Why is Dictionary preferred over Hashtable?

 

In most programming languages, dictionaries are preferred over hashtables. What are the reasons behind that?

For what it's worth, a Dictionary is (conceptually) a hash table.

If you meant "why do we use the Dictionary<TKey, TValue> class instead of the Hashtable class?", then it's an easy answer: Dictionary<TKey, TValue> is a generic type, Hashtable is not. That means you get type safety with Dictionary<TKey, TValue> , because you can't insert any random object into it, and you don't have to cast the values you take out.

Interestingly, the Dictionary<TKey, TValue> implementation in the .NET Framework is based on the Hashtable , as you can tell from this comment in its source code:

The generic Dictionary was copied from Hashtable's source

Source

Dictionary <<<>>> Hashtable differences:

  • Generic <<<>>> Non-Generic
  • Needs own thread synchronization <<<>>> Offers thread safe version through Synchronized() method
  • Enumerated item: KeyValuePair <<<>>> Enumerated item: DictionaryEntry
  • Newer (> .NET 2.0 ) <<<>>> Older (since .NET 1.0 )
  • is in System.Collections.Generic <<<>>> is in System.Collections
  • Request to non-existing key throws exception <<<>>> Request to non-existing key returns null
  • potentially a bit faster for value types <<<>>> bit slower (needs boxing/unboxing) for value types

    • Dictionary / Hashtable similarities:

  • Both are internally hashtables == fast access to many-item data according to key
  • Both need immutable and unique keys
  • Keys of both need own GetHashCode() method

    • Similar .NET collections (candidates to use instead of Dictionary and Hashtable):

  • ConcurrentDictionary - thread safe (can be safely accessed from several threads concurrently)
  • HybridDictionary - optimized performance (for few items and also for many items)
  • OrderedDictionary - values can be accessed via int index (by order in which items were added)
  • SortedDictionary - items automatically sorted
  • StringDictionary - strongly typed and optimized for strings

    • Because Dictionary is a generic class ( Dictionary<TKey, TValue> ), so that accessing its content is type-safe (ie you do not need to cast from Object , as you do with a Hashtable ).

    Compare 

    var customers = new Dictionary<string, Customer>();
...
Customer customer = customers["Ali G"];

    to 

    var customers = new Hashtable();
...
Customer customer = customers["Ali G"] as Customer;

    However, Dictionary is implemented as Hashtable inside, so technically it works the same way.

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