Difference between Dictionary and Hashtable
C#CollectionsC# Problem Overview
> 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?
C# Solutions
Solution 1 - C#
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)
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<TKey, TValue>
.
Solution 2 - C#
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<string, int> pair in objHashTable)
//{
// lblDisplay.Text = pair.Value + " " + lblDisplay.Text;
//}
}
The following is for dictionary
public void MethodDictionary()
{
Dictionary<string, int> dictionary = new Dictionary<string, int>();
dictionary.Add("cat", 2);
dictionary.Add("dog", 1);
dictionary.Add("llama", 0);
dictionary.Add("iguana", -1);
//dictionary.Add(1, -2); // Compilation Error
foreach (KeyValuePair<string, int> pair in dictionary)
{
lblDisplay.Text = pair.Value + " " + lblDisplay.Text;
}
}
Solution 3 - C#
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
Solution 4 - C#
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: https://stackoverflow.com/questions/380595/multimap-in-c-3-0/380601#380601
Solution 5 - C#
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<=hash.Count;i++)//=>No error for index 0
{
//Can be accessed through indexers
Console.WriteLine(hash[i]);
}
Console.WriteLine(hash["Ten"]);//=> No error in Has Table
here no error for key 0 & also for key "ten"(note: t is small)
//Strict type declaration
Dictionary<int,string> dictionary= new Dictionary<int, string>();
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 <= 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 & also for key 10 as both are inexistent in dictionary, runtime error, while try to acess.
Solution 6 - C#
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.
Solution 7 - C#
ILookup
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
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
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.:)