In C#, a HashSet is a powerful and efficient collection type designed for storing unique elements. It provides fast lookup, insertion, and removal operations, making it ideal for scenarios where you need to maintain a set of distinct items.

Key Features of HashSet

Stores only unique elements
Offers O(1) average time complexity for add, remove, and contains operations
Implements the ISet<T> interface
Allows for set operations like union, intersection, and difference

Creating and Using a HashSet

To use a HashSet in C#, you first need to include the System.Collections.Generic namespace. Here's a basic example of creating and using a HashSet:


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("Cherry");
        fruits.Add("Apple"); // Duplicate, won't be added

        Console.WriteLine($"Count: {fruits.Count}"); // Output: Count: 3

        foreach (string fruit in fruits)
        {
            Console.WriteLine(fruit);
        }
    }
}

In this example, we create a HashSet of strings and add some fruit names. Notice that the duplicate "Apple" is not added, maintaining the uniqueness of elements in the set.

Common HashSet Operations

HashSet provides several useful methods for set operations:

1. Checking for Existence


bool containsBanana = fruits.Contains("Banana"); // Returns true

2. Removing Elements


bool removed = fruits.Remove("Cherry"); // Returns true if Cherry was in the set

3. Set Operations


HashSet<string> set1 = new HashSet<string> { "A", "B", "C" };
HashSet<string> set2 = new HashSet<string> { "B", "C", "D" };

set1.UnionWith(set2); // set1 now contains { "A", "B", "C", "D" }
set1.IntersectWith(set2); // set1 now contains { "B", "C" }
set1.ExceptWith(set2); // set1 now contains { "A" }

Performance Considerations

HashSet is optimized for fast lookups and uniqueness checks. It's particularly useful when you need to:

Quickly determine if an element exists in a collection
Ensure no duplicates are stored
Perform set operations efficiently

However, keep in mind that HashSet does not maintain insertion order. If order is important, consider using a C# List in combination with additional logic to ensure uniqueness.

HashSet vs. Other Collections

Feature	HashSet	List	Dictionary
Uniqueness	Enforced	Not enforced	Keys unique
Lookup speed	O(1) average	O(n)	O(1) average
Ordered	No	Yes	No

Conclusion

C# HashSet is a versatile and efficient collection for managing unique elements. Its fast operations and set-based functionality make it an excellent choice for many scenarios in C# programming. By understanding its strengths and use cases, you can leverage HashSet to write more efficient and cleaner code.

For more advanced collection types in C#, you might want to explore C# Dictionaries or C# Queue and Stack.