C# 3.0 Pocket Reference, 2nd Edition

LINQ (C# 3.0)

LINQ allows you to write structured type-safe queries over local object collections and remote data sources. LINQ is a new feature of C# 3.0 and .NET Framework 3.5.

LINQ lets you query any collection implementing IEnumerable<>, whether an array, list, XML DOM, or remote data source (such as a table in SQL Server). LINQ offers the benefits of both compile-time type checking and dynamic query composition.

Note

A good way to experiment with LINQ is to download LINQPad at www.linqpad.net. LINQPad lets you interactively query local collections and SQL databases in LINQ without any setup.

LINQ Fundamentals

The basic units of data in LINQ are sequences and elements. A sequence is any object that implements the generic IEnumerable interface and an element is each item in the sequence. In the following example, names is a sequence, and Tom, Dick, and Harry are elements:

	string[] names = { "Tom", "Dick", "Harry" };

A sequence such as this we call a local sequence because it represents a local collection of objects in memory.

A query operator is a method that transforms a sequence. A typical query operator accepts an input sequence and emits a transformed output sequence. In the Enumerable class in System.Linq, there are around 40 query operators; all implemented as static extension methods. These are called standard query operators.

Note

LINQ also supports sequences that can be dynamically fed from a remote data source such as a SQL Server. These sequences additionally implement the ...