C# in Depth

342 CHAPTER 12 LINQ beyond collectionsmany different things that an XElement could iterate through. XElement contains anumber of axis methods that are used as query sources. If you’re familiar with XPath,the idea of an axis will no doubt be familiar to you. Here are the axis methods useddirectly for querying, each of which returns an appropriate IEnumerable:■ Ancestors ■ DescendantNodes■ AncestorsAndSelf ■ DescendantNodesAndSelf■ Annotations ■ Elements■ Attributes ■ ElementsAfterSelf■ Descendants ■ ElementsBeforeSelf■ DescendantsAndSelfAll of these are fairly self-explanatory (and the MSDN documentation provides moredetails). There are useful overloads to retrieve only nodes with an appropriate name:calling Descendants("user") on an XElement will return all user elements underneaththe element you call it on, for instance. A number of extension methods alsomake these axes available to whole sequences of nodes: the result is a concatenatedsequence as if the method has been called on each node in turn.As well as these calls returning sequences, there are some methods that return asingle result—Attribute and Element are the most important, returning the namedattribute and the first descendant element with the specified name, respectively.One aspect of XAttribute that is particularly relevant to querying is the set of theexplicit conversions from an XAttribute to any number of other types, such as int,string, and DateTime. These are important for both filtering and projecting results.Enough talk! Let’s see some code. We’ll start off simply, just displaying the userswithin our XML structure, as shown in listing 12.17.Listing 12.17Displaying the users within an XML structureXElement root = XmlSampleData.GetElement();var query = from user in root.Element("users").Elements()select new { Name=(string)user.Attribute("name"),UserType=(string)user.Attribute("type") };foreach (var user in query){Console.WriteLine ("{0}: {1}", user.Name, user.UserType);}After creating the data at the start, we navigate down to the users element, and ask itfor its direct child elements. This two-step fetch could be shortened to justroot.Descendants("user"), but it’s good to see the more rigid navigation so youcan use it where necessary. It’s also more robust in the face of changes to the documentstructure, such as another (unrelated) user element being added elsewhere inthe document.The rest of the query expression is merely a projection of an XElement into ananonymous type. I’ll admit that we’re cheating slightly with the user type: we’ve kept itas a string instead of calling Enum.Parse to convert it into a proper UserType value.The latter approach works perfectly well—but it’s quite longwinded when you onlyLicensed to Rhona Hadida