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C# Object-Oriented Programming (OOP)

C# Hashtable (Hash Table)

HashTable is a non-generic collection that stores key-value pairs, similar to the generic Dictionary<TKey, TValue> collection. It optimizes search by calculating the hash code of each key and storing it in different buckets internally, then matching the hash code of the specified key when accessing the value.

HashTable Features
Hashtable stores key-value pairs.
Belongs to the System.Collection namespace.
Implements the IDictionary interface.
Keys must be unique and cannot be null.
Values can be null or duplicated.
Elements are stored as DictionaryEntry objects.

Create a hash table (HashTable)

The following example demonstrates how to create a hash table and add elements.

Example: Create and add elements

Hashtable numberNames = new Hashtable();
numberNames.Add(1, "One");　//Add keys using the Add() method/Value
numberNames.Add(2, "Two");
numberNames.Add(3, "Three");
//The following causes a runtime exception: The key has already been added. run-time exception: key already added.
//numberNames.Add(3, "Three");　
foreach(DictionaryEntry de in numberNames)
　　　　Console.WriteLine("Key: {0}, Value: {"1}"
//Using a collection-initializer syntax to create a Hashtable
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}"

The Hashtable collection can include all elements of a dictionary, as shown below.

Example: Add a dictionary to a hash table

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 a hash table

By passing a key to the indexer, you can retrieve the value of an existing key from the Hashtable. Since Hashtable is a non-generic collection, a value type must be typed when retrieving it.

Example: Update a hash table

//Using a collection-initializer syntax to create a Hashtable
var cities = new Hashtable(){
	{"UK", "London, Manchester, Birmingham"},
	{"USA", "Chicago, New York, Washington"},
	{"India", "Mumbai, New Delhi, Pune"}
}
　　　　
string citiesOfUK = (string) cities["UK"];　//Convert to string
string citiesOfUSA = (string) cities["USA"];　//Convert to string
Console.WriteLine(citiesOfUK);
Console.WriteLine(citiesOfUSA);
cities["UK"] = "Liverpool, Bristol";　//　Update the value of UK
cities["USA"] = "Los Angeles, Boston";　//Update the value of USA
if(!cities.ContainsKey("France")){
　　　　cities["France"] = "Paris";
}

Delete elements from the hash table

The Remove() method deletes the key-value pair that matches the specified value in the Hashtable. If the specified key is not found in the Hashtable, a KeyNotFoundException is thrown, so the ContainsKey() method is used to check for the existing key before deletion.

The Clear() method can delete all elements at once.

Example: Delete an element from a hash table

var cities = new Hashtable(){
	{"UK", "London, Manchester, Birmingham"},
	{"USA", "Chicago, New York, Washington"},
	{"India", "Mumbai, New Delhi, Pune"}
}
cities.Remove("UK");　//　Remove UK
//Remove UK　//Throw a runtime exception: KeyNotFoundException
if(cities.ContainsKey("France")){　//　Check before retrieving the key
　　　　Remove UK
}
cities.Clear();　//Remove All Elements

Hashtable Class Hierarchy

The following figure illustrates the hierarchy of the Hashtable class.

C# Hash Table

C# FileInfo C# Dictionary(Dictionary)