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Specifying Namespaces While Constructing Elements
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When you create an XML tree programmatically, you need to specify namespace information associated with it using the XNamespace class. To illustrate how this is done, we will develop an application like the one shown in Figure 13-14.
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Figure 13-14. Application that creates an XML tree with a namespace
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C HA P TER 13 WO RK ING W IT H LI NQ T O XM L
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The application consists of four text boxes for entering a namespace name, a namespace prefix (if any), a first name, and a last name. Clicking the Create XML Tree button creates XElement and XAttribute instances as required and displays the resultant XML tree in a message box. The Click event handler of the button is shown in Listing 13-23. Listing 13-23. Using XNamespace to Specify Namespace Information private void button1_Click(object sender, EventArgs e) { XNamespace ns = textBox1.Text; XElement root = new XElement(ns + "employee", new XElement(ns + "firstname",textBox3.Text), new XElement(ns + "lastname",textBox4.Text) ); if (textBox2.Text == "") { root.SetAttributeValue("xmlns", ns); } else { root.SetAttributeValue(XNamespace.Xmlns + textBox2.Text, ns); } MessageBox.Show(root.ToString()); } The code creates an instance of the XNamespace class and sets the namespace name as indicated in the text box. Notice that the = operator is overloaded, and hence you can assign a string value directly to its instance. The <employee> element is then created. While specifying an element name, the code appends the element name to the namespace. If a namespace prefix is not specified, an attribute with the name xmlns is added to the root element. This way, the code defines a default namespace for the XML tree. If a namespace prefix is specified, that prefix is appended to the xmlns URI (see http://www.w3.org/2000/xmlns/ for more information on xmlns). Notice the use of the XNamespace.Xmlns property, which returns an XNamespace object corresponding to the xmlns URI. Finally, the resultant XML tree is shown using a message box. Figure 13-15 shows a sample generation of an XML tree.
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Figure 13-15. XML tree with namespace information
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CHAPTER 13 WORKING WITH LINQ TO XML
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Validating XML Documents
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XML documents are often validated against XSD schemas to ensure correctness of data. In 5, you learned validation techniques using XmlDocument, XmlReader, and XPathNavigator. Now it s time to see how LINQ to XML can be used to validate XML documents against XSD schemas. The XDocument as well as XElement classes provide an extension method named Validate() that does the job of validating the XML data against the XSD schema. Remember that, in order to access the Validate() extension method, you must import the System.Xml.Schema namespace in your code.
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Note Extension methods enable you to add methods to an existing class without inheriting or modifying the original class. They are a special kind of static methods that are called as if they were instance methods on the extended class.
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To illustrate the use of the Validate() method, we will create an application like the one shown in Figure 13-16.
Figure 13-16. Application that validates XDocument against an XSD schema The application consists of two text boxes and one button. The text boxes accept a path to the XML document that is to be validated and a path to the XSD schema file. Clicking the Validate button loads the XML document in an XDocument instance and validates the document against the supplied schema. Errors during validation (if any) are reported to the user. In this example, we will use the Employees.xsd schema file that we developed in 5 to validate the Employees.xml file. Just to give you a quick recap of the structure of the XSD schema file, Listing 13-24 shows a part of it. Listing 13-24. Partial Markup from Employees.xsd < xml version="1.0" encoding="utf-8" > <xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified"
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xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xs:element name="employees"> <xs:complexType> <xs:sequence> <xs:element name="employee" type="EmployeeType" minOccurs="0" maxOccurs="unbounded" /> </xs:sequence> </xs:complexType> </xs:element> <xs:complexType name="EmployeeType"> <xs:all> <xs:element name="firstname" type="NameSimpleType" /> <xs:element name="notes" type="NotesSimpleType" /> <xs:element name="lastname" type="NameSimpleType" /> <xs:element name="homephone" type="PhoneSimpleType" /> </xs:all> ... The Click event handler of the Validate button is shown in Listing 13-25. Listing 13-25. Validating Against the XSD Schema private void button1_Click(object sender, EventArgs e) { XDocument doc = XDocument.Load(textBox1.Text); XmlSchemaSet schema=new XmlSchemaSet(); schema.Add(null,textBox2.Text); ValidationEventHandler handler = new ValidationEventHandler(MyHandler); doc.Validate(schema, handler); } public void MyHandler(object sender, ValidationEventArgs e) { MessageBox.Show(e.Message); } The code loads the source XML document in an instance of the XDocument class. It then creates an XmlSchemaSet and adds the specified XSD schema to it using the Add() method. The Validate() extension method of the XDocument class accepts two parameters: XmlSchemaSet and an event handler that gets called if validation fails. The event handler is of type ValidationEventHandler. The MyHandler() method acts as the event handler in our example. The validation is triggered by calling the Validate() method. If there are any validation errors, the MyHandler() event handler gets invoked. The MyHandler() event handler receives a parameter of type ValidationEventArgs. The Message property of the ValidationEventArgs class provides information about the validation errors. In order to test the code, remove the employeeid attribute from the first <employee> element of the Employees.xml file and run the application. Figure 13-17 shows a validation error message after a sample run.
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