Query and transform XML with LINQ to XML in Visual C#

Drawing QR-Code in Visual C# Query and transform XML with LINQ to XML

Before moving on to our next LINQ to XML axis method, let s revisit the XML fragment that we re working with and talk about what s next. We started our journey by selecting the first category under the root element in listing 10.1. The following XML fragment is the result:

<category name=".NET"> <books> <book>CLR via C#</book> <book>Essential .NET</book> </books> </category>

Once we have the XElement in hand, we output the name of the category to the console with the help of the Attribute query axis method. Next we need to query the category XElement for all the book elements contained within it. Unfortunately, we need to select multiple elements, so we can t use the Element method that we ve already learned about. It looks like it s time to learn about the Elements axis method.

The Elements axis method is similar to the Element query axis method; the primary difference is that rather than returning the first matching XElement, Elements returns all matches. Given this, it shouldn t be surprising that Elements returns an IEnumerable of XElement objects, rather than a single XElement. Like Element, Elements accepts an XName as a parameter. In our case, we re looking for all <book> elements so we ll provide book as our parameter to Elements. Since the <book> elements aren t directly under the category XElement that we selected in sections 10.1.1 and 10.1.2, we ll need to select the <books> element with the Element query axis method, and then call Elements as shown in listing 10.4.

XElement root = XElement.Load("categorizedBooks.xml"); XElement dotNetCategory = root.Element("category"); XAttribute name = dotNetCategory.Attribute("name"); XElement books = dotNetCategory.Element("books"); IEnumerable<XElement> bookElements = books.Elements("book"); Console.WriteLine((string) dotNetcategory); foreach(XElement bookElement in bookElements) { Console.WriteLine(" - " + (string)bookElement); }

When we run the code, we get the following results:

In addition to allowing us to find all elements with a given name, the Elements method also has a parameterless overload that can be used to retrieve all the children of an XElement. In the listing, we could have called the parameterless version of Elements since the <books> element only contains <book> elements as children. By leveraging the Element, Attribute, and Elements axis methods, we ve successfully read a set of details out of our sample XML and accomplished our goal. We didn t set our sights that high, but nevertheless we ve learned about three essential LINQ to XML axis methods that we ll use when constructing more complex LINQ to XML queries. It s important to remember that Elements only searches the elements that are direct children of the XElement that it s called on. Sometimes rather than needing just the children of the current element, we want to look at all the elements that exist at any level beneath the current element. It s for these scenarios that the LINQ to XML API provides the Descendants axis method.

The Descendants axis method works in the same way as the Elements method, but instead of limiting the elements returned to those that are direct children of the current element, Descendants will traverse all the elements underneath the current element. The Descendants axis method is helpful when you want to retrieve all the elements with a particular XName, but you re not sure where in the tree they live. The Descendants axis method has two overloads. The first overload accepts an XName and returns all elements anywhere underneath the current element with the provided XName. To retrieve every descendant, regardless of XName, you can call Descendants without any parameters. We re once again going to use the XML we introduced in listing 10.1. This time, instead of looking for all the books within a single category, we d like to return every book, no matter what category it s in. Since the book elements exist at different levels within the XML, we can t use the Elements axis method. Instead, we ll use the Descendants axis method. To retrieve every book within our XML, we can write the code shown in listing 10.5.