Differences Between Hashtable and Dictionary in C#

Both Hashtable and Dictionary are used to store data in key-value pairs in C#, but they have several differences in terms of performance, usage, and compatibility. Here’s a breakdown of the key differences:

1. Type Safety

• Hashtable: It is not type-safe. Keys and values are stored as objects, meaning that you need to cast them back to their original types when retrieving data.
• Dictionary: It is type-safe. The Dictionary<TKey, TValue> class allows you to specify the types of keys and values, making it strongly typed and eliminating the need for casting.

2. Namespace

• Hashtable: Belongs to the System.Collections namespace.
• Dictionary: Belongs to the System.Collections.Generic namespace.

3. Generics

• Hashtable: Does not support generics. You must cast objects to their correct types when retrieving values.
• Dictionary: Supports generics, allowing you to define the specific types for both keys and values (e.g., Dictionary<int, string>).

4. Performance

• Hashtable: Slightly slower due to boxing and unboxing of objects.
• Dictionary: Faster because it avoids boxing/unboxing and is more optimized for modern usage.

5. Null Keys

• Hashtable: Allows a null key.
• Dictionary: Does not allow null as a key. It throws an ArgumentNullException if you try to insert a null key.

6. Thread-Safety

• Hashtable: Thread-safe for single writers and multiple readers. However, in a multi-threaded environment with writes, synchronization needs to be handled manually.
• Dictionary: Not thread-safe. For thread-safe operations, you need to use ConcurrentDictionary or handle synchronization yourself.

7. Use Case

• Hashtable: It’s part of older collections in .NET and is rarely used in modern development due to the lack of type safety and performance.
• Dictionary: It is preferred for modern development, especially when type safety and performance are important.

8. Sorting

• Hashtable: Does not maintain any order for keys or values.
• Dictionary: Also does not maintain any order, but you can use a SortedDictionary if you need sorted keys.

Summary Table: Hashtable vs Dictionary

Interview Questions:

1. Why would you choose a Dictionary over a Hashtable in C#?
   • A Dictionary is preferred over a Hashtable because it is strongly typed, offers better performance, and is designed to work well with generics in modern C# applications.
2. What are the performance implications of using Hashtable compared to Dictionary?
   • Hashtable requires boxing and unboxing for value types, which makes it slower compared to Dictionary, which benefits from being strongly typed and not requiring these operations.
3. Can you store a null key in a Dictionary?
   • No, Dictionary does not allow null keys and will throw an ArgumentNullException. However, a Hashtable allows a null key.
4. When would you use a Hashtable instead of a Dictionary?
   • You might use a Hashtable in legacy code or in situations where you are working with older frameworks. However, for most modern applications, Dictionary is a better option.
5. What should you use for a thread-safe dictionary in multi-threaded applications?
   • In a multi-threaded environment, you should use ConcurrentDictionary, which is thread-safe for both read and write operations.

Conclusion

In most modern applications, Dictionary<TKey, TValue> is preferred over Hashtable due to its performance, type safety, and generic support. Hashtable is largely considered outdated but may still be found in legacy systems. For thread-safe operations, consider using ConcurrentDictionary.

Reference:

http://www.c-sharpcorner.com/blogs/differences-between-hashtable-and-dictionary1