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Inside Visual Studio 2008

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LINQ Facilities

Most Windows and web applications are centered on some sort of data repository, usually a relational database. To retrieve data, you have to use a different API each time. It can be SQL for relational databases, XQuery and XPath for XML documents, some interface (ICollection, for instance) for collections and arrays. The main goal of LINQ is unifying this model by providing an ad hoc framework. Conveniently, this query framework is wired to some new keywords in C# and VB:


int[] fiboNumbers = new int[] {0,1,1,2,3,5,8,13,21,34};
var data = from n in fiboNumbers
           where n % 2 == 0
           select n;


As weird as it may seem, this is C# code that compiles in Visual Studio 2008. The new keywords are from, where, and select. Their meaning is really close to the meaning that analogous keywords have in SQL. Translated to human language, the query assigns to the data variable all elements in the queryable object (a collection, in this case) that match the specified condition (all even numbers in this case). Here's another LINQ expression that uses lambdas:


var data = (from c in customers
            where c.Country == "USA"
            select c).SelectMany(c => c.Orders);


A lambda expression is characterized by the => operator, prefixed by input parameters, and followed by the parametric expression to evaluate. The preceding code selects all U.S. customers and flattens the result set to a list of orders. Without the SelectMany keyword and the input lambda, you'll get an array of collections of orders—not a list of order objects.

LINQ operators work on objects that implement a particular interface—IQueryable. .NET 3.5 counts a number of these objects. They are wrappers for well-known data containers such as collections, XML documents, DataSets, and SQL Server databases. The model is extensible too, so third-parties can provide their own LINQ engine to query Oracle databases or perhaps the filesystem.

The most interesting of LINQ queryable objects is the object that wraps a SQL Server 200x database. It is a user-defined class that inherits from a base class named DataContext. Visual Studio 2008 provides an ad hoc visual tool called "O/R designer"; see Figure 4. After you set up a database connection in the Server Explorer window, you drop tables and stored procedures in the designer and get a dynamically generated class to work with. Next, you simply instantiate this class and use it as the queryable object:


NorthwindDataContext dataContext = 
     new NorthwindDataContext();
var data = from c in dataContext.Customers
     where c.Country == "Spain" 
     select c;


[Click image to view at full size]

Figure 4: O/R designer modeling the SQL Server's Northwind database.

You can then go through the returned result set using a for/each statement or just bind the object to a data-bound control.


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