LINQ TO XML

LINQ to XML refers to methods that let a program move data between XML objects and other data-containing objects. For example, using LINQ to XML you can select customer data and use it to build an XML document.

LINQ provides a new selection of XML elements. These classes contained in the System.Xml.Linq namespace correspond to the classes in the System.Xml namespace. The names of the new classes begin with “X” instead of “Xml.” For example, the LINQ class representing an element is XElement whereas the System.Xml class is XmlElement.

The LINQ versions of the XML classes provide many of the same features as the System.Xml versions, but they also provide support for new LINQ features.

The following section describes one of the most visible features of the LINQ XML classes: XML literals. The two sections after that introduce methods for using LINQ to move data into and out of XML objects.

XML Literals

In addition to features similar to those provided by the System.Xml classes, the System.Xml.Linq classes provide new LINQ-oriented features. One of the most visible of those features is the ability to use XML literal values. For example, the following code creates an XDocument object that contains three Customer elements:

Dim xml_literal As XElement =
    <AllCustomers>
        <Customer FirstName="Ann" LastName="Archer">100.00</Customer>
        <Customer FirstName="Ben" LastName="Best">-24.54</Customer>
        <Customer FirstName="Carly" LastName="Cant">62.40</Customer>
    </AllCustomers>

Visual Basic LINQ translates this literal into an XML object hierarchy holding a root element named AllCustomers that contains three Customer elements. Each Customer element has two attributes, FirstName and LastName.

To build the same hierarchy using System.Xml objects would take a lot more work. The CustomersToXml example program, which is available for download on the book’s website, includes a System.Xml version in addition to the previous LINQ literal version. The System.Xml version takes 26 lines of code and is much harder to read than the LINQ literal version.

Other LINQ XML classes such as XDocument, XComment, XCdata, and XProcessingInstruction also have literal formats, although usually it’s easier to use an XElement instead of an XDocument, and the others are usually contained in an XElement or XDocument.

The Visual Basic code editor also provides some extra enhancements to make writing XML literals easier. For example, if you type a new XML tag, when you type the closing “>” character the editor automatically adds a corresponding closing tag. If you type “<Customer>” the editor adds the “</Customer>” tag. Later if you change a tag’s name, the code editor automatically changes the corresponding closing tag.

Together these LINQ XML literal tools make building hard-coded XML data much easier than it is using the System.Xml objects.