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
Hashtablestores key-value pairs.- Comes under
System.Collectionsnamespace. - 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.
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.
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.
//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
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.
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.
