How to Use HashSet in C#: Syntax, Example, and Tips

In C#, use HashSet<T> to store unique elements without duplicates. You create a HashSet, add items with Add(), and check membership with Contains().

Syntax

The HashSet<T> class stores unique elements of type T. You create it with new HashSet<T>(). Use Add(item) to add elements, Remove(item) to delete, and Contains(item) to check if an element exists.

csharp

HashSet<string> set = new HashSet<string>();
set.Add("apple");
bool hasApple = set.Contains("apple");
set.Remove("apple");

Example

This example shows how to create a HashSet, add items, check for duplicates, and print all unique items.

csharp

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        HashSet<string> fruits = new HashSet<string>();

        fruits.Add("apple");
        fruits.Add("banana");
        fruits.Add("apple"); // duplicate, will not be added

        Console.WriteLine("Fruits in the set:");
        foreach (var fruit in fruits)
        {
            Console.WriteLine(fruit);
        }

        Console.WriteLine($"Contains 'banana'? {fruits.Contains("banana")}");
    }
}

Output

Fruits in the set:
apple
banana
Contains 'banana'? True

Common Pitfalls

Common mistakes include trying to add duplicates (which HashSet ignores silently) and expecting ordered items (HashSet does not keep order). Also, modifying a set while iterating causes errors.

csharp

/* Wrong: Adding duplicates has no effect */
HashSet<int> numbers = new HashSet<int>();
numbers.Add(1);
numbers.Add(1); // ignored, no error but no duplicate added

/* Wrong: Modifying during foreach causes error */
// foreach (var n in numbers) {
//     numbers.Add(2); // throws InvalidOperationException
// }

/* Right: Collect items to add, then add after loop */
var toAdd = new List<int> { 2 };
foreach (var n in numbers) {
    // do something
}
foreach (var n in toAdd) {
    numbers.Add(n);
}

Quick Reference

• Create: new HashSet<T>()
• Add: Add(item)
• Remove: Remove(item)
• Check: Contains(item)
• Count: Count property
• Clear all: Clear()

Key Takeaways

HashSet stores unique elements and ignores duplicates automatically.

Use Add, Remove, and Contains methods to manage items in the set.

HashSet does not maintain order of elements.

Avoid modifying a HashSet while iterating over it to prevent errors.

HashSet is efficient for fast lookups and uniqueness checks.