Why Dictionary is preferred over hashtable in C#?

Question

In most of programming languages, we preferred using a dictionary over a hashtable . What are the reasons behind it?

Answer 1

Because Dictionary is a generic class ( Dictionary<TKey, TValue> ), 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<string, Customer>();
...
Customer customer = customers["Ali G"];

to

var customers = new Hashtable();
...
Customer customer = customers["Ali G"] as Customer;

Answer 2

In .NET, the difference between Dictionary<,> 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).

Answer 3

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 types. When creating a Dictionary instance, you must specify the data types for both the key and value.

Answer 4

FWIW, a Dictionary is a hash table.

If you meant "why do we use the Dictionary class instead of the Hashtable class?", then it's an easy answer: Dictionary is a generic type, Hashtable is not. That means you get type safety with Dictionary, because you can't insert any random object into it, and you don't have to cast the values you take out.

Answer 5

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.

Answer 6

People are saying that a Dictionary is the same as a hash table.

This is not necessarily true. A hash table is an implementation of a dictionary. A typical one at that, and it may be the default one in .NET, but it's not by definition the only one.

You could equally well implement a dictionary with a linked list or a search tree, it just wouldn't be as efficient (for some metric of efficient).

Answer 7

one more difference that i can figure out is we can not use dictionary (generics) with web services the reason is no web service standard supports genrics standard.

Answer 8

This is not necessarily true. A hash table is an implementation of a dictionary. A typical one at that, and it may be the default one in .NET, but it's not by definition the only one.

I'm not sure that this is required by the ECMA standard, but the MSDN documentation very clearly calls it out as being implemented as a hashtable. They even provide the SortedList class for times when an alternative is more reasonable.

Answer 9

Notice that MSDN says: "Dictionary<(Of <(TKey, TValue>)>) class is implemented as a hash table" not "Dictionary<(Of <(TKey, TValue>)>) class is implemented as a HashTable" Dictionary is NOT implemented as a HashTable, but 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.

Answer 10

According to what I see by using 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.

Answer 11

When you declare a variable as Dictionary(Of K, T), the new Dictionary type that is created only accepts keys of the one type and values of the other.

http://w3vb.net/vb-net-data-structures/vb-net-collections/generic-dictionary/example-dictionaryof-k-t-generic-in-vb-net