Day 7: Collections & LINQ Basics

Welcome to Day 7. Arrays in C# are fixed size, which makes them fast, but annoying when you don’t know exactly how many items you’ll have.

Enter the .NET Collections.

The Mighty List

List<T> is your go-to collection. It can grow and shrink dynamically. The <T> stands for “Type”, meaning it is a Generic collection. You specify the type when you create it.

// Using the System.Collections.Generic namespace
List<string> names = new List<string>();

names.Add("Alice");
names.Add("Bob");

Console.WriteLine(names[0]);     // Alice
names.Remove("Bob");             // Removes bob
Console.WriteLine(names.Count);  // 1

You can initialize them quickly, just like arrays:

var numbers = new List<int> { 10, 20, 30, 40 };

Dictionary<TKey, TValue>

Dictionaries (similar to Maps in JS / Hashes in Ruby) store key-value pairs. They offer incredibly fast lookups by key.

// Key: String (Username), Value: Int (Score)
var playerScores = new Dictionary<string, int>();

playerScores.Add("Alice_123", 9500);
// Or the shorthand syntax
playerScores["Bob_the_Builder"] = 4200;

Console.WriteLine(playerScores["Alice_123"]); // 9500

Enter LINQ (Language Integrated Query)

Here’s where C# flexes its muscles. LINQ is a massive feature that lets you query collections using a declarative syntax, similar to SQL.

It completely eliminates massive nested loops and if statements when filtering, sorting, or mapping data.

Make sure you have using System.Linq; at the top of your file!

var scores = new List<int> { 45, 92, 12, 88, 75, 99 };

// 1. Filtering (Where)
var passingScores = scores.Where(score => score >= 50);

// 2. Sorting (OrderBy)
var highToLow = scores.OrderByDescending(score => score);

// 3. Mapping (Select) - Like Array.map() in JS!
var curvedScores = scores.Select(score => score + 5);

// 4. Aggregations (Max, Min, Sum, Average)
int maxScore = scores.Max();
double avg = scores.Average();

Chaining LINQ

You can chain these commands together to create insanely powerful, readable queries!

// Find all passing scores, add a 5 point curve, and order them highest to lowest!
var finalGrades = scores
    .Where(s => s >= 50)
    .Select(s => s + 5)
    .OrderByDescending(s => s)
    .ToList(); // ToList executes the query immediately

Challenge for Day 7

Create a Product class with a Name and Price. Create a List<Product> with 5 items. Use LINQ to:

Find all products under $50.
Select only the Names of the products over $100.

Tomorrow: Delegates, Events & Lambdas.