Difference between Dictionary and Hashtable

Question

Duplicate of: Why Dictionary is preferred over hashtable in C#?

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What is the difference between Dictionary and Hashtable. How to decide which one to use?

Answer 1

Simply, Dictionary<TKey,TValue> 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<TKey,TValue> (and the other generic collections)

Answer 2

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: http://stackoverflow.com/questions/380595/multimap-in-c-3-0/380601#380601

Answer 3

hopefully answers to this question provides you a good answer

Answer 4

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.

Answer 5

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

Answer 6

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.:)