C# HashSet of Objects

In C#, HashSet<T> is a generic collection designed to manage unique elements efficiently. When dealing with objects, a HashSet can ensure uniqueness based on the object's properties by leveraging the GetHashCode and Equals methods. This article will explore how to create a HashSet of objects, implement proper equality comparisons, and manage the collection effectively.

Why Use a HashSet for Objects?

Uniqueness: Guarantees that no duplicate objects exist based on specified properties.
Performance: Offers constant-time O(1) operations for adding and searching, making it suitable for large collections.

Implementing a HashSet of Objects

Creating a Custom Object

To store objects in a HashSet, a class must override GetHashCode and Equals to ensure proper uniqueness comparisons. Here's an example with a custom Person class:

using System;
using System.Collections.Generic;

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int Age { get; set; }

    // Override GetHashCode to create a unique hash code
    public override int GetHashCode()
    {
        return HashCode.Combine(FirstName, LastName, Age);
    }

    // Override Equals to define when two objects are considered equal
    public override bool Equals(object obj)
    {
        if (obj is Person otherPerson)
        {
            return FirstName == otherPerson.FirstName &&
                   LastName == otherPerson.LastName &&
                   Age == otherPerson.Age;
        }
        return false;
    }
}

Using HashSet with the Custom Object

Now that the Person class correctly implements equality comparison, you can use it within a HashSet.

public class HashSetObjectsExample
{
    public static void Main()
    {
        // Create a HashSet of Person objects
        HashSet<Person> people = new HashSet<Person>
        {
            new Person { FirstName = "John", LastName = "Doe", Age = 30 },
            new Person { FirstName = "Jane", LastName = "Doe", Age = 28 },
            new Person { FirstName = "John", LastName = "Doe", Age = 30 } // Duplicate
        };

        // Add a new person
        people.Add(new Person { FirstName = "Alice", LastName = "Smith", Age = 25 });

        // Attempt to add a duplicate
        bool added = people.Add(new Person { FirstName = "Jane", LastName = "Doe", Age = 28 });
        Console.WriteLine("Was the duplicate person added? " + added); // Outputs: False

        // Display all unique people
        Console.WriteLine("Unique people in the HashSet:");
        foreach (Person person in people)
        {
            Console.WriteLine($"{person.FirstName} {person.LastName}, Age: {person.Age}");
        }
    }
}

Handling Custom Comparisons

Custom Comparer: Implement IEqualityComparer<T> if objects need to be compared differently without modifying their internal code.

Practical Applications

Entity Management: Avoid duplicate entries in collections representing unique entities like users or products.
Filtering Data: Use a HashSet to filter out duplicate or redundant objects efficiently.
Set Operations: Perform union, intersection, or difference operations on sets of objects.

Conclusion

Using a HashSet for objects in C# allows developers to manage unique collections effectively by overriding GetHashCode and Equals methods. Understanding and implementing these methods correctly will help ensure accurate comparisons and optimal performance.

c# hashset of objects