C# 0 The Complete Reference

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LINQ in C# is essentially a language within a language. As a result, the subject of LINQ is 

quite large, involving many features, options, and alternatives. Although this chapter describes 

LINQ in significant detail, it is not possible to explore all facets, nuances, and applications of this 

powerful feature. To do so would require an entire book of its own. Instead, this chapter focuses 

on the core elements of LINQ and presents numerous examples. It is important to understand 

that we are just now at the beginning of the LINQ revolution. Going forward, LINQ is a 

subsystem that you will want to study in greater detail.

What Is LINQ?

LINQ stands for 

Language-Integrated Query.

 It encompasses a set of features that let you 

retrieve information from a data source. As you may know, the retrieval of data constitutes an 

important part of many programs. For example, a program might obtain information from a 

customer list, look up product information in a catalog, or access an employee’s record. In 

many cases, such data is stored in a database that is separate from the application. For 

example, a product catalog might be stored in a relational database. In the past, interacting 

with such a database would involve generating queries using Structured Query Language 

(SQL). Other sources of data, such as XML, required their own approach. Therefore, prior to 

C# 3.0, support for such queries was not built into C#. LINQ changes this.

LINQ adds to C# the ability to generate queries for any LINQ-compatible data source. 

Furthermore, the syntax used for the query is the same, no matter what data source is used. 

This means that the syntax used to query data in a relational database is the same as that 

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used to query data stored in an array, for example. It is no longer necessary to use SQL or 

any other non-C# mechanism. The query capability is fully integrated into the C# language.

In addition to using LINQ with SQL, LINQ can be used with XML files and ADO.NET 

Datasets. Perhaps equally important, it can also be used with C# arrays and collections 

(described in Chapter 24). Therefore, LINQ gives you a uniform way to access data. This is a 

powerful, innovative concept. It is not only changing the way that data is accessed, but it also 

offers a new way to think about and approach old problems. In the future, many programming 

solutions will be crafted in terms of LINQ. Its effects will not be limited to just database access.

LINQ is supported by a set of interrelated features, including the query syntax added to 

the C# language, lambda expressions, anonymous types, and extension methods. Lambda 

expressions are described in Chapter 15. The others are examined here.