C# 0 The Complete Reference

EqualityComparer

 and 

KeyComparer

. Two other properties called 

hcp

 and 

comparer

are flagged as obsolete.

Hashtable

 stores key/value pairs in the form of a 

DictionaryEntry

 structure, but most 

of the time, you won’t be aware of it directly because the properties and methods work with 

keys and values individually. For example, when you add an element to a 

Hashtable

, you 

call

Add( )

, which takes two arguments: the key and the value.

It is important to note that 

Hashtable

 does not guarantee the order of its elements. This 

is because the process of hashing does not usually lend itself to the creation of sorted tables.

Here is an example that demonstrates 

Hashtable

:

// Demonstrate Hashtable.

using System;

using System.Collections;

class HashtableDemo {

  static void Main() {

    // Create a hash table.

    Hashtable ht = new Hashtable();

    // Add elements to the table.

    ht.Add("house", "Dwelling");

    ht.Add("car", "Means of transport");

    ht.Add("book", "Collection of printed words");

    ht.Add("apple", "Edible fruit");

    // Can also add by using the indexer.

    ht["tractor"] = "Farm implement";

    // Get a collection of the keys.

    ICollection c = ht.Keys;

    // Use the keys to obtain the values.

    foreach(string str in c)

      Console.WriteLine(str + ": " + ht[str]);

  }

}

Method

Description

public vir tual bool ContainsKey(object 

k

)

Returns true if 

k

 is a key in the invoking 

Hashtable

.

Returns false other wise.

public vir tual bool ContainsValue(object 

v

)

Returns true if 

v

 is a value in the invoking 

Hashtable

.

Returns false other wise.

public vir tual IDictionar yEnumerator

    GetEnumerator( )

Returns an 

IDictionaryEnumerator

 for the invoking 

Hashtable

.

public static Hashtable

    Synchronized(Hashtable 

ht

)

Returns a synchronized version of the 

Hashtable

passed in 

ht.

T

ABLE

 24-5 

Several Commonly Used Methods Defi ned by 

Hashtable

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The output from this program is shown here:

book: Collection of printed words

tractor: Farm implement

apple: Edible fruit

house: Dwelling

car: Means of transport

As the output shows, the key/value pairs are not stored in sorted order. Notice how the 

contents of the hash table