LINQ, Lambda Expressions, and Functional Programming

LINQ (Language-Integrated Query)

LINQ makes a query a first-class language construct. You write queries against strongly typed collections of objects. The actual execution of the query is deferred until you iterate over the query variable (e.g., by using foreach, .ToArray() or .Count(). This concept is referred to as deferred execution. Remember this to avoid processing a collection of items more than once.

 

LINQ methods include many of the common SQL database methods, like, Where, Select, Join, Union, Distinct, GroupBy, Average, Sum, Min, Max, Count, All, Any, Contains, and more.

 

LINQ may be expressed either in comprehension syntax or as calls to extension methods.

Comprehension Syntax

IEnumerable<int> numQuery =

            from num in numbers

            where (num % 2) == 0

            select num;

 

Extension Method with Lambda Expression

IEnumerable<int> numQuery = numbers.Where(num => (num % 2) == 0);

 

http://msdn.microsoft.com/en-us/library/vstudio/bb397897.aspx

Lambda Expressions

By using lambda expressions, you can write local functions that can be passed as arguments or returned as the value of function calls.

 

For example, the lambda expression

 

c => c.City == "London"

 

is the same as

 

delegate (Customer c)
{
    return c.City == "London";
}

 

Lambda expressions are particularly helpful for writing LINQ query expressions.

 

http://msdn.microsoft.com/en-us/library/vstudio/bb397687.aspx

Functional Programming

LINQ technology is a form of declarative, functional programming.

 

Functional programming is a form of declarative programming. Mainstream languages, such as C#, C++, and Java, were designed to primarily support imperative (procedural) programming. With an imperative approach, a developer writes code that describes in exacting detail the steps that the computer must take to accomplish the goal. This is sometimes referred to as algorithmic programming. In contrast, a functional approach involves composing the problem as a set of functions to be executed. You define carefully the input to each function, and what each function returns.

The following table describes some of the general differences between these two approaches. 

Characteristic	Imperative approach	Functional approach
Programmer focus	How to perform tasks (algorithms)	What information is desired and what transformations are required
Order of execution	Important	Low importance
Primary flow control	Loops, conditionals, and function (method) calls	Function calls, including recursion
Primary manipulation unit	Instances of structures or classes	Functions as first-class objects and data collections

 

http://msdn.microsoft.com/en-us/library/bb669144.aspx

 

BTW, F# is a functional language.