Difference between Dictionary and Hashtable

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Possible Duplicate    Why Dictionary is preferred over hashtable in C         What is the difference between Dictionary and Hashtable  How to decide which one to use

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ILookup Interface is used in  net 3 5 with linq   The HashTable is the base class that is weakly type  the DictionaryBase abstract class is stronly typed and uses internally a HashTable   I found a a strange thing about Dictionary  when we add the multiple  entries in Dictionary  the order in which the entries are added is maintained  Thus if I apply a foreach on the Dictionary  I will get the records in the same order I have inserted them   Whereas  this is not true with normal HashTable  as when I add same records in Hashtable the order is not maintained  As far as my knowledge goes  Dictionary is based on Hashtable  if this is true  why my Dictionary maintains the order but HashTable does not   As to why they behave differently  it s because Generic Dictionary implements a hashtable  but is not based on System Collections Hashtable  The Generic Dictionary implementation is based on allocating key-value-pairs from a list  These are then indexed with the hashtable buckets for random access  but when it returns an enumerator  it just walks the list in sequential order - which will be the order of insertion as long as entries are not re-used   shiv govind Birlasoft

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Want to add a difference   Trying to acess a inexistent key gives runtime error in Dictionary but no problem in hashtable as it returns null instead of error   e g            No strict type declaration         Hashtable hash   new Hashtable            hash Add 1   One            hash Add 2   Two            hash Add 3   Three            hash Add 4   Four            hash Add 5   Five             hash Add 6   Six            hash Add 7   Seven            hash Add 8   Eight            hash Add 9   Nine            hash Add  Ten   10     No error as no strict type          for int i 0 i lt  hash Count i       gt No error for index 0                         Can be accessed through indexers             Console WriteLine hash i                      Console WriteLine hash  Ten        gt  No error in Has Table   here no error for key 0  amp  also for key  ten  note  t is small     Strict type declaration         Dictionary lt int string gt  dictionary  new Dictionary lt int  string gt             dictionary Add 1   One            dictionary Add 2   Two            dictionary Add 3   Three            dictionary Add 4   Four            dictionary Add 5   Five            dictionary Add 6   Six            dictionary Add 7   Seven            dictionary Add 8   Eight            dictionary Add 9   Nine              dictionary Add  Ten   10     error as only key  value pair of type int  string can be added            for i 0  key doesn t  exist error         for  int i   1  i  lt   dictionary Count  i                            Can be accessed through indexers             Console WriteLine dictionary i                        Error   The given key was not present in the dictionary            Console WriteLine dictionary 10      here error for key 0  amp  also for key 10 as both are inexistent in dictionary  runtime error  while try to acess

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There is one more important difference between a HashTable and Dictionary  If you use indexers to get a value out of a HashTable  the HashTable will successfully return null for a non-existent item  whereas the Dictionary will throw an error if you try accessing a item using a indexer which does not exist in the Dictionary

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Simply  Dictionary lt TKey TValue gt  is a generic type  allowing    static typing  and compile-time verification  use without boxing   If you are  NET 2 0 or above  you should prefer Dictionary lt TKey TValue gt   and the other generic collections   A subtle but important difference is that Hashtable supports multiple reader threads with a single writer thread  while Dictionary offers no thread safety  If you need thread safety with a generic dictionary  you must implement your own synchronization or  in  NET 4 0  use ConcurrentDictionary lt TKey  TValue gt

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The Hashtable class is a specific type of dictionary class that uses an integer value  called a hash  to aid in the storage of its keys  The Hashtable class uses the hash to speed up the searching for a specific key in the collection  Every object in  NET derives from the Object class  This class supports the GetHash method  which returns an integer that uniquely identifies the object  The Hashtable class is a very efficient collection in general  The only issue with the Hashtable class is that it requires a bit of overhead  and for small collections  fewer than ten elements  the overhead can impede performance      There is Some special difference between two which must be considered       HashTable  is non-generic collection  the biggest overhead of this   collection is that it does boxing automatically for your values  and   in order to get your original value you need to perform unboxing     these to decrease your application performance as penalty       Dictionary  This is generic type of collection where no implicit   boxing  so no need to unboxing you will always get your original   values which you were stored so it will improve  your application   performance       the Second Considerable difference is       if  your were trying to access a value on from hash table on the basis   of key that does not exist it will return null But in the case of   Dictionary it will give you KeyNotFoundException

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Lets give an example that would explain the difference between hashtable and dictionary   Here is a method that implements hashtable  public void MethodHashTable         Hashtable objHashTable   new Hashtable        objHashTable Add 1  100         int     objHashTable Add 2 99  200      float     objHashTable Add  A   300       char     objHashTable Add  4   400       string      lblDisplay1 Text   objHashTable 1  ToString        lblDisplay2 Text   objHashTable 2 99  ToString        lblDisplay3 Text   objHashTable  A   ToString        lblDisplay4 Text   objHashTable  4   ToString             ----------- Not Possible for HashTable ----------       foreach  KeyValuePair lt string  int gt  pair in objHashTable                    lblDisplay Text   pair Value         lblDisplay Text              The following is for dictionary    public void MethodDictionary           Dictionary lt string  int gt  dictionary   new Dictionary lt string  int gt         dictionary Add  cat   2       dictionary Add  dog   1       dictionary Add  llama   0       dictionary Add  iguana   -1          dictionary Add 1  -2      Compilation Error      foreach  KeyValuePair lt string  int gt  pair in dictionary                lblDisplay Text   pair Value         lblDisplay Text

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Dictionary is typed  so valuetypes don t need boxing   a Hashtable isn t  so valuetypes need boxing   Hashtable has a nicer way of obtaining a value than dictionary IMHO  because it always knows the value is an object  Though if you re using  NET 3 5  it s easy to write an extension method for dictionary to get similar behavior    If you need multiple values per key  check out my sourcecode of MultiValueDictionary here  multimap in  NET

[c#] Difference between Dictionary and Hashtable

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