Why is Dictionary preferred over Hashtable in C

Question

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

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The Hashtable is a loosely-typed data structure  so you can add keys and values of any type to the Hashtable  The Dictionary class is a type-safe Hashtable implementation  and the keys and values are strongly typed  When creating a Dictionary instance  you must specify the data types for both the key and value

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Notice that the documentation says   quot the Dictionary lt  Of  lt  TKey  TValue gt   gt   class is implemented as a hash table quot   not  quot the Dictionary lt  Of  lt  TKey  TValue gt   gt   class is implemented as a HashTable quot  Dictionary is NOT implemented as a HashTable  but it is implemented following the concept of a hash table  The implementation is unrelated to the HashTable class because of the use of Generics  although internally Microsoft could have used the same code and replaced the symbols of type Object with TKey and TValue  In  NET 1 0 Generics did not exist  this is where the HashTable and ArrayList originally began

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For what it s worth  a Dictionary is  conceptually  a hash table   If you meant  why do we use the Dictionary lt TKey  TValue gt  class instead of the Hashtable class    then it s an easy answer  Dictionary lt TKey  TValue gt  is a generic type  Hashtable is not  That means you get type safety with Dictionary lt TKey  TValue gt   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 lt TKey  TValue gt  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

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Collections  amp  Generics are useful for handling group of objects  In  NET  all the collections objects comes under the interface IEnumerable  which in turn has ArrayList Index-Value    amp  HashTable Key-Value   After  NET framework 2 0  ArrayList  amp  HashTable were replaced with List  amp  Dictionary  Now  the Arraylist  amp  HashTable are no more used in nowadays projects   Coming to the difference between HashTable  amp  Dictionary  Dictionary is generic where as Hastable is not Generic  We can add any type of object to HashTable  but while retrieving we need to cast it to the required type  So  it is not type safe  But to dictionary  while declaring itself we can specify the type of key and value  so there is no need to cast while retrieving   Let s look at an example   HashTable  class HashTableProgram       static void Main string   args                Hashtable ht   new Hashtable            ht Add 1   One            ht Add 2   Two            ht Add 3   Three            foreach  DictionaryEntry de in ht                        int Key    int de Key    Casting             string value   de Value ToString      Casting             Console WriteLine Key         value                        Dictionary   class DictionaryProgram       static void Main string   args                Dictionary lt int  string gt  dt   new Dictionary lt int  string gt             dt Add 1   One            dt Add 2   Two            dt Add 3   Three            foreach  KeyValuePair lt int  String gt  kv in dt                        Console WriteLine kv Key         kv Value

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The Hashtable is a loosely-typed data structure  so you can add keys and values of any type to the Hashtable  The Dictionary class is a type-safe Hashtable implementation  and the keys and values are strongly typed  When creating a Dictionary instance  you must specify the data types for both the key and value

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People are saying that a Dictionary is the same as a hash table   This is not necessarily true  A hash table is one way to implement a dictionary  A typical one at that  and it may be the default one in  NET in the Dictionary class  but it s not by definition the only one   You could equally well implement a dictionary using a linked list or a search tree  it just wouldn t be as efficient  for some metric of efficient

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Dictionary    It returns throws Exception if we try to find a key which does not exist  It is faster than a Hashtable because there is no boxing and unboxing  Only public static members are thread safe  Dictionary is a generic type which means we can use it with any data type  When creating  must specify the data types for both keys and values    Example  Dictionary lt string  string gt   lt NameOfDictionaryVar gt                    new Dictionary lt string  string gt     Dictionay is a type-safe implementation of Hashtable  Keys and Values are strongly typed    Hashtable    It returns null if we try to find a key which does not exist  It is slower than dictionary because it requires boxing and unboxing  All the members in a Hashtable are thread safe  Hashtable is not a generic type  Hashtable is loosely-typed data structure  we can add keys and values of any type

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Dictionary    It returns throws Exception if we try to find a key which does not exist  It is faster than a Hashtable because there is no boxing and unboxing  Only public static members are thread safe  Dictionary is a generic type which means we can use it with any data type  When creating  must specify the data types for both keys and values    Example  Dictionary lt string  string gt   lt NameOfDictionaryVar gt                    new Dictionary lt string  string gt     Dictionay is a type-safe implementation of Hashtable  Keys and Values are strongly typed    Hashtable    It returns null if we try to find a key which does not exist  It is slower than dictionary because it requires boxing and unboxing  All the members in a Hashtable are thread safe  Hashtable is not a generic type  Hashtable is loosely-typed data structure  we can add keys and values of any type

User · Answer

In  NET  the difference between Dictionary lt   gt  and HashTable is primarily that the former  is a generic type  so you get all the benefits of generics in terms of static type checking  and reduced boxing  but this isn t as big as people tend to think in terms of performance - there is a definite memory cost to boxing  though

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The Extensive Examination of Data Structures Using C  article on MSDN states that there is also a difference in the  collision resolution strategy   The Hashtable class uses a technique referred to as rehashing       Rehashing works as follows  there is a set of hash different functions    H1     Hn  and when inserting or retrieving an item from the hash   table  initially the H1 hash function is used  If this leads to a   collision  H2 is tried instead  and onwards up to Hn if needed    The Dictionary uses a technique referred to as chaining       With rehashing  in the event of a collision the hash is recomputed  and the new slot corresponding to a hash is tried   With chaining  however  a secondary data structure is utilized to hold   any collisions  Specifically  each slot in the Dictionary has an array   of elements that map to that bucket  In the event of a collision  the   colliding element is prepended to the bucket s list

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Dictionary  lt  lt  lt     Hashtable differences    Generic   lt  lt  lt      Non-Generic Needs own thread synchronization  lt  lt  lt     Offers thread safe version through Synchronized   method Enumerated item  KeyValuePair  lt  lt  lt     Enumerated item  DictionaryEntry Newer     NET 2 0   lt  lt  lt     Older  since  NET 1 0  is in System Collections Generic  lt  lt  lt     is in System Collections  Request to non-existing key throws exception  lt  lt  lt     Request to non-existing key returns null potentially a bit faster for value types  lt  lt  lt     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

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One more difference that I can figure out is   We can not use Dictionary lt KT VT   generics  with web services  The reason is no web service standard supports the generics standard

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Because Dictionary is a generic class   Dictionary lt TKey  TValue gt     so that accessing its content is type-safe  i e  you do not need to cast from Object  as you do with a Hashtable    Compare  var customers   new Dictionary lt string  Customer gt         Customer customer   customers  Ali G      to  var customers   new Hashtable        Customer customer   customers  Ali G   as Customer    However  Dictionary is implemented as hash table internally  so technically it works the same way

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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 provides 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   An additional difference is that when we add the multiple entries in Dictionary  the order in which the entries are added is maintained  When we retrieve the items from Dictionary we will get the records in the same order we have inserted them  Whereas Hashtable doesn t preserve the insertion order

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A Hashtable object consists of buckets that contain the elements of the collection  A bucket is a virtual subgroup of elements within the Hashtable  which makes searching and retrieving easier and faster than in most collections   The Dictionary class has the same functionality as the Hashtable class  A Dictionary 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   For further reading  Hashtable and Dictionary Collection Types

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HashTable   Key value will be converted into an object  boxing  type while storing into the heap   Key value needs to be converted into the desired type while reading from the heap   These operations are very costly  We need to avoid boxing unboxing as much as possible   Dictionary   Generic variant of HashTable   No boxing unboxing  No conversions required

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In  NET  the difference between Dictionary lt   gt  and HashTable is primarily that the former  is a generic type  so you get all the benefits of generics in terms of static type checking  and reduced boxing  but this isn t as big as people tend to think in terms of performance - there is a definite memory cost to boxing  though

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Since  NET Framework 3 5 there is also a HashSet lt T gt  which provides all the pros of the Dictionary lt TKey  TValue gt  if you need only the keys and no values   So if you use a Dictionary lt MyType  object gt  and always set the value to null to simulate the type safe hash table you should maybe consider switching to the HashSet lt T gt

User · Answer

FYI  In  NET  Hashtable is thread safe for use by multiple reader threads and a single writing thread  while in Dictionary public static members are thread safe  but any instance members are not guaranteed to be thread safe   We had to change all our Dictionaries back to Hashtable because of this

User · Answer

For what it s worth  a Dictionary is  conceptually  a hash table   If you meant  why do we use the Dictionary lt TKey  TValue gt  class instead of the Hashtable class    then it s an easy answer  Dictionary lt TKey  TValue gt  is a generic type  Hashtable is not  That means you get type safety with Dictionary lt TKey  TValue gt   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 lt TKey  TValue gt  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

User · Answer

HashTable   Key value will be converted into an object  boxing  type while storing into the heap   Key value needs to be converted into the desired type while reading from the heap   These operations are very costly  We need to avoid boxing unboxing as much as possible   Dictionary   Generic variant of HashTable   No boxing unboxing  No conversions required

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 provides 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   An additional difference is that when we add the multiple entries in Dictionary  the order in which the entries are added is maintained  When we retrieve the items from Dictionary we will get the records in the same order we have inserted them  Whereas Hashtable doesn t preserve the insertion order

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According to what I see by using  NET Reflector    Serializable  ComVisible true   public abstract class DictionaryBase   IDictionary  ICollection  IEnumerable          Fields     private Hashtable hashtable          Methods     protected DictionaryBase        public void Clear            Take note of these lines    Fields private Hashtable hashtable    So we can be sure that DictionaryBase uses a HashTable internally

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Because Dictionary is a generic class   Dictionary lt TKey  TValue gt     so that accessing its content is type-safe  i e  you do not need to cast from Object  as you do with a Hashtable    Compare  var customers   new Dictionary lt string  Customer gt         Customer customer   customers  Ali G      to  var customers   new Hashtable        Customer customer   customers  Ali G   as Customer    However  Dictionary is implemented as hash table internally  so technically it works the same way

User · Answer

Because Dictionary is a generic class   Dictionary lt TKey  TValue gt     so that accessing its content is type-safe  i e  you do not need to cast from Object  as you do with a Hashtable    Compare  var customers   new Dictionary lt string  Customer gt         Customer customer   customers  Ali G      to  var customers   new Hashtable        Customer customer   customers  Ali G   as Customer    However  Dictionary is implemented as hash table internally  so technically it works the same way

User · Answer

FYI  In  NET  Hashtable is thread safe for use by multiple reader threads and a single writing thread  while in Dictionary public static members are thread safe  but any instance members are not guaranteed to be thread safe   We had to change all our Dictionaries back to Hashtable because of this

User · Answer

People are saying that a Dictionary is the same as a hash table   This is not necessarily true  A hash table is one way to implement a dictionary  A typical one at that  and it may be the default one in  NET in the Dictionary class  but it s not by definition the only one   You could equally well implement a dictionary using a linked list or a search tree  it just wouldn t be as efficient  for some metric of efficient

User · Answer

In  NET  the difference between Dictionary lt   gt  and HashTable is primarily that the former  is a generic type  so you get all the benefits of generics in terms of static type checking  and reduced boxing  but this isn t as big as people tend to think in terms of performance - there is a definite memory cost to boxing  though

User · Answer

Notice that the documentation says   quot the Dictionary lt  Of  lt  TKey  TValue gt   gt   class is implemented as a hash table quot   not  quot the Dictionary lt  Of  lt  TKey  TValue gt   gt   class is implemented as a HashTable quot  Dictionary is NOT implemented as a HashTable  but it is implemented following the concept of a hash table  The implementation is unrelated to the HashTable class because of the use of Generics  although internally Microsoft could have used the same code and replaced the symbols of type Object with TKey and TValue  In  NET 1 0 Generics did not exist  this is where the HashTable and ArrayList originally began

User · Answer

In most programming languages  dictionaries are preferred over hashtables   I don t think this is necessarily true  most languages have one or the other  depending on the terminology they prefer   In C   however  the clear reason  for me  is that C  HashTables and other members of the System Collections namespace are largely obsolete  They were present in c  V1 1   They have been replaced from C  2 0 by the Generic classes in the System Collections Generic namespace

User · Answer

For what it s worth  a Dictionary is  conceptually  a hash table   If you meant  why do we use the Dictionary lt TKey  TValue gt  class instead of the Hashtable class    then it s an easy answer  Dictionary lt TKey  TValue gt  is a generic type  Hashtable is not  That means you get type safety with Dictionary lt TKey  TValue gt   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 lt TKey  TValue gt  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

User · Answer

FYI  In  NET  Hashtable is thread safe for use by multiple reader threads and a single writing thread  while in Dictionary public static members are thread safe  but any instance members are not guaranteed to be thread safe   We had to change all our Dictionaries back to Hashtable because of this

User · Answer

One more difference that I can figure out is   We can not use Dictionary lt KT VT   generics  with web services  The reason is no web service standard supports the generics standard

User · Answer

Dictionary lt  gt  is a generic type and so it s type safe   You can insert any value type in HashTable and this may sometimes throw an exception  But Dictionary lt int gt  will only accept integer values and similarly Dictionary lt string gt  will only accept strings   So  it is better to use Dictionary lt  gt  instead of HashTable

User · Answer

The Hashtable is a loosely-typed data structure  so you can add keys and values of any type to the Hashtable  The Dictionary class is a type-safe Hashtable implementation  and the keys and values are strongly typed  When creating a Dictionary instance  you must specify the data types for both the key and value

User · Answer

Since  NET Framework 3 5 there is also a HashSet lt T gt  which provides all the pros of the Dictionary lt TKey  TValue gt  if you need only the keys and no values   So if you use a Dictionary lt MyType  object gt  and always set the value to null to simulate the type safe hash table you should maybe consider switching to the HashSet lt T gt

User · Answer

Dictionary  lt  lt  lt     Hashtable differences    Generic   lt  lt  lt      Non-Generic Needs own thread synchronization  lt  lt  lt     Offers thread safe version through Synchronized   method Enumerated item  KeyValuePair  lt  lt  lt     Enumerated item  DictionaryEntry Newer     NET 2 0   lt  lt  lt     Older  since  NET 1 0  is in System Collections Generic  lt  lt  lt     is in System Collections  Request to non-existing key throws exception  lt  lt  lt     Request to non-existing key returns null potentially a bit faster for value types  lt  lt  lt     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

User · Answer

In most programming languages  dictionaries are preferred over hashtables   I don t think this is necessarily true  most languages have one or the other  depending on the terminology they prefer   In C   however  the clear reason  for me  is that C  HashTables and other members of the System Collections namespace are largely obsolete  They were present in c  V1 1   They have been replaced from C  2 0 by the Generic classes in the System Collections Generic namespace

User · Answer

In  NET  the difference between Dictionary lt   gt  and HashTable is primarily that the former  is a generic type  so you get all the benefits of generics in terms of static type checking  and reduced boxing  but this isn t as big as people tend to think in terms of performance - there is a definite memory cost to boxing  though

User · Answer

People are saying that a Dictionary is the same as a hash table   This is not necessarily true  A hash table is one way to implement a dictionary  A typical one at that  and it may be the default one in  NET in the Dictionary class  but it s not by definition the only one   You could equally well implement a dictionary using a linked list or a search tree  it just wouldn t be as efficient  for some metric of efficient

User · Answer

The Extensive Examination of Data Structures Using C  article on MSDN states that there is also a difference in the  collision resolution strategy   The Hashtable class uses a technique referred to as rehashing       Rehashing works as follows  there is a set of hash different functions    H1     Hn  and when inserting or retrieving an item from the hash   table  initially the H1 hash function is used  If this leads to a   collision  H2 is tried instead  and onwards up to Hn if needed    The Dictionary uses a technique referred to as chaining       With rehashing  in the event of a collision the hash is recomputed  and the new slot corresponding to a hash is tried   With chaining  however  a secondary data structure is utilized to hold   any collisions  Specifically  each slot in the Dictionary has an array   of elements that map to that bucket  In the event of a collision  the   colliding element is prepended to the bucket s list

User · Answer

The Hashtable is a loosely-typed data structure  so you can add keys and values of any type to the Hashtable  The Dictionary class is a type-safe Hashtable implementation  and the keys and values are strongly typed  When creating a Dictionary instance  you must specify the data types for both the key and value

User · Answer

Collections  amp  Generics are useful for handling group of objects  In  NET  all the collections objects comes under the interface IEnumerable  which in turn has ArrayList Index-Value    amp  HashTable Key-Value   After  NET framework 2 0  ArrayList  amp  HashTable were replaced with List  amp  Dictionary  Now  the Arraylist  amp  HashTable are no more used in nowadays projects   Coming to the difference between HashTable  amp  Dictionary  Dictionary is generic where as Hastable is not Generic  We can add any type of object to HashTable  but while retrieving we need to cast it to the required type  So  it is not type safe  But to dictionary  while declaring itself we can specify the type of key and value  so there is no need to cast while retrieving   Let s look at an example   HashTable  class HashTableProgram       static void Main string   args                Hashtable ht   new Hashtable            ht Add 1   One            ht Add 2   Two            ht Add 3   Three            foreach  DictionaryEntry de in ht                        int Key    int de Key    Casting             string value   de Value ToString      Casting             Console WriteLine Key         value                        Dictionary   class DictionaryProgram       static void Main string   args                Dictionary lt int  string gt  dt   new Dictionary lt int  string gt             dt Add 1   One            dt Add 2   Two            dt Add 3   Three            foreach  KeyValuePair lt int  String gt  kv in dt                        Console WriteLine kv Key         kv Value

User · Answer

Because Dictionary is a generic class   Dictionary lt TKey  TValue gt     so that accessing its content is type-safe  i e  you do not need to cast from Object  as you do with a Hashtable    Compare  var customers   new Dictionary lt string  Customer gt         Customer customer   customers  Ali G      to  var customers   new Hashtable        Customer customer   customers  Ali G   as Customer    However  Dictionary is implemented as hash table internally  so technically it works the same way

User · Answer

People are saying that a Dictionary is the same as a hash table   This is not necessarily true  A hash table is one way to implement a dictionary  A typical one at that  and it may be the default one in  NET in the Dictionary class  but it s not by definition the only one   You could equally well implement a dictionary using a linked list or a search tree  it just wouldn t be as efficient  for some metric of efficient

User · Answer

According to what I see by using  NET Reflector    Serializable  ComVisible true   public abstract class DictionaryBase   IDictionary  ICollection  IEnumerable          Fields     private Hashtable hashtable          Methods     protected DictionaryBase        public void Clear            Take note of these lines    Fields private Hashtable hashtable    So we can be sure that DictionaryBase uses a HashTable internally

User · Answer

A Hashtable object consists of buckets that contain the elements of the collection  A bucket is a virtual subgroup of elements within the Hashtable  which makes searching and retrieving easier and faster than in most collections   The Dictionary class has the same functionality as the Hashtable class  A Dictionary 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   For further reading  Hashtable and Dictionary Collection Types

User · Answer

FYI  In  NET  Hashtable is thread safe for use by multiple reader threads and a single writing thread  while in Dictionary public static members are thread safe  but any instance members are not guaranteed to be thread safe   We had to change all our Dictionaries back to Hashtable because of this

User · Answer

For what it s worth  a Dictionary is  conceptually  a hash table   If you meant  why do we use the Dictionary lt TKey  TValue gt  class instead of the Hashtable class    then it s an easy answer  Dictionary lt TKey  TValue gt  is a generic type  Hashtable is not  That means you get type safety with Dictionary lt TKey  TValue gt   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 lt TKey  TValue gt  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

User · Answer

Dictionary lt  gt  is a generic type and so it s type safe   You can insert any value type in HashTable and this may sometimes throw an exception  But Dictionary lt int gt  will only accept integer values and similarly Dictionary lt string gt  will only accept strings   So  it is better to use Dictionary lt  gt  instead of HashTable

[c#] Why is Dictionary preferred over Hashtable in C#?

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