What are the differences between Hashtable and HashSet <T> in C#?

Hashtable and HashSet<T> are both collection classes in C#, but they have different characteristics and usage scenarios. Here are the key differences between them:

1. Data Structure:

   • Hashtable: It uses a hash table data structure to store key-value pairs. Each entry in a Hashtable is a combination of a key and its associated value.
   • HashSet<T>: It uses a hash table data structure internally to store unique elements of type T. It does not store key-value pairs; it only contains elements.

2. Key-Value vs. Unique Elements:

   • Hashtable: It stores data in the form of key-value pairs, where each key is unique within the collection.
   • HashSet<T>: It stores unique elements of type T. Duplicate elements are not allowed in a HashSet<T>.

3. Type Safety:

   • Hashtable: It is not type-safe since it stores objects as keys and values. You need to cast the values retrieved from a Hashtable to their appropriate types.
   • HashSet<T>: It is type-safe. You specify the type of elements (T) that the HashSet<T> can contain, and it enforces type safety at compile time.

4. Performance:

   • Hashtable: Retrieval and insertion operations have an average time complexity of O(1) if a good hash function and reasonable load factor are maintained.
   • HashSet<T>: Similar to Hashtable, retrieval and insertion operations in a HashSet<T> have an average time complexity of O(1).

5. Use Cases:

   • Hashtable: It is suitable for scenarios where you need to associate keys with values and retrieve values quickly based on keys. However, it is considered a legacy collection, and for new development, you may prefer to use Dictionary<TKey, TValue>.
   • HashSet<T>: It is useful when you need to store a collection of unique elements and perform set operations like union, intersection, and difference efficiently.

6. Thread Safety:

   • Neither Hashtable nor HashSet<T> provides inherent thread safety. If thread safety is required, you must synchronize access to the collection manually using techniques such as locking or by using thread-safe collections like ConcurrentDictionary<TKey, TValue> or ConcurrentHashSet<T>.

Feature	Hashtable	HashSet<T>
Data Structure	Uses a hash table to store key-value pairs	Uses a hash table to store unique elements
Key-Value vs. Unique Elements	Stores key-value pairs where keys are unique	Stores unique elements of type T
Type Safety	Not type-safe; stores objects as keys and values	Type-safe; enforces type safety at compile time
Performance	O(1) average time complexity for retrieval and insertion	O(1) average time complexity for retrieval and insertion
Use Cases	Suitable for associating keys with values and fast value retrieval	Useful for storing unique elements and performing set operations efficiently
Thread Safety	Not inherently thread-safe; requires manual synchronization	Not inherently thread-safe; requires manual synchronization if thread safety is required

In summary, while both Hashtable and HashSet<T> provide fast retrieval and insertion operations using hash table data structures, they serve different purposes. Use Hashtable when you need key-value pairs with unique keys, and use HashSet<T> when you need a collection of unique elements of type T. Additionally, prefer generic collections like Dictionary<TKey, TValue> and HashSet<T> over non-generic collections like Hashtable for type safety and clarity.