C# LINQ: Query Collections Like a Pro

What is LINQ?

LINQ (Language Integrated Query) lets you query any IEnumerable collection — arrays, lists, databases, XML — using a readable, declarative syntax. It was introduced in C# 3.0.

Basic LINQ with Method Syntax

using System;
using System.Collections.Generic;
using System.Linq;

var numbers = new List<int> { 5, 2, 8, 1, 9, 3, 7, 4, 6 };

// Filter (Where)
var evens = numbers.Where(n => n % 2 == 0).ToList();
// [2, 8, 4, 6]

// Sort (OrderBy)
var sorted = numbers.OrderBy(n => n).ToList();
// [1, 2, 3, 4, 5, 6, 7, 8, 9]

// Transform (Select)
var squares = numbers.Select(n => n * n).ToList();
// [25, 4, 64, 1, 81, 9, 49, 16, 36]

// Aggregate
int sum = numbers.Sum();         // 45
int max = numbers.Max();         // 9
double avg = numbers.Average();  // 5.0

LINQ Query Syntax

var result = from n in numbers
             where n > 4
             orderby n descending
             select n;
// [9, 8, 7, 6, 5]

LINQ on Objects

var people = new List<Person> {
    new("Alice", 30),
    new("Bob", 25),
    new("Charlie", 35),
    new("Dave", 25)
};

// Filter and project
var names = people
    .Where(p => p.Age >= 30)
    .Select(p => p.Name)
    .ToList();
// ["Alice", "Charlie"]

// GroupBy
var byAge = people
    .GroupBy(p => p.Age)
    .Select(g => new { Age = g.Key, Count = g.Count() });
// Age 30: 1, Age 25: 2, Age 35: 1

First, Single, Any, All

var nums = new[] { 1, 2, 3, 4, 5 };

bool hasEven   = nums.Any(n => n % 2 == 0);   // true
bool allPos    = nums.All(n => n > 0);         // true
int first      = nums.First(n => n > 3);       // 4
int? firstOrNull = nums.FirstOrDefault(n => n > 10); // null

Chaining LINQ

var topScorers = students
    .Where(s => s.Score >= 80)
    .OrderByDescending(s => s.Score)
    .Take(3)
    .Select(s => s.Name)
    .ToList();