C# - Hashtable

The Hashtable is a non-generic collection that stores key-value pairs, similar to generic Dictionary<TKey, TValue> collection. It optimizes lookups by computing the hash code of each key and stores it in a different bucket internally and then matches the hash code of the specified key at the time of accessing values.

Hashtable Characteristics

  • Hashtable stores key-value pairs.
  • Comes under System.Collection namespace.
  • Implements IDictionary interface.
  • Keys must be unique and cannot be null.
  • Values can be null or duplicate.
  • Values can be accessed by passing associated key in the indexer e.g. myHashtable[key]
  • Elements are stored as DictionaryEntry objects.

Creating a Hashtable

The following example demonstrates creating a Hashtable and adding elements.

Example: Create and Add Elements
Hashtable numberNames = new Hashtable();
numberNames.Add(1,"One"); //adding a key/value using the Add() method
numberNames.Add(2,"Two");
numberNames.Add(3,"Three");

//The following throws run-time exception: key already added.
//numberNames.Add(3, "Three"); 

foreach(DictionaryEntry de in numberNames)
    Console.WriteLine("Key: {0}, Value: {1}", de.Key, de.Value);
		
//creating a Hashtable using collection-initializer syntax
var cities = new Hashtable(){
	{"UK", "London, Manchester, Birmingham"},
	{"USA", "Chicago, New York, Washington"},
	{"India", "Mumbai, New Delhi, Pune"}
};
		
foreach(DictionaryEntry de in cities)
    Console.WriteLine("Key: {0}, Value: {1}", de.Key, de.Value);

The Hashtable collection can include all the elements of Dictionary, as shown below.

Example: Add Dictionary in Hashtable
Dictionary<int, string> dict = new Dictionary<int, string>();
dict.Add(1, "one");
dict.Add(2, "two");
dict.Add(3, "three");

Hashtable ht = new Hashtable(dict);

Update Hashtable

You can retrieve the value of an existing key from the Hashtable by passing a key in indexer. The Hashtable is a non-generic collection, so you must type cast values while retrieving it.

Example: Update Hashtable
//creating a Hashtable using collection-initializer syntax
var cities = new Hashtable(){
	{"UK", "London, Manchester, Birmingham"},
	{"USA", "Chicago, New York, Washington"},
	{"India", "Mumbai, New Delhi, Pune"}
};
    
string citiesOfUK = (string) cities["UK"]; //cast to string
string citiesOfUSA = (string) cities["USA"]; //cast to string

Console.WriteLine(citiesOfUK);
Console.WriteLine(citiesOfUSA);

cities["UK"] = "Liverpool, Bristol"; // update value of UK key
cities["USA"] = "Los Angeles, Boston"; // update value of USA key
//cities["France"] = "Paris"; //throws run-time exception: KeyNotFoundException

if(cities.ContainsKey("France")){
    cities["France"] = "Paris";
}

Remove Elements in Hashtable

The Remove() method removes the key-value that match with the specified in the Hashtable. It throws the KeyNotfoundException if the specified key not found in the Hashtable, so check for an existing key using the ContainsKey() method before removing.

Use the Clear() method to remove all the elements in one shot.

Example: Remove Elements from Hashtable
var cities = new Hashtable(){
	{"UK", "London, Manchester, Birmingham"},
	{"USA", "Chicago, New York, Washington"},
	{"India", "Mumbai, New Delhi, Pune"}
};

cities.Remove("UK"); // removes UK 
//cities.Remove("France"); //throws run-time exception: KeyNotFoundException

if(cities.ContainsKey("France")){ // check key before removing it
    cities.Remove("France");
}

cities.Clear(); //removes all elements

Hashtable Class Hierarchy

The following diagram illustrates the Hashtable class hierarchy.

C# Hashtable