LINQ

Over the last few months I've written a series of blog posts that covered some of the new language features that are coming with the Visual Studio and .NET Framework Orcas release. Here are pointers to those posts:

• Automatic Properties, Object Initializer and Collection Initializers
• Extension Methods
• Lambda Expressions
• Query Syntax
• Anonymous Types

The language features above help make querying data a first-class programming concept. We call this overall querying programming model LINQ, which stands for .NET Language Integrated Query.

Developers can use LINQ with any data source. They can express efficient query behavior in their programming language of choice, optionally transform/shape data query results into whatever format they want, and then easily manipulate the results. LINQ-enabled languages can provide full type-safety and compile-time checking of query expressions, and development tools can provide full intellisense, debugging, and rich refactoring support when writing LINQ code.

LINQ supports a very rich extensibility model that facilitates the creation of efficient domain-specific operators for data sources. The Orcas version of the .NET Framework ships with built-in libraries that enable LINQ support against objects, XML, and databases.

What Is LINQ-to-SQL?
LINQ-to-SQL is an O/RM (object/relational mapping) implementation that ships in the .NET Framework Orcas release, and lets you model a relational database using .NET classes. You can then query the database using LINQ, as well as update/insert/delete data from it.

LINQ-to-SQL fully supports transactions, views, and stored procedures. It also provides an easy way to integrate data validation and business logic rules into your data model.

Modeling Databases Using LINQ to SQL
Orcas ships with a LINQ-to-SQL designer that provides an easy way to model and visualize a database as a LINQ-to-SQL object model. My next article will cover in more depth how to use this designer (you can also watch this video I made in January to see me build a LINQ-to-SQL model from scratch using it).

Using the LINQ-to-SQL designer I can easily create a representation like the Northwind sample database below: (Figure 1)

My LINQ-to-SQL design-surface above defines four entity classes: Product, Category, Order, and OrderDetail. The properties of each class map to the columns of a corresponding table in the database. Each instance of a class entity represents a row in the database table. (Figure 2)
The arrows between the four entity classes above represent associations/relationships between the different entities. These are typically modeled using primary-key/foreign-key relationships in the database. The direction of the arrows on the design-surface indicate whether the association is a one-to-one or one-to-many relationship. Strongly-typed properties will be added to the entity classes based on this. For example, the Category class above has a one-to-many relationship with the Product class. This means it will have a Categories property, which is a collection of Product objects in that category. The Product class then has a Categoryproperty that points to a Category class instance that represents the Category to which the Product belongs.

The right-hand method pane in the LINQ-to-SQL design surface above contains a list of stored procedures that interact with our database model. In the sample above I added a single "GetProductsByCategory" SPROC. It takes a categoryID as an input argument and returns a sequence of Product entities as a result. We'll look at how to call this SPROC in a code sample below.

Understanding the DataContext Class
When you press the "save" button in the LINQ-to-SQL designer surface, Visual Studio will persist the .NET classes that represent the entities and database relationships that we modeled. For each LINQ-to-SQL designer file added to our solution, a custom DataContext class will also be generated. This DataContext class is the main conduit by which we'll query entities from the database as well as apply changes. The DataContext class created will have properties that represent each Table we modeled in the database, as well as methods for each Stored Procedure we added.

For example, below is the NorthwindDataContext class that is persisted based on the model we designed above:

LINQ-to-SQL Code Examples
Once we've modeled our database using the LINQ-to-SQL designer, we can then easily write code to work against it. Below are a few code examples that show off common data tasks:

1)  QUERY PRODUCTS FROM THE DATABASE
The code below uses LINQ query syntax to retrieve an IEnumerable sequence of Product objects. Note how the code is querying across the Product/Category relationship to retrieve only those products in the "Beverages" category.