Hash table and Dictionary

Question

Hi Friends What is the difference between hash table and dictionary.why we use hash table and dictionary. Thanks!

Senthilkumar · Answer

Hi The HashTable belongs to the system.Collections namespace and Dictionary belongs to the System.Collections.Generic namesapce and it can be used from C#2.0 above. both uses the key and value combination. But the Dictionary has some advantages over HashTable. The dictionary is static typing (and compile-time verification) and uses without boxing and unboxing. It is type specific and strongly typed. It will not throw any error at the run time. The performance wise it will be good because it does not need the type casting. For example HashTable follows 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(); Dictionary follows Dictionary dictionary = new Dictionary (); dictionary.Add(cat 2); dictionary.Add(dog 1); dictionary.Add(llama 0); dictionary.Add(iguana -1); //dictionary.Add(1 -2); // Compilation Error foreach (KeyValuePair pair in 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 Differences: Generic >> Non-Generic Needs own thread synchronization >> Offers thread safe version through Synchronized() method Enumerated item: KeyValuePair >> Enumerated item: DictionaryEntry Newer (> .NET 2.0) >> Older (since .NET 1.0) is in System.Collections.Generic >> is in System.Collections Request to non-existing key throws exception >> Request to non-existing key returns null http://stackoverflow.com/questions/301371/why-dictionary-is-preferred-over-hashtable-in-c

Sam Hobbs · Answer

The most important difference as Senthilkumar says is that the Dictionary(TKey TValue) Class is in the System.Collections.Generic namespace and the Hashtable Class is in the System.Collections namespace. That is why the Dictionary is  static typing . The advantages of the Dictionary class that Senthilkumar explains are advantages for all collections in the System.Collections.Generic namespace. Hash tables were originally created in a time when main memory was much more expensive than it is now; it is probably accurate to say that main memory and disk storage was extremely more expensive. Hash tables can be efficient when the amount of main memory is very limited. Note that the original IBM PC was limted to 1 MB of storage. Most computers now have at least a thousand times more main memory. Hash tables were originally designed long before the original IBM PC existed in a time when memory was more expensive.

MuthuMari M · Answer

Thank you !!!