Introducing LINQ to XML

LINQ to XML provides the power of the DOM and the expressiveness of XPath and XQuery through LINQ extension methods to manage and query in-memory XML nodes. It uses the latest enhancements of .NET languages, such as anonymous methods, generics, nullable types, and so forth. Consider the following introductory sample XML document:

<?xml version="1.0" encoding="UTF-16" standalone="yes"?>
<customer id="C01>
  <firstName>Paolo</firstName>

  <lastName>Pialorsi</lastName>
  <addresses>
    <address type="email">paolo@devleap.it</address>
    <address type="url">http://www.devleap.it/</address>

    <address type="home">Brescia - Italy</address>
  </addresses>
</customer>

Listing 6-1 shows how you can build such a document with LINQ to XML syntax using C#.

Listing 6-1: A sample of XML functional construction

XDocument customer =
    new XDocument(
        new XDeclaration("1.0", "UTF-16", "yes"),
        new XElement("customer",
            new XAttribute("id", "C01"),
            new XElement("firstName", "Paolo"),
            new XElement("lastName", "Pialorsi"),
            new XElement("addresses",
                new XElement("address",
                    new XAttribute("type", "email"),
                        "paolo@devleap.it"),
                new XElement("address",
                    new XAttribute("type", "url"),
                        "http://www.devleap.it/"),
                new XElement("address",
                    new XAttribute("type", "home"),
                        "Brescia - Italy"))));

The preceding example is called “functional construction,” and it reveals how it is simple and intuitive to define an XML nodes hierarchy with the LINQ to XML API. As you can see, the code layout describes the final hierarchical structure using nested constructors. A standard DOM approach requires you to declare and collect many objects, while this new syntax uses a single “hierarchical” statement to achieve this goal, allowing the developer to focus on the final output XML structure rather than on DOM rules.

As you saw in Chapter 3, “Visual Basic 9.0 Language Features,” Visual Basic 9.0 language enhancements introduce a new feature called XML literals that makes it easier to write and read XML contents. In fact, the previous example, when written in Microsoft Visual Basic 9.0, looks like the excerpt shown in Listing 6-2.

Listing 6-2: A sample of Visual Basic 9.0 XML literals

Dim customerXml As XDocument = _
  <?xml version="1.0" encoding="UTF-16" standalone="yes"?>
    <customer id="C01">
      <firstName>Paolo</firstName>

      <lastName>Pialorsi</lastName>
      <addresses>
        <address type="email">paolo@devleap.it</address>
        <address type="url">http://www.devleap.it/</address>

        <address type="home">Brescia - Italy</address>
      </addresses>
    </customer>

The power and expressiveness of this syntax is truly evident. We write pure XML code, even if it is inside a Visual Basic 9.0 piece of code. Also, querying XML with LINQ to XML is really simple. In the following sample code, we enumerate all the address tags of a given customer, using C#:

foreach(XElement a in customer.Descendants("addresses").Elements()) {
    Console.WriteLine(a);
}

Using Visual Basic 9.0, this query can be written using a more intuitive syntax, like the following one:

Dim address As XElement
For Each address In xCustomer.<customer>.<addresses>.<address>
    Console.WriteLine(address.Value)
Next

We will come back to this fancy syntax later, but now just notice that once again we are using an XML-like syntax within Visual Basic 9.0 code. From the next section to the end of this chapter, we will cover all the features of LINQ to XML.