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Language Integrated Query (LINQ)

A feature in C# that lets you query collections (arrays, lists, databases, XML, etc.) using a simple and readable syntax.

Basic LINQ query: 

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int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// Find even numbers using LINQ
var evens = from num in numbers 
            where num % 2 == 0 
            select num;

foreach (var num in evens)
{
    Console.WriteLine(num);
}

Same as: 

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int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

var evens = numbers.Where(num => num % 2 == 0);

foreach (var num in evens)
{
    Console.WriteLine(num);
}

Example with objects: 

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class Person 
{
    public string Name { get; set; }
    public int Age { get; set; }
}

var people = new List<Person>
{
    new Person { Name = "Alice", Age = 25 },
    new Person { Name = "Bob", Age = 30 },
    new Person { Name = "Charlie", Age = 20 }
};

// Find people older than 25, ordered by age
var result = people.Where(p => p.Age > 25)
                   .OrderBy(p => p.Age)
                   .Select(p => p.Name);

foreach (var name in result)
{
    Console.WriteLine(name);
}

LINQ Methods (Method Syntax)

LINQ has built-in extension methods to make queries shorter:

Method Description Example
Where Filters data list.Where(x => x > 5)
Select Transforms each element list.Select(x => x * 2)
OrderBy Sorts in ascending order list.OrderBy(x => x)
OrderByDescending Sorts in descending order list.OrderByDescending(x => x)
First /FirstOrDefault Gets the first element list.FirstOrDefault()
Last /LastOrDefault Gets the last element list.LastOrDefault()
Count Counts elements list.Count()
Sum Adds up all values list.Sum()