Collections in C# – List, Dictionary, HashSet and Queue

1Working with C# Collections

Hi friends, in this article we will learn collections in C#. Collections store and manage groups of objects. Unlike arrays, many collections can grow or shrink dynamically. C# provides different collection types for different requirements. We will explore List, Dictionary, HashSet and Queue. These are commonly used in APIs, shopping applications, banking systems and enterprise software.

2Think About a Shopping Application

Imagine an online shopping application. You maintain a list of products, look up product prices using product IDs, avoid duplicate coupon codes and manage customer orders in the order they arrive.

These needs match four collections: List, Dictionary, HashSet and Queue. Choosing the right collection makes your application easier to maintain.

3Four Collections You Should Know

List

A List stores items in order and allows duplicates. You can access items using an index and add or remove items dynamically.

Dictionary

A Dictionary stores key-value pairs. Each key must be unique. It is useful for looking up a value using a key.

HashSet

A HashSet stores unique items. Adding an existing item does not create a duplicate.

Queue

A Queue follows FIFO, meaning First In, First Out. The first item added is the first item removed.

4Build a Collection Demo

Let's create all four collections and perform common operations.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        var products = new List<string>
            { "Laptop", "Mouse" };
        products.Add("Keyboard");
        Console.WriteLine(products.Count);

        var prices = new Dictionary<int, string>
        {
            { 101, "Laptop" },
            { 102, "Mouse" }
        };
        Console.WriteLine(prices[101]);

        var codes = new HashSet<string>();
        codes.Add("SAVE10");
        codes.Add("SAVE10");
        Console.WriteLine(codes.Count);

        var orders = new Queue<string>();
        orders.Enqueue("Order A");
        orders.Enqueue("Order B");
        Console.WriteLine(orders.Dequeue());
    }
}

We use Add to insert List, Dictionary and HashSet items. Queue uses Enqueue to add and Dequeue to remove the next item.

5Check the Collection Results

3
Laptop
1
Order A

The List contains three products. Dictionary finds Laptop using key 101. HashSet keeps only one coupon. Queue removes Order A first.

6Watch Each Collection Work

Follow the operations as items are added, looked up, deduplicated and removed from the queue.

7Choose Collections with Confidence

Each collection solves a different problem. List maintains ordered items, Dictionary provides key-based lookup, HashSet prevents duplicates and Queue processes items in arrival order.

• Use List for dynamic ordered data.
• Use Dictionary for unique keys and fast lookups.
• Use HashSet for unique values and Queue for FIFO processing.

8Questions Interviewers May Ask