Further Exploration with LINQ - Page 2
       by Granville Barnett  |  24 September 2007

In the previous page, you learned how to use LINQ with SQL stored procedures by looking at a few examples. In this page, let's look at how LINQ works with XML.

LINQ to XML
I have mentioned that LINQ can be used against several data stores, and so far we have only covered in-memory collections and relational data with SQL Server 2005. However, we can also use LINQ with XML data. Dubbed LINQ to XML, LINQ allows us to not only query XML in a truly unique way, but also create XML documents in a very expressive manner. You can find the LINQ extensions for XML in the System.Xml.Linq namespace.

Let’s dive in and create a sample XML document to represent a book and its related data:

XElement book = new XElement("book",new XAttribute("publisher", "MS Press"), new XElement("title", "CLR Via C#"),

           new XElement("authors",

           new XElement("author", "Jeffrey Richter")));

 

Console.WriteLine(book);

You can see the result produced by this code in the following image:

[ your XML document recreated ]

One of the things about which I was skeptical at first was the whacky indentation used when defining an XML document using some of the new types like XElement and XAttribute. This indentation is not compulsory, however it is remarkably clear to read when reviewing your code, so I would encourage you to preserve this indentation, if only for code clarity.

Querying XML Data
We will now take the opportunity to look at a few queries of XML data. For all the queries we load an XML document which contains several book elements, using the same definition as outlined in above. Just like any query, the simplest is to drag everything down; we will take the same approach and select all the book elements from the XML document as shown in this snippet:

XElement doc = XElement.Load("Books.xml");

 

IEnumerable<XElement> query = doc.Descendants("book");

 

foreach (XElement book in query) {

Console.WriteLine(book);

}

I’m sure that you will agree with me that it is dead easy to query XML using LINQ – let’s now take the opportunity to look at a query that selects only the books of a certain publisher. Before we actually look at some code we need to come to terms with types in XML documents – there are none! For this reason we are provided with casts for all major value and reference types (int, double, float, string, etc…).

For example, to select only “MS Press” books use:

XElement doc = XElement.Load("Books.xml");

 

IEnumerable<XElement> query = doc.Descendants("book").Where(x => (string)x.Attribute("publisher") == "MS Press");

 

foreach (XElement book in query) {

Console.WriteLine(book);

}

The result of this query can be seen in the following image:

[ the output produced by our query ]

Adding Data to the XML Document
Adding XML to the existing XML document is very simple, we need only construct our XML using a mixture of XElement, and XAttribute types (there are many more you can use other than these two!) and then add them to the document. The following adds a new book:

XElement doc = XElement.Load("Books.xml");

 

XElement myBook = new XElement("book", new XAttribute("publisher", "Addison-Wesley"), new XElement("title", "Exceptional C++ Style"), new XElement("authors", new XElement("author", "Herb Sutter")));

 

doc.Add(myBook);

doc.Save("Books.xml");

As with LINQ to SQL, where we have to call the SubmitChanges method of the DataContext to commit the changes to the database, in LINQ to XML, no changes are made to the loaded XML document until that document is saved.

Onwards to the next page.

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