Hash table and Dictionary

MuthuMari M

14y4.9kVisual Basic .NET

Hi Friends,

What is the difference between hash table and dictionary.why we use hash table and dictionary.

Thanks!

3 Replies

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Senthilkumar14y ago
Accepted 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
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Sam Hobbs14y ago
The most important difference, as Senthilkumar says, is that theDictionary(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.
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MuthuMari M14y ago
Thank you !!!
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