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

User · Accepted Answer

System Collections Generic Dictionary lt TKey  TValue gt  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 lt TKey  TValue gt

User · Answer

Dictionaries have the advantage of being a generic type  which makes it type safe and a bit faster due to lack of need for boxing  The following comparison table  constructed using the answers found in a similar SO question post  illustrates some of the other reasons that support dictionaries over hash tables  or vice versa

User · Answer

Another important difference is that Hashtable is thread safe  Hashtable has built in multiple reader single writer  MR SW  thread safety which means Hashtable allows ONE writer together with multiple readers without locking  In the case of Dictionary there is no thread safety  if you need thread safety you must implement your own synchronization  To elaborate further   Hashtable  provide some thread-safety through the Synchronized property  which returns a thread-safe wrapper around the collection  The wrapper works by locking the entire collection on every add or remove operation  Therefore  each thread that is attempting to access the collection must wait for its turn to take the one lock  This is not scalable and can cause significant performance degradation for large collections  Also  the design is not completely protected from race conditions  The  NET Framework 2 0 collection classes like  List lt T gt   Dictionary lt TKey  TValue gt   etc do not provide any thread synchronization  user code must provide all synchronization when items are added or removed on multiple threads concurrently If you need type safety as well thread safety  use concurrent collections classes in the  NET Framework  Further reading here

User · Answer

MSDN Article   The Dictionary lt TKey  TValue gt  class has the same   functionality as the Hashtable class  A Dictionary lt TKey  TValue gt    of a specific type  other than Object  has better performance than a   Hashtable for value types because the elements of Hashtable are of   type Object and  therefore  boxing and unboxing typically occur if   storing or retrieving a value type     Link  http   msdn microsoft com en-us library 4yh14awz v vs 90  aspx

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If you care about reading that will always return the objects in the order they are inserted in a Dictionary  you may have a look at   OrderedDictionary - values can be accessed via an integer index  by order in which items were added  SortedDictionary - items are automatically sorted

User · Answer

Dictionary is faster than hashtable as dictionary is a generic strong type  Hashtable is slower as it takes object as data type which leads to boxing and unboxing

User · Answer

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

User · Answer

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

User · Answer

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  Memory allocation     Time used for inserting     Time for searching an item

User · Answer

Differences between Hashtable and Dictionary Dictionary   Dictionary returns error if we try to find a key which does not exist  Dictionary faster than a Hashtable because there is no boxing and unboxing  Dictionary is a generic type which means we can use it with any data type   Hashtable   Hashtable returns null if we try to find a key which does not exist  Hashtable slower than dictionary because it requires boxing and unboxing  Hashtable is not a generic type

[c#] .NET HashTable Vs Dictionary - Can the Dictionary be as fast?

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