Is LINQ evaluated immediately?

Many LINQ methods such as Where and Select use deferred execution: the query runs when you iterate the result or call ToList or ToArray, not when you write it.

When should I use an array instead of a List?

Use an array for a fixed number of items known up front. Use List when you need to add or remove items.

C# Collections and LINQ Basics

Introduction

Real programs work with groups of data. C# provides generic collections such as List and Dictionary, and LINQ (Language Integrated Query) adds readable methods for filtering, transforming, and sorting them. This tutorial covers both.

Prerequisites

The .NET SDK installed
Understanding of lambdas such as n => n * 2 is helpful

Step-by-Step Guide

Step 1: Store items in a List

List<T> is a resizable, ordered collection. Using a collection initializer, you can fill it when you create it.

var numbers = new List<int> { 5, 3, 8, 1, 9, 2 };
numbers.Add(7);

Console.WriteLine(numbers.Count);   // 7
Console.WriteLine(numbers[0]);      // 5

Step 2: Look up values with a Dictionary

Dictionary<TKey, TValue> maps keys to values. Use TryGetValue to read a key that might not exist.

var ages = new Dictionary<string, int>
{
    ["Ava"] = 30,
    ["Ben"] = 25
};

if (ages.TryGetValue("Cy", out var age))
{
    Console.WriteLine(age);
}
else
{
    Console.WriteLine("Not found");
}

Step 3: Filter, transform, and sort with LINQ

LINQ methods work on any collection and can be chained. Add using System.Linq if your project doesn't include it implicitly.

var evens = numbers.Where(n => n % 2 == 0).ToList();
var squares = numbers.Select(n => n * n).ToList();
var sorted = numbers.OrderBy(n => n).ToList();

Console.WriteLine(string.Join(", ", sorted));   // 1, 2, 3, 5, 7, 8, 9

Step 4: Calculate aggregates

LINQ also provides Sum, Average, Min, Max, and Count.

Console.WriteLine(numbers.Sum());       // 35
Console.WriteLine(numbers.Max());       // 9
Console.WriteLine(numbers.Average());   // 5

FAQs

Is LINQ evaluated immediately?: Many LINQ methods such as Where and Select use deferred execution: the query runs when you iterate the result or call ToList or ToArray, not when you write it.
When should I use an array instead of a List?: Use an array for a fixed number of items known up front. Use List when you need to add or remove items.

Summary

List and Dictionary cover most storage needs, and LINQ turns loops into short, readable queries. Remember that many LINQ queries run lazily until you enumerate them.