Introduction to LINQ - Page 1
       by Granville Barnett  |  15 September 2007

Language Integrated Query (LINQ) comes as part of the future revisions of both the C# and VB.NET compilers, introducing a standard set of operators that can be used to query several different data stores such as SQL Server and XML.

In the past developers have had to spend many hours creating trivial data access layers like that in the listing below so that applications can interact more fluently with their chosen data store:

//...

public List<Book> GetBooks()

{

List<Book> bookList = new List<Book>();

using (SqlConnection sqlConn =

new SqlConnection(_conn))
 

using (SqlCommand cmd = new SqlCommand("SelectBooks", sqlConn))

{

cmd.CommandType =

CommandType.StoredProcedure;

sqlConn.Open();

SqlDataReader sqlRdr = cmd.ExecuteReader();
 

while (sqlRdr.Read())

{

bookList.Add(new Book((int)sqlRdr.GetInt32(0),

             (string)sqlRdr.GetString(1),

             (int)sqlRdr.GetInt32(2),

             (string)sqlRdr.GetString(3)));

}

}

return bookList;

}

// ...

Typically these data access layers would use SQL queries and map their results to a collection of custom types. This was fun the first time we did it many years ago, but quite frankly we are now starting to question our own sanity – mention a data access layer (DAL) again, and we might just scream!

In this article we will discuss how LINQ for SQL can help us create more intuitive, flexible DAL’s with a lot less code. Unfortunately we cannot just dive in without first setting the scene.

By the time you have read this article you will be familiar with key concepts introduced in LINQ and LINQ to SQL. With this new-found knowledge, you will be able to construct more powerful DAL’s for your applications to interact with.

Advantages of using LINQ for SQL
Many developers have been intimidated by complex Object Relation Mapping (ORM) software due to the complexities of replicating a conceptual model of their database schema. One of the major advantages of using LINQ to SQL is that the complexity is taken away from the developer; new types have been included in ADO.NET Next (the next version of ADO.NET), which easily allow you to express relationships between entities. Developers will benefit from being able to step through queries while debugging, and get intellisense when constructing queries, resulting in a more comfortable and robust programming environment.

Furthermore, the public May CTP of LINQ includes a number of tools for us to use which automate the process of constructing our conceptual entity view of our database schema.
For more information on mapping to relational databases using objects refer to Patterns of Enterprise Application Architecture by Martin Fowler.

Understanding LINQ
Before we apply LINQ queries to entities we must first familiarize ourselves with exactly what LINQ is. In this section we will look at code examples of LINQ in action as well as provide a core understanding of the new features in C# 3.0 which LINQ uses extensively.

Overview of LINQ
In the following diagram we can see that there are several components to LINQ, the middle tier of technologies which include LINQ, LINQ for XML, and LINQ for SQL can be utilized by both C# and VB.NET and potentially other languages.

[ The structure of LINQ ]

Each middle tier component provides us with the required functionality to interact with that component’s data store.

Tools to Get Started
Getting up and running with LINQ is simple – it requires a one-off download of the LINQ May CTP. The CTP includes:

• C# 3.0/VB.NET 9.0 compiler (includes LINQ, LINQ for SQL and LINQ for XML namespaces)
• Language reference update for Visual Studio 2005
• Project Templates for Visual Studio (includes Console Application, Win Forms, WPF, and Class Library)
• Two tools for automating the creation of entities (LINQ for Relational Data (DLINQ) recently renamed to LINQ for SQL), these include the DLINQ objects item (visual designer for Visual Studio 2005) and SqlMetal which is a command line utility.
• Sample applications

The May CTP of LINQ is a required installation for running any code in this article - especially using Visual Studio 2005. If you have the patience, you can also download and install the Visual Studio 2008 Beta edition, but some of the steps will be different if you try to follow the instructions found in this article.

In the next page let's get started by creating a simple LINQ application.

Onwards to the next page.

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