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Listing 5-17. Validating a Node Explicitly private void button2_Click(object sender, EventArgs e) { XmlNode node = doc.SelectSingleNode("//employee[@employeeid='" + comboBox1.SelectedItem + "']"); if (node != null) { node.ChildNodes[0].InnerText = textBox1.Text; node.ChildNodes[1].InnerText = textBox2.Text; node.ChildNodes[2].InnerText = textBox3.Text; XmlCDataSection notes = doc.CreateCDataSection(textBox4.Text); node.ChildNodes[3].ReplaceChild(notes, node.ChildNodes[3].ChildNodes[0]); } doc.Validate(OnValidationError,node); if (!isError) { doc.Save(Application.StartupPath + "/employees.xml"); } } Note the code marked in bold. The Validate() method of XmlDocument can validate the entire document or just a node against a previously specified schema or DTD. The Validate() method accepts two parameters: the name of a function that matches the ValidationEventHandler delegate signature and the XmlNode to validate. There is one more overload of Validate() that takes just a function matching the ValidationEventHandler delegate signature and validates the entire document. The OnValidationError function is shown in Listing 5-18. Listing 5-18. The OnValidationError Function void OnValidationError(object sender, ValidationEventArgs e) { MessageBox.Show(e.Message); isError = true; } The function simply shows the error message in a message box and sets a class-level Boolean variable isError to true. This variable is checked to decide whether to save the document. To test the application, modify the phone number of any employee to more than 20 characters and click the Update button. You should see a message box similar to the one shown in Figure 5-19.
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CHAPTER 5 VA LIDATIN G XML DOCUMEN TS
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Figure 5-19. Validation error during saving changes
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Using XPath Navigator to Validate XML Documents
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You might be using XPathNavigator to read XML documents and may wish to perform validation on those documents. Recollect from 4 that XPathNavigator can be obtained by using the XmlDocument or XPathDocument classes. The XPathNavigator obtained by using XmlDocument is editable, whereas that obtained by using XPathDocument is read-only. The XPathNavigator class by itself does not allow you to validate data, but you can use the underlying XmlDocument or XPathDocument classes to perform the validation. We have already seen the validation performed by using XmlDocument, but just like XmlDocument, the XPathDocument class allows you to validate XML documents against a schema or DTD during loading. In this case, the constructor of XPathDocument can accept a validating XmlReader to perform the validation. Because the XPathNavigator obtained by using XPathDocument is read-only, there is no way to revalidate the XML document after it has been loaded. Listing 5-19 shows how the XPathNavigator obtained by using XPathDocument can perform the validation. Listing 5-19. Validating by Using XPathNavigator XmlReaderSettings settings = new XmlReaderSettings(); settings.ValidationType = ValidationType.Schema; settings.Schemas.Add("", "C:\Bipin\employees.xsd"); settings.ValidationEventHandler += new ValidationEventHandler(OnValidationError); XmlReader reader = XmlReader.Create("C:\Bipin\employees.xml", settings); XPathDocument doc = new XPathDocument(reader); XPathNavigator navigator = doc.CreateNavigator(); The code creates an XmlReaderSettings object as before and adds a schema to use for validation. The Create() method of XmlReader accepts this XmlReaderSettings object and returns an XmlReader instance. The XmlReader instance is then supplied to the constructor of the XPathDocument class. The CreateNavigator() method of XPathDocument finally creates the XPathNavigator that can be used to navigate through the XML document.
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This chapter introduced you to DTDs and schemas. You learned how to create a DTD and a schema. You also learned to use Visual Studio tools for creating XSD schemas. The .NET Framework s Schema Object Model (SOM) is an extensive collection of objects that allows you to create XSD schemas programmatically. You learned some of the important and commonly used classes from the SOM hierarchy.
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Creating a schema or DTD is just half of the story. The other half involves actually validating your XML documents against the specified schema or DTD. To validate an XML document against a schema or DTD, you can use several approaches that is, with the XmlReader, XmlDocument, and XPathNavigator classes. All the approaches essentially rely on two classes: XmlReaderSettings and XmlReader. The former class configures the XmlReader to perform validation. It also attaches an event handler for handling validation events. The latter actually reads the document and notifies you of validation errors.
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