Generic Collections (List, Dictionary, HashSet, Stack/Queue) Masterclass

⚡ C# 12 & .NET 8 🟢 Chapter 13 of 35 📂 Phase 5: Strings, Arrays & Collections 📅 2026 Edition

📌 Covered in this chapter: Generic Collections · List · Dictionary · HashSet · Queue (FIFO) · Stack (LIFO) · O(1) Hash Lookups

Welcome to Phase 5 (Chapter 13): C# Generic Collections (List, Dictionary, HashSet, Stack & Queue) Masterclass! Unlike fixed-size arrays, collections dynamically grow and shrink as data is added or removed. According to official Microsoft documentation, generic collections in System.Collections.Generic improve type safety and performance by eliminating boxing/unboxing overhead.

1List<T> & Dictionary<TKey, TValue>

List<T> is a dynamic array that grows automatically when capacity is reached. Dictionary<TKey, TValue> is a high-speed Hash Table that provides O(1) key lookups.

C# — List<T> and Dictionary ▶ Run in Compiler

using System.Collections.Generic;

// 1. List — Dynamic Resizable Array
List<string> courses = new() { "C#", "ASP.NET Core", "SQL" };
courses.Add("Azure Cloud");
courses.Remove("SQL");

Console.WriteLine($"Course count: {courses.Count}");
foreach (string c in courses) Console.WriteLine($"Course: {c}");

// 2. Dictionary — Fast O(1) Key-Value Lookup
Dictionary<int, string> employees = new()
{
    { 101, "Alice" },
    { 102, "Bob" },
    { 103, "Charlie" }
};

if (employees.TryGetValue(102, out string? empName))
{
    Console.WriteLine($"ID 102 Name: {empName}");
}

2HashSet<T>, Queue<T> & Stack<T>

Other generic collections suit specific algorithm patterns:

C# — HashSet, Queue, and Stack ▶ Run in Compiler

// 1. HashSet — Unordered Collection of Unique Elements
HashSet<int> uniqueIds = new() { 1, 2, 2, 3, 3, 3 };
Console.WriteLine($"Unique ID count: {uniqueIds.Count}"); // 3

// 2. Queue — FIFO (First-In, First-Out)
Queue<string> ticketQueue = new();
ticketQueue.Enqueue("User 1");
ticketQueue.Enqueue("User 2");
Console.WriteLine($"Processing: {ticketQueue.Dequeue()}"); // User 1

// 3. Stack — LIFO (Last-In, First-Out)
Stack<string> undoStack = new();
undoStack.Push("Action 1");
undoStack.Push("Action 2");
Console.WriteLine($"Undo Action: {undoStack.Pop()}"); // Action 2

3Technical FAQs

Q1: Why are Generic Collections preferred over ArrayList?

Generic collections like List<int> store items without casting them to object, eliminating Boxing/Unboxing and enforcing compile-time type safety.

Q2: How do I choose the right collection?

Use List<T> for ordered sequential access; Dictionary<K,V> for fast key lookup; HashSet<T> for uniqueness; Queue<T> for FIFO processing; and Stack<T> for LIFO undo/redo operations.