Introduction

Introduction

What is LINQ?

The acronym LINQ stands for Language Integrated Query. LINQ syntax is roughly derived from SQL and was introduced with .NET 3.5

The LINQ paradigm describes a method for programmatically accessing and manipulating any data, independent of the source. LINQ integrates query capabilities directly into the C# language. It provides a consistent query experience for in-memory objects (LINQ to Objects), relational databases (LINQ to SQL), and XML (LINQ to XML).

Getting started with basic LINQ queries

Query 1: From a give array of strings, below query filters the ones with the letter ‘a’ in it

var words = new string[] { "falcon", "eagle", "sky", "tree", "water" };

// Query syntax
var res = from word in words
          where word.Contains('a')
          select word;

foreach (var word in res)
    Console.WriteLine(word);

Console.ReadKey();

Things to note:

• The above LINQ query is written in what is called as query syntax

• The above snippet will not compile without using System.Linq; It is the namespace required to use LINQ in .NET framework

• We can use this snippet in any kind of .NET project, like Console, WinForms or Web based projects

• We have used the var keyword to hold the results of the query, since it is convenient

• The actual return type would be IEnumerable<string>

• The IEnumerable interface is central to LINQ. We can use LINQ on any object that implements IEnumerable

• We can even create our own classes that implements IEnumerable, and those classes will instantly "inherit" all LINQ functionality!

We can re-write the same using what is called method syntax as shown below

var words = new string[] { "falcon", "eagle", "sky", "tree", "water" };

// Method syntax
var res = words.Where(word => word.Contains("a")); //Lambda expression

foreach (var word in res)
    Console.WriteLine(word);

Console.ReadKey();

We can re-write the same replacing var with IEnumerable as shown below:

var words = new string[] { "falcon", "eagle", "sky", "tree", "water" };

IEnumerable<string> res = words.Where(word => word.Contains("a"));

foreach (var word in res)
    Console.WriteLine(word);

Console.ReadKey();

In all cases, the o/p will be

falcon
eagle
water

Query 2: From a given array of integers, below query filters the even ones in it

int[] nums = { 1, 2, 3, 4, 5, 6, 7, 8, 9};

var res = nums.Where(n => n % 2 == 0);

foreach (var num in res)
    Console.WriteLine(num);

Console.ReadKey();

In the queries above, since we have used LINQ on in-memory objects, we call this mode as LINQ-To-Objects. Similarly, we can use LINQ on any other data, be it in the XML form, SQL Data (LINQ-To-SQL) or Entities (LINQ-To-Entities)

Pros & cons of LINQ

Let’s check some advantages of using LINQ:

• Improves code readability

• Provides compile-time object type-checking

• Provides IntelliSense support for generic collection

• LINQ queries can be reused

• Provides in-built methods to write less code and expedite development

• Provides common query syntax for various data sources

There are also disadvantages of using LINQ:

• Difficult to write complex queries as SQL

• Performance degradation if queries are not written accurately

• Require to recompile, and redeploy every time a change is made to the query

• Doesn’t take full advantage of SQL features such as cached execution plan for stored procedure

Additional Reading

https://docs.microsoft.com/en-us/dotnet/standard/linq/