C# Hashtable vs. HashSet

In C#, both Hashtable and HashSet<T> are collections that use hashing to store and manage data efficiently. However, they serve different purposes and have distinct characteristics. This article explores the differences between Hashtable and HashSet<T>, providing practical use cases and examples to help you choose the best collection for your requirements.

Hashtable

Key Features

• Key-Value Storage: Stores key-value pairs, where each key maps to a unique value.
• Non-Generic: Keys and values are stored as object, meaning any type can be stored.
• Null Handling: Supports both null keys and values.
• Thread Safety: Limited thread safety through SyncRoot.

Example Usage

using System;
using System.Collections;

public class HashtableExample
{
    public static void Main()
    {
        // Create a hashtable and add key-value pairs
        Hashtable employees = new Hashtable
        {
            { "E101", "Alice" },
            { "E102", "Bob" },
            { "E103", "Charlie" }
        };

        // Add a new key-value pair
        employees["E104"] = "Diana";

        // Retrieve a value using a key
        Console.WriteLine($"Employee E101: {employees["E101"]}");

        // Iterate through all key-value pairs
        Console.WriteLine("\nAll Employees:");
        foreach (DictionaryEntry entry in employees)
        {
            Console.WriteLine($"{entry.Key}: {entry.Value}");
        }
    }
}

HashSet<T>

Key Features

• Unique Elements: Stores only unique values, rejecting duplicates.
• Generic: Strongly typed, meaning all values must be of a specified type.
• Performance: Faster lookup, insertion, and deletion due to hash-based indexing.
• No Key-Value: Stores only values without a corresponding key.

Example Usage

using System;
using System.Collections.Generic;

public class HashSetExample
{
    public static void Main()
    {
        // Create a hashset and add elements
        HashSet<string> colors = new HashSet<string> { "Red", "Green", "Blue" };

        // Add a new element, duplicates are ignored
        colors.Add("Yellow");
        bool addedAgain = colors.Add("Red"); // Returns false since it's a duplicate

        // Check if a value exists
        bool containsGreen = colors.Contains("Green");

        // Iterate through all unique values
        Console.WriteLine("\nUnique Colors:");
        foreach (string color in colors)
        {
            Console.WriteLine(color);
        }

        Console.WriteLine($"Was 'Red' added again? {addedAgain}");
        Console.WriteLine($"Does set contain 'Green'? {containsGreen}");
    }
}

Key Differences

Structure:

• Hashtable: Stores key-value pairs.
• HashSet<T>: Stores unique values only.

Generic vs. Non-Generic:

• Hashtable: Non-generic, requires casting.
• HashSet<T>: Generic, provides type safety.

Duplicates:

• Hashtable: Allows duplicate values (but not keys).
• HashSet<T>: Disallows duplicates.

Conclusion

Hashtable and HashSet<T> serve different purposes in C# applications. Use Hashtable for key-value pairs where you need to retrieve data by key, and HashSet<T> when you only need a collection of unique values.