What is LINQ?

• LINQ: Language Integrated Query
  • Language Integrated: means that LINQ is part of programming language syntax
  • Query: a query is an expression that retrieves data from a data source
  • Data source: NET collections, streams, SQL databases, in-memory representations, XML etc
• LINQ is a Microsoft programming model and methodology that essentially adds formal query capabilities into .NET-based programming languages
• LINQ offers a compact, expressive, and intelligible syntax for manipulating data
• LINQ is uniform query syntax to retrieve data providing a single querying interface for different types of data sources
• LINQ is a way of providing a general purpose “querying” mechanism in the CLR
• Result from a LINQ operation:
  • IEnumerable<T>
  • IQueryable<T>

There 2 types of LINQ syntax:

• Query syntax
• Method syntax

Consist of three distinct actions:

• Obtain the data source
• Create the query
• Execute the query

Query syntax

// 1. Data source
int[] numbers = new int[7] { 0, 1, 2, 3, 4, 5, 6 };

// 2. Query creation. numQuery is an IEnumerable<int>
var numQuery = from num in numbers
			   where (num % 2) == 0
			   select num;
			   
// 3. Query execution
foreach (int num in numQuery)
{

}

Method syntax

• Method syntax uses lambda expression
• Lambda expression: syntactic sugar for passing lambda as argument to higher order function

// 1. Data source
int[] numbers = { 5, 10, 8, 3, 6, 12};

// 2. Query creation. numQuery is an IEnumerable<int>
IEnumerable<int> numQuery = numbers.Where(num => num % 2 == 0).OrderBy(n => n);

// 3. Query execution
foreach (int i in numQuery)
{
    Console.Write(i + " ");
}