.NET HashTable Vs Dictionary - Can the Dictionary be as fast?

Question

I am trying to figure out when and why to use a Dictionary or a HashTable. I have done a bit of a search on here and have found people talking about the generic advantages of the Dictionary which I totally agree with, which leads the boxing and unboxing advantage for a slight performance gain.

But I have also read the Dictionary will not always return the objects in the order they are inserted, thing it is sorted. Where as a HashTable will. As I understand it this leads to the HashTable being far faster for some situations.

My question is really, what might those situations be? Am I just wrong in my assumptions above? What situations might you use to choose one above the other, (yes the last one is a bit ambiguous).

Answer 1

System.Collections.Generic.Dictionary<TKey, TValue> and System.Collections.Hashtable classes both maintain a hash table data structure internally. None of them guarantee preserving the order of items.

Leaving boxing/unboxing issues aside, most of the time, they should have very similar performance.

The primary structural difference between them is that Dictionary relies on chaining (maintaining a list of items for each hash table bucket) to resolve collisions whereas Hashtable uses rehashing for collision resolution (when a collision occurs, tries another hash function to map the key to a bucket).

There is little benefit to use Hashtable class if you are targeting for .NET Framework 2.0+. It's effectively rendered obsolete by Dictionary<TKey, TValue>.

Answer 2

Both are effectively the same class (you can look at the disassembly). HashTable was created first before .Net had generics. Dictionary, however is a generic class and gives you strong typing benefits. I would never use HashTable since Dictionary costs you nothing to use.

Answer 3

Another important difference is that the Hashtable type supports lock-free multiple readers and a single writer at the same time, while Dictionary does not.

Answer 4

I guess it doesn't mean anything to you now. But just for reference for people stopping by

Performance Test - SortedList vs. SortedDictionary vs. Dictionary vs. Hashtable