LINQ Pocket Reference

Projecting into an X-DOM

You can also use LINQ queries to project into an X-DOM. The source can be anything over which LINQ can query, such as:

• LINQ to SQL Tables

• A local collection

• Another X-DOM

Regardless of the source, the strategy is the same in using LINQ to emit an X-DOM: you first write a functional construction expression that produces the desired X-DOM shape, and then build a LINQ query around the expression.

For instance, suppose we wanted to retrieve customers from a database into the following XML:

	<customers>
	  <customer id="1">
	    <name>Sue</name>
	    <buys>3</buys>
	  </customer>
	  ...
	</customers>

We start by writing a functional construction expression for the X-DOM using simple literals:

	var customers =
	  new XElement ("customers",
	    new XElement ("customer", new XAttribute ("id", 1),
	      new XElement ("name", "Sue"),
	      new XElement ("buys", 3)
	    )
	  );

We then turn this into a projection and build a LINQ query around it:

	var customers =
	  new XElement ("customers",
	    from c in dataContext.Customers
	    select
	      new XElement ("customer",
	        new XAttribute ("id", c.ID),
	        new XElement ("name", c.Name),
	        new XElement ("buys", c.Purchases.Count)
	      )
	    );

Here’s the result:

	<customers>
	  <customer id="1">
	    <name>Tom</firstname>
	    <buys>3</buys>
	  </customer>
	  <customer id="2">
	    <name>Harry</firstname>
	    <buys>2</buys>
	  </customer>
	    ...
	</customers>

The outer query in this case defines the line at which the query transitions from being a remote LINQ to SQL query to a local LINQ to enumerable query. XElement’s constructor doesn’t know ...