How to Create a HashTable Collection in C#?

The Hashtable is a non-generic collection in C# that stores key-value pairs, similar to a generic Dictionary collection. Hashtable is defined in the System.Collections namespace and computes hash codes for keys to optimize lookups by storing them in different internal buckets.

In this tutorial, we will see how to create a Hashtable collection in C# using different approaches.

Key Features of Hashtable

• Stores key-value pairs as DictionaryEntry objects

• Keys must be unique and non-null, but values can be null or duplicate

• Keys are immutable objects that provide hash functions

• Values can be accessed using keys in an indexer like arrays

• Elements can be of the same or different types

• Implements IDictionary, ICollection, IEnumerable, and other interfaces

Syntax

Following is the syntax for creating a Hashtable

using System.Collections;

Hashtable hashtableName = new Hashtable();

Adding elements using the Add() method

hashtableName.Add(key, value);

Common Hashtable Constructors

Using Default Constructor

Example

using System;
using System.Collections;

class MyHashTable {
   static public void Main() {
      // Create hashtable using the default constructor
      Hashtable indianNumberSystem = new Hashtable();
      
      // Add key/value pairs using the Add() method
      indianNumberSystem.Add(1, "Ones"); 
      indianNumberSystem.Add(10, "Tens");
      indianNumberSystem.Add(100, "Hundred");
      indianNumberSystem.Add(1000, "Thousand");
      indianNumberSystem.Add(10000, "Ten Thousand");
      indianNumberSystem.Add(100000, "Lac");
      indianNumberSystem.Add(1000000, "Ten Lac");
      indianNumberSystem.Add(10000000, "Crore");
      
      // Display HashTable contents
      Console.WriteLine("Key-Value pairs from Indian Number System:");
      foreach(DictionaryEntry element in indianNumberSystem) {
         Console.WriteLine("{0} : {1}", element.Key, element.Value);
      }      
   }
}

The output of the above code is

Key-Value pairs from Indian Number System:
100 : Hundred
1000 : Thousand
10 : Tens
1000000 : Ten Lac
100000 : Lac
10000000 : Crore
10000 : Ten Thousand
1 : Ones

Using Collection Initializer

Example

using System;
using System.Collections;

class MyHashTable {
   static public void Main() {
      // Create hashtable with collection initializer
      Hashtable cityHashtable = new Hashtable() {
         {"USA", "New York"},
         {"France", "Paris"},
         {"UK", "London"},
         {"India", "Mumbai"},
         {"Germany", "Berlin"}
      };
      
      Console.WriteLine("Countries and Cities:");
      foreach(DictionaryEntry element in cityHashtable) {
         Console.WriteLine("{0} : {1}", element.Key, element.Value);
      }      
   }
}

The output of the above code is

Countries and Cities:
Germany : Berlin
France : Paris
USA : New York
UK : London
India : Mumbai

Creating from Existing Dictionary

Example

using System;
using System.Collections;

class MyHashTable {
   static public void Main() {
      // Create initial hashtable
      Hashtable originalTable = new Hashtable() {
         {"C#", "Programming Language"},
         {"HTML", "Markup Language"}
      };
      
      // Create new hashtable from existing one
      Hashtable copiedTable = new Hashtable(originalTable);
      copiedTable.Add("SQL", "Database Language");
      
      Console.WriteLine("Copied and Extended Hashtable:");
      foreach(DictionaryEntry element in copiedTable) {
         Console.WriteLine("{0} : {1}", element.Key, element.Value);
      }
   }
}

The output of the above code is

Copied and Extended Hashtable:
HTML : Markup Language
C# : Programming Language
SQL : Database Language

Conclusion

Hashtable in C# provides an efficient way to store key-value pairs with fast lookup capabilities through hashing. While it's a non-generic collection, it remains useful for scenarios requiring mixed data types or when working with legacy code that doesn't use generics.