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Synchronizing DataSet Objects with XML

Access and Manipulate XML Data: Transform DataSet data into XML data.

One area in which the .NET Framework's use of XML is especially innovative is in connecting databases with XML. You already know that ADO.NET provides a complete in-memory representation of the structure and data of a relational database through its DataSet object. What the System.Xml namespace adds to this picture is the capability to automatically synchronize a DataSet object with an equivalent XML file. In this section of the chapter, you'll learn about the classes and techniques that make this synchronization possible.

The XmlDataDocument Class

The XmlDocument class is useful for working with XML via the DOM, but it's not a data-enabled class. To bring the DataSet class into the picture, you need to use an XmlDataDocument class, which inherits from the XmlDocument class. Table 2.5 shows the additional members that the XmlDataDocument class adds to the XmlDocument class.

Table 2.5 Additional Members of the XmlDataDocument Class

Member

Type

Description

DataSet

Property

Retrieves a DataSet object representing the data in the XmlDataDocument object

GetElementFromRow()

Method

Retrieves an XmlElement object representing a specified DataRow object

GetRowFromElement()

Method

Retrieves a DataRow object representing a specified XmlElement object

Load()

Method

Loads the XmlDataDocument object and synchronizes it with a DataSet object


Synchronizing a DataSet Object with an XmlDataDocument Object

The reason the XmlDataDocument class exists is to allow you to exploit the connections between XML documents and DataSet objects. You can do this by synchronizing the XmlDataDocument object (and hence the XML document that it represents) with a particular DataSet object. When you synchronize an XmlDataDocument object with DataSet object, any changes made in one are automatically reflected in the other. You can start the synchronization process with any of these objects:

  • An XmlDataDocument object

  • A full DataSet object

  • A schema-only DataSet object

I'll demonstrate these three options in the remainder of this section.

Starting with an XmlDataDocument Object

One way to synchronize a DataSet object and an XmlDataDocument object is to start with the XmlDataDocument object and to retrieve the DataSet object from its DataSet property. Step-by-Step 2.4 demonstrates this technique.

STEP BY STEP 2.4 - Retrieving a DataSet Object from an XmlDataDocument Object

  1. Add a new form to the project. Name the new form StepByStep2_4.cs.

  2. Add a Button control (btnLoadXml) and a DataGrid control (dgXML) to the form.

  3. Switch to the code view and add the following using directives:

    using System.Data;
    using System.Xml;
  4. Double-click the Button control and add the following code to handle the button's Click event:

    private void btnLoadXml_Click(
      object sender, System.EventArgs e)
    {
      // Create a new XmlTextReader on the file
      XmlTextReader xtr = new
        XmlTextReader(@"..\..\Books.xml");
      // Create an object to synchronize
      XmlDataDocument xdd = new XmlDataDocument();
      // Retrieve the associated DataSet
      DataSet ds = xdd.DataSet;
      // Initialize the DataSet by reading the schema
      // from the XML document
      ds.ReadXmlSchema(xtr);
      // Reset the XmlTextReader
      xtr.Close();
      xtr = new XmlTextReader(@"..\..\Books.xml");
      // Tell it to ignore whitespace
      xtr.WhitespaceHandling = WhitespaceHandling.None;
      // Load the synchronized object
      xdd.Load(xtr);
      // Display the resulting DataSet
      dgXML.DataSource = ds;
      dgXML.DataMember = "Book";
      // Clean up
      xtr.Close();
    }
  5. Insert the Main() method to launch the form. Set the form as the startup object for the project.

  6. Run the project. Click the button. The code will load the XML file and then display the corresponding DataSet object on the DataGrid control, as shown in Figure 2.4.

Figure 2.4Figure 2.4 You can synchronize a DataSet object with an XmlDataDocument object.

The code in Step-by-Step 2.4 performs some extra setup to make sure that the DataSet object can hold the data from the XmlDataDocument object. Even when you're creating the DataSet object from the XmlDataDocument object, you must still explicitly create the schema of the DataSet object before it will contain data. That's necessary because in this technique you can also use a DataSet object that represents only a portion of the XmlDataDocument object. In this case, the code takes advantage of the ReadXmlSchema() method of the DataSet object to automatically construct a schema that matches the XML document. Because the XmlTextReader object is designed for forward-only use, the code closes and reopens this object after reading the schema so that it can also be used to read the data.

TIP

Automatic Schema Contents When you use the ReadXmlSchema() method of the DataSet object to construct an XML schema for the DataSet, both elements and attributes within the XML document become DataColumn objects in the DataSet object.

Starting with a Full DataSet Object

A second way to end up with a DataSet object synchronized to an XmlDataDocument object is to start with a DataSet object. Step-by-Step 2.5 demonstrates this technique.

STEP BY STEP 2.5 - Creating an XmlDataDocument Object from a DataSet Object

  1. Add a new form to the project. Name the new form StepByStep2_5.cs.

  2. Add a Button control (btnLoadDataSet) and a ListBox control (lbNodes) to the form.

  3. Open Server Explorer.

  4. Expand the tree under Data Connections to show a SQL Server data connection that points to the Northwind sample database. Expand the Tables node of this database. Drag and drop the Employees table to the form to create a SqlConnection object and a SqlDataAdapter object on the form.

  5. Select the SqlDataAdapter object. Click the Create DataSet hyperlink beneath the Properties Window. Name the new DataSet dsEmployees and click OK.

  6. Add a new class to the project. Name the new class Utility.cs. Alter the code in Utility.cs as follows:

    using System;
    using System.Windows.Forms;
    using System.Xml;
    using System.Text;
    
    namespace _320C02
    {
      public class Utility
      {
        public Utility()
        {
        }
        public void XmlToListBox(
          XmlDocument xd, ListBox lb)
        {
          // Get the document root
          XmlNode xnodRoot = xd.DocumentElement;
          // Walk the tree and display it
          XmlNode xnodWorking;
          if(xnodRoot.HasChildNodes)
          {
            xnodWorking = xnodRoot.FirstChild;
            while(xnodWorking != null)
            {
              AddChildren(xnodWorking, lb, 0);
              xnodWorking =
                xnodWorking.NextSibling;
            }
          }
        }
    
        public void AddChildren(XmlNode xnod,
          ListBox lb, Int32 intDepth)
        {
          // Adds a node to the ListBox,
          // together with its children. intDepth
          // controls the depth of indenting
    
          StringBuilder sbNode =
            new StringBuilder();
          // Only process Text and Element nodes
          if((xnod.NodeType == XmlNodeType.Element)
            ||(xnod.NodeType == XmlNodeType.Text))
          {
            sbNode.Length = 0;
            for(int intI=1;
              intI <= intDepth ; intI++)
            {
              sbNode.Append(" ");
            }
            sbNode.Append(xnod.Name + " ");
            sbNode.Append(
              xnod.NodeType.ToString());
            sbNode.Append(": " + xnod.Value);
            lb.Items.Add(sbNode.ToString());
    
            // Now add the attributes, if any
            XmlAttributeCollection atts =
              xnod.Attributes;
            if(atts != null)
            {
              for(int intI = 0;
                intI < atts.Count; intI++)
              {
                sbNode.Length = 0;
                for (int intJ = 1;
                  intJ <= intDepth + 1;
                  intJ++)
                {
                  sbNode.Append(" ");
                }
                sbNode.Append(
                  atts[intI].Name + " ");
                sbNode.Append(atts[
                  intI].NodeType.ToString());
                sbNode.Append(": " +
                  atts[intI].Value);
                lb.Items.Add(sbNode);
              }
            }
            // And recursively walk
            // the children of this node
            XmlNode xnodworking;
            if (xnod.HasChildNodes)
            {
              xnodworking = xnod.FirstChild;
              while (xnodworking != null)
              {
                AddChildren(xnodworking, lb,
                  intDepth + 1);
                xnodworking =
                  xnodworking.NextSibling;
              }
            }
          }
        }
      }
    }
  7. Switch to the code view of the form StepByStep2_5.cs and add the following using directives:

    using System.Data;
    using System.Xml;
  8. Double-click the Button control and add the following code to handle the button's Click event:

    private void btnLoadDataSet_Click(
      object sender, System.EventArgs e)
    {
      // Fill the DataSet
      sqlDataAdapter1.Fill(dsEmployees1, "Employees");
      // Retrieve the associated document
      XmlDataDocument xd =
        new XmlDataDocument(dsEmployees1);
      // Display it in the ListBox
      Utility util = new Utility();
      util.XmlToListBox(xd, lbNodes);
    }
  9. Insert the Main() method to launch the form. Set the form as the startup object for the project.

  10. Run the project. Click the button. The code loads the DataSet object from the Employees table in the SQL Server database. It then converts the DataSet object to XML and displays the results in the ListBox, as shown in Figure 2.5.

Figure 2.5Figure 2.5 The XmlDataDocument object allows structured data to be retrieved and manipulated through a DataSet object.

Starting with an XML Schema

The third method to synchronize the two objects is to follow a three-step process:

  1. Create a new DataSet object with the proper schema to match an XML document, but no data.

  2. Create the XmlDataDocument object from the DataSet object.

  3. Load the XML document into the XmlDataDocument object.

Step-by-Step 2.6 demonstrates this technique.

STEP BY STEP 2.6 - Synchronization Starting with an XML Schema

  1. Add a new form to the project. Name the new form StepByStep2_6.cs.

  2. Add a Button control (btnLoadXml), a DataGrid control (dgXML), and a ListBox control (lbNodes) to the form.

  3. Add a new XML Schema file to the project from the Add New Item dialog box. Name the new file Books.xsd.

  4. Switch to the XML view of the schema file and add this code:

    <?xml version="1.0" encoding="utf-8" ?>
    <xs:schema id="Books" xmlns=""
     xmlns:xs="http://www.w3.org/2001/XMLSchema">
      <xs:element name="Books">
        <xs:complexType>
          <xs:choice maxOccurs="unbounded">
            <xs:element name="Book">
              <xs:complexType>
                <xs:sequence>
                  <xs:element name="Author"
                   type="xs:string" />
                  <xs:element name="Title"
                   type="xs:string" />
                </xs:sequence>
                <xs:attribute name="Pages"
                 type="xs:string" />
              </xs:complexType>
            </xs:element>
          </xs:choice>
        </xs:complexType>
      </xs:element>
    </xs:schema>
  5. Switch to the code view of the form StepByStep2_5.cs and add the following using directives:

    using System.Data;
    using System.Xml;
  6. You need the Utility.cs class created in Step-by-Step 2.5, so create it now if you didn't already create it.

  7. Double-click the Button control and add the following code to handle the button's Click event:

    private void btnLoadXml_Click(
      object sender, System.EventArgs e)
    {
      // Create a dataset with the desired schema
      DataSet ds = new DataSet();
      ds.ReadXmlSchema(@"..\..\Books.xsd");
      // Create a matching document
      XmlDataDocument xdd = new XmlDataDocument(ds);
      // Load the XML
      xdd.Load(@"..\..\Books.xml");
      // Display the XML via the DOM
      Utility util = new Utility();
      util.XmlToListBox(xdd, lbNodes);
      // Display the resulting DataSet
      dgXML.DataSource = ds;
      dgXML.DataMember = "Book";
    }
  8. Insert the Main() method to launch the form. Set the form as the startup object for the project.

  9. Run the project. Click the button. The code loads the DataSet object and the XmlDataDocument object. It then displays the DataSet object in the DataGrid control and the XML document content in the ListBox control, as shown in Figure 2.6.

Figure 2.6Figure 2.6 You can start synchronization with a schema file to hold only the desired data in the DataSet object.

The advantage to using this technique is that you don't have to represent the entire XML document in the DataSet schema; the schema needs to include only the XML elements that you want to work with. For example, in this case the DataSet object does not contain the Publisher column, even though the XmlDataDocument includes that column (as you can verify by inspecting the information in the ListBox control).

Guided Practice Exercise 2.1

As you might guess, the XmlTextReader is not the only class that provides a connection between the DOM and XML documents stored on disk. There's a corresponding XmlTextWriter class that is designed to take an XmlDocument object and write it back to a disk file.

For this exercise, you should write a form that enables the user to open an XML file and edit the contents of the XML file on a DataGrid control. Users who are finished editing should be able to click a button and save the edited file back to disk. You can use the WriteTo() method of an XmlDocument or XmlDataDocument object to write that object to disk through an XmlTextWriter object.

How would you design such a form?

You should try working through this problem on your own first. If you get stuck, or if you'd like to see one possible solution, follow these steps:

  1. Add a new form GuidedPracticeExercise2_1.cs to your Visual C# .NET project.

  2. Add two Button controls (btnLoadXml and btnSaveXml) and a DataGrid control (dgXML) to the form.

  3. Switch to the code view and add the following using directives:

    using System.Data;
    using System.Xml;
  4. Double-click the form and add the following code in the class definition:

    // Define XmlDataDocument object and DataSet
    // object at the class level
    XmlDataDocument xdd = new XmlDataDocument();
    DataSet ds;
    
    private void GuidedPracticeExercise2_1_Load(
      object sender, System.EventArgs e)
    {
      // Retrieve the associated DataSet and store it
      ds = xdd.DataSet;
    }
  5. Double-click both the Button controls and add the following code in their Click event handlers:

    private void btnLoadXml_Click(
      object sender, System.EventArgs e)
    {
      // Create a new XmlTextReader on the file
      XmlTextReader xtr = new
        XmlTextReader(@"..\..\Books.xml");
      // Initialize the DataSet by reading the schema
      // from the XML document
      ds.ReadXmlSchema(xtr);
      // Reset the XmlTextReader
      xtr.Close();
      xtr = new XmlTextReader(@"..\..\Books.xml");
      // Tell it to ignore whitespace
      xtr.WhitespaceHandling = WhitespaceHandling.None;
      // Load the synchronized object
      xdd.Load(xtr);
      // Display the resulting DataSet
      dgXML.DataSource = ds;
      dgXML.DataMember = "Book";
      // Clean up
      xtr.Close();
    }
    
    private void btnSaveXml_Click(
      object sender, System.EventArgs e)
    {
      // Create a new XmlTextWriter on the file
      XmlTextWriter xtw = new XmlTextWriter(
        @"..\..\Books.xml",
        System.Text.Encoding.UTF8);
      // And write the document to it
      xdd.WriteTo(xtw);
      xtw.Close();
    }
  6. Insert the Main() method to launch the form. Set the form as the startup object for the project.

  7. Run the project. Click the Load XML button to load the Books.xml file to the DataGrid control. Change some data on the DataGrid control, and then click the Save XML button. Close the form. Open the Books.xml file to see the changed data.

The code in this exercise keeps the XmlDataDocument and DataSet objects at the class level so that they remain in scope while the user is editing data. To make the data available for editing, a DataSet object is synchronized to an XmlDataDocument object and then that DataSet object is bound to a DataGrid control. As the user makes changes on the DataGrid control, those changes are automatically saved back to the DataSet object, which in turn keeps the XmlDataDocument object synchronized to reflect the changes. When the user clicks the Save XML button, the contents of the XmlDataDocument object are written back to disk.

If you have difficulty following this exercise, review the section "Synchronizing DataSet Objects with XML." After doing that review, try this exercise again.

REVIEW BREAK

  • The XmlDataDocument class is a subclass of the XmlDocument class that can be synchronized with a DataSet object.

  • You can start the synchronization process with the XmlDataDocument object or with the DataSet object, or you can use a schema file to construct both objects.

  • Changes to one synchronized object are automatically reflected in the other.

  • You can use an XmlTextWriter object to persist an XmlDocument object back to disk.

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