The way data can be retrieved in .NET. LINQ provides a uniform way to retrieve data from any object that implements the IEnumerable<T> interface. With LINQ, arrays, collections, relational data, and XML are all potential data sources.

 
Why LINQ?

With LINQ, you can use the same syntax to retrieve data from any data source:

var query = from e in employees
where e.id == 1
select e.name

The middle level represents the three main parts of the LINQ project:

LINQ to Objects is an API that provides methods that represent a set of standard query operators (SQOs) to retrieve data from any object whose class implements the IEnumerable<T> interface. These queries are performed against in-memory data.

LINQ to ADO.NET augments SQOs to work against relational data. It is composed of three parts.

LINQ to SQL (formerly DLinq) is use to query relational databases such as Microsoft SQL Server. LINQ to DataSet supports queries by using ADO.NET data sets and data tables. LINQ to Entities is a Microsoft ORM solution, allowing developers to use Entities (an ADO.NET 3.0 feature) to declaratively specify the structure of business objects and use LINQ to query them.

LINQ to XML (formerly XLinq) not only augments SQOs but also includes a host of XML-specific features for XML document creation and queries.

What You Need to Use LINQ

LINQ is a combination of extensions to .NET languages and class libraries that support them. To use it, you’ll need the following:

Obviously LINQ, which is available from the new Microsoft .NET Framework 3.5 that you can download at http://go.microsoft.com/?linkid=7755937.

You can speed up your application development time with LINQ using Visual Studio 2008, which offers visual tools such as LINQ to SQL designer and the Intellisense  support with LINQ’s syntax.

Optionally, you can download the Visual C# 2008 Expression Edition tool at www.microsoft.com/vstudio/express/download. It is the free edition of Visual Studio 2008 and offers a lot of LINQ support such as Intellisense and LINQ to SQL designer. To use LINQ to ADO.NET, you need SQL