What is HashSet in C#?

A HashSet is a collection that stores only unique elements using a hash table, this is used to enable faster retrieval of elements compared to other collection types.

C# Program to show the use of HashSet in C#:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Create a new HashSet of strings
        HashSet<string> cars = new HashSet<string>();

        // Add elements to the HashSet
        cars.Add("Audi");
        cars.Add("Ford");
        cars.Add("Mazda");
        cars.Add("Tesla");
        cars.Add("Mazda");// Duplicate element, will not be added

        // Display the elements in the HashSet
        Console.WriteLine("Cars in the HashSet:");
        foreach (var car in cars)
        {
            Console.WriteLine(car);
        }

        // Check if an element exists in the HashSet
        string carToCheck = "Ford";
        if (cars.Contains(carToCheck))
        {
            Console.WriteLine($"\n{carToCheck} is in the HashSet.");
        }
        else
        {
            Console.WriteLine($"\n{carToCheck} is not in the HashSet.");
        }

        // Remove an element from the HashSet
        cars.Remove("Tesla");
        Console.WriteLine("\nAfter removing 'Tesla':");
        foreach (var car in cars)
        {
            Console.WriteLine(car);
        }

        // Get the number of elements in the HashSet
        Console.WriteLine($"\nNumber of elements in the HashSet: {cars.Count}");

        // Clear all elements from the HashSet
        cars.Clear();
        Console.WriteLine($"\nNumber of elements in the HashSet after clearing: {cars.Count}");
        Console.ReadKey();
    }
}

We have the below operations in the above program,

• Creating a HashSet: The HashSet<string> named cars is created to store a collection of unique strings.
• Adding Elements: Elements are added to the HashSet using the Add method. If an element already exists, it will not be added again.
• Displaying Elements: The elements in the HashSet are displayed using a foreach loop.
• Checking for Existence: The Contains method checks if a specific element exists in the HashSet.
• Removing Elements: The Remove method removes an element from the HashSet.
• Getting the Count: The Count property returns the number of elements in the HashSet.
• Clearing the HashSet: The Clear method removes all elements from the HashSet.

Output:

Cars in the HashSet:
Audi
Ford
Mazda
Tesla

Ford is in the HashSet.

After removing 'Tesla':
Audi
Ford
Mazda

Number of elements in the HashSet: 3

Number of elements in the HashSet after clearing: 0