Menu

Tech

English

What is a HashSet?

Part of the Logic & Flow section of Coddy's C# journey — lesson 56 of 66.

A HashSet is a collection in C# that stores unique elements in no particular order. It's part of the System.Collections.Generic namespace and provides high-performance set operations.

First, include the required namespace:

using System.Collections.Generic;

Create an empty HashSet:

HashSet<string> fruits = new HashSet<string>();

This creates a HashSet that can store string values.

Unlike arrays or lists, HashSets:

Don't allow duplicate elements
Don't maintain insertion order

Provide fast lookup operations (O(1) complexity)
Are ideal for when you need to ensure uniqueness

If you try to add a duplicate element, the HashSet will ignore it:

fruits.Add("Apple");
fruits.Add("Apple"); // This is ignored

After executing the above code, the set still contains only one "Apple".

Cheat sheet

A HashSet is a collection that stores unique elements with no particular order and provides fast lookup operations.

Include the required namespace:

using System.Collections.Generic;

Create a HashSet:

HashSet<string> fruits = new HashSet<string>();

Key characteristics:

No duplicate elements allowed
No insertion order maintained
Fast lookup operations (O(1) complexity)
Duplicate additions are ignored

Adding elements:

fruits.Add("Apple");
fruits.Add("Apple"); // Ignored - no duplicates

Try it yourself

This lesson doesn't include a code challenge.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Logic & Flow

1Multi-dimensional Arrays

2D Arrays Basics Declaring and Initializing 2D Accessing 2D Array Elements Nested Loops with 2D Arrays Jagged Arrays Common Matrix Operations Recap - Multi-dimensional

4Flow Control Techniques

Early Returns Guard Clauses Jump Statements (goto)Break and Continue Flatten Nested Conditionals

7Logical Operators Advanced

Short-Circuit Evaluation Conditional Logical Operators Operator Precedence Recap - Advanced Operators

10HashMap Part 2

HashMap Methods Nested HashMap Recap - Manage Warehouse Recap - HashMap Operations

2Advanced Decision Making

Multiple Conditions Complex Boolean Logic If vs. Switch Comparison Nested Switch Statements Recap - Advanced Decisions

5Exception Handling

Try-Catch Basics Exception Types Multiple Catch Blocks Working with Files Finally Block Using vs. Try-Finally Custom Exceptions Recap - Error Handling

8Data Analysis System

Data Collection Setup Data Entry Logic

Project lesson

Project lesson

Project lesson

Report Generation

Project lesson

Project lesson

11HashSet Part 1

What is a HashSet?Adding an Element Removing an Element Checking if an Element Exists Empty and Size Recap - HashSet