeList lstActors = elmVideo.ChildNodes;

                // Visit each item of the collection
                // looking for an element named Actors
                foreach (XmlNode nodActor in lstActors)
                {
                    // If you find an element named Actors
                    if (nodActor.Name == "actors")
                    {
                        // Create a new element named Actor
                        // Specify its name as Actor
                        XmlElement elmNew = xmlDoc.CreateElement("actor");
                        // Create the text of the new element
                        XmlText txtActor = xmlDoc.CreateTextNode("Tom Selleck");
                        // Add the new Actor element to the Actors node
                        elmVideo.LastChild.AppendChild(elmNew);
                        // Specify the text of the new node
                        elmVideo.LastChild.LastChild.AppendChild(txtActor);

                        // Add other Actor nodes
                        elmNew = xmlDoc.CreateElement("actor");
                        txtActor = xmlDoc.CreateTextNode("Paulina Porizkova");
                        elmVideo.LastChild.AppendChild(elmNew);
                        elmVideo.LastChild.LastChild.AppendChild(txtActor);

                        elmNew = xmlDoc.CreateElement("actor");
                        txtActor = xmlDoc.CreateTextNode("William Daniels");
                        elmVideo.LastChild.AppendChild(elmNew);
                        elmVideo.LastChild.LastChild.AppendChild(txtActor);

                        // Save the file
                        xmlDoc.Save(strFilename);

                        // Stop, in this example, we don't expect another Actors node
                        break;
                    }
                }
            }
        }
    }
}

This would produce:

<?xml version="1.0" encoding="utf-8"?>
<videos>
  <video>
    <title>The Distinguished Gentleman</title>
    <director>Jonathan Lynn</director>
    <length>112 Minutes</length>
    <format>DVD</format>
    <rating>R</rating>
    <category>Comedy</category>
  </video>
  <video>
    <title>Her Alibi</title>
    <director>Bruce Beresford</director>
    <length>94 Minutes</length>
    <format>DVD</format>
    <rating>PG-13</rating>
    <actors>
      <actor>Tom Selleck</actor>
      <actor>Paulina Porizkova</actor>
      <actor>William Daniels</actor>
    </actors>
  </video>
</videos>

Inserting an Element Referencing a Sibling

Instead of simply adding a new node at the end of child nodes, you can specify any other position you want. For example, you may want the new node to precede an existing child node. To support this operation, the XmlNode class provides the InsertBefore() method. Its syntax is:

public virtual XmlNode InsertBefore(XmlNode newChild, XmlNode refChild);

The first argument of this method is the new node that will be added. The second argument is the sibling that will succeed the new node. Consider the following version of our Videos.xml file:

<?xml version="1.0" encoding="utf-8"?>
<videos>
    <video>
      <title>The Distinguished Gentleman</title>
      <director>Jonathan Lynn</director>
      <length>112 Minutes</length>
      <format>DVD</format>
      <rating>R</rating>
      <category>Comedy</category>
    </video>
    <video>
	<title>Fatal Attraction</title>
	<director>Adrian Lyne</director>
	<length>119 Minutes</length>
	<format>DVD</format>
	<rating>R</rating>
    </video>
</videos>

Imagine you want to create a new category element below the director element whose name is Adrian Lyne. You can first get a list of videos. Inside of each video, check the nodes and find out whether the video has a director node whose text is Adrian Lyne. Once you find that node, you can add the new element after it. Here is an example:

private void btnDocument_Click(object sender, EventArgs e)
{
    string strFilename = "videos.xml";
    XmlDocument xmlDoc = new XmlDocument();

    if (File.Exists(strFilename))
    {
        xmlDoc.Load(strFilename);

        // Get a reference to the root node
        XmlElement elmRoot = xmlDoc.DocumentElement;

        // Create a list of the videos
        XmlNodeList lstVideos = xmlDoc.GetElementsByTagName("video");

        // visit each video
        foreach (XmlNode node in lstVideos)
        {
            // Within a video, create a list of its children
            XmlNodeList lstChildren = node.ChildNodes;

            // Visit each child node
            foreach (XmlNode dir in lstChildren)
            {
                // If the child node is (a director and its name is) Adrian Lyne
                if (dir.InnerText == "Adrian Lyne")
                {
                    // Create an element named Category
                    XmlElement elmNew = xmlDoc.CreateElement("category");
                    // Specify the text of the new element
                    elmNew.InnerText = "Drama";

                    // Insert the new node below the Adrian Lyne node Director
                    node.InsertAfter(elmNew, dir);

                    // Save the file
                    xmlDoc.Save(strFilename);

                    // Stop
                    break;
                }
            }
        }
    }
}

This would produce:

<?xml version="1.0" encoding="utf-8"?>
<videos>
  <video>
    <title>The Distinguished Gentleman</title>
    <director>Jonathan Lynn</director>
    <length>112 Minutes</length>
    <format>DVD</format>
    <rating>R</rating>
    <category>Comedy</category>
  </video>
  <video>
    <title>Fatal Attraction</title>
    <director>Adrian Lyne</director>
    <category>Drama</category>
    <length>119 Minutes</length>
    <format>DVD</format>
    <rating>R</rating>
  </video>
</videos>

In the same way, you can insert a new node after a child of a child (or of a child of a child of a child) of any node.

If you want to new node to be positioned after an existing child node, you can call the XmlNode.InsertAfter() method. Its syntax is:

public virtual XmlNode InsertAfter(XmlNode newChild, XmlNode refChild);

College Park Auto Parts (Lesson 4) Locate the year of a part and insert after

Deleting Elements

 

Deleting an Element

If you have a node you don't want or don't need anymore in the file, you can delete it. To delete a node, the XmlNode class provides the RemoveChild() method. Its syntax is:

public virtual XmlNode RemoveChild(XmlNode oldChild);

This method takes as argument the node to delete. If the node exists, it would be deleted and the method would return it. If the node doesn't exist, nothing would happen. To effectively use this method, you should first locate the particular node you want to delete. You can look for it using any of the logics we have applied so far. Once you find the node, you can then delete it. Imagine you want to delete a node whose name is Director and whose value is Bruce Beresford. Here is an example of calling this method to perform the operation:

private void btnDocument_Click(object sender, EventArgs e)
{
    string strFilename = "videos.xml";
    XmlDocument xmlDoc = new XmlDocument();

    if (File.Exists(strFilename))
    {
        xmlDoc.Load(strFilename);

        // Get a reference to the root node
        XmlElement elmRoot = xmlDoc.DocumentElement;

        // Create a list of the videos
        XmlNodeList lstVideos = xmlDoc.GetElementsByTagName("video");

        // visit each video
        foreach (XmlNode node in lstVideos)
        {
            // Within a video, get a list of its children
            XmlNodeList lstChildren = node.ChildNodes;

            // Visit each child node
            foreach (XmlNode dir in lstChildren)
            {
                // If the child node is Bruce Beresford
                if (dir.InnerText == "Adrian Lyne")
                {
                    node.RemoveChild(dir);

                    // Save the file
                    xmlDoc.Save(strFilename);

                    // Stop
                    break;
                }
            }
        }
    }
}

Clearing an Element of its Children

To delete all child nodes of a node, you can call the XmlNode.RemoveAll() method. Its syntax is:

public virtual void RemoveAll();

When called, this method will remove all child nodes, if any, of their parent node.

 

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