LINQ Queries with Xml Literals

Xml literals provide an alternative syntax for LINQ to Xml queries in Visual Basic code. Let’s retake the first Xml document used in the “System.Xml.Linq Namespace” section, which looks like the following but with a slight modification:

There is a Person element that we want to be excluded. The goal is querying all Contact elements whose age is greater than 40. Instead of recurring to the classical syntax, you can write the following code:

The preceding code uses new symbols for querying documents known as Xml Axis Properties. Table 28.3 summarizes Xml axis.

Table 28.3 Xml Axis

In other words, the preceding query can be described by this sentence: “Process all Contacts’ children Contact elements.” The difference with the Xml Axis Descendants Properties can be explained with another example. Consider the following document that is just a revalidation of the previous one:

Now each Contact element has subelements. If you wanted to get a collection of all LastName elements for all elements, you could use the Xml Axis Descendants property as follows:

It’s worth mentioning that such a query also includes results from the Person element, because it exposes a LastName attribute. So if you need to filter results depending on the root element, you should invoke the Axis Descendants property. Notice also how in both the previous code examples an explicit conversion is required when you need a comparison against non-String data. In this particular case the comparison is done against an integer number (40); therefore, you can invoke the Integer.Parse method—because you expect that the Age attribute contains the string representation of a number. Xml Axis properties provide therefore a simplified and cleaner way for querying Xml documents. Just remember that, as discussed in the previous section, you will need helper classes for mapping each XElement content into a .NET type to provide data-binding features to your code.