How to Create a HashTable Collection in C#?

The HashTable is a non−generic collection in C#. It stores key-value pairs and is similar to a generic “Dictionary” collection. HashTable is defined in

System. Collections. namespace.

HashTable computes the hash code of each key and stores it in a different bucket internally. The hash code is then matched to the hash code of the specified key when the values are accessed. Thus the lookups are optimized by HashTable.

In this tutorial, we will see how we can create a HashTable collection in C#.

Hashtable Features

Before we jump to creating a HashTable, let us see some of the salient features of the HashTable collection in C#.

HashTable collection stores key−value pairs.

HashTable is a part of Systems. collections namespace in C# and implements the IDictionary interface. Elements of HashTable are stored as DictionaryEntry objects.

The HashTable keys must not be null and are unique. However, values can be null or duplicate.

Values in the HashTable can be accessed by using the key in the indexer just like the way we access array values.

Keys in HashTable are immutable objects. Each of these key objects provides a Hash function.

A typical Hashtable class implements the IDictionary, ICollection, ISerializable, IEnumerable, IDeserializationCallback, and ICloneable interfaces of C#.

Elements stored in HashTable can be of the same type or different types.

Keeping these salient features in mind, let us now discuss how to create a HashTable in C#.

How to Create a HashTable Collection in C#?

The HashTable class of C# provides 16 overloaded constructors to create a HashTable.

The following table shows the HashTable constructors that we will be using in this article.

Constructor	Description
Hashtable()	Initialization of new, an empty instances of the HashTable class with default initial capacity, hash code provider, comparator, and load factor.
Hashtable(IDictionary)	Creates a new instance of the Hashtable class and initializes it with the contents of the specified dictionary.

Note − To know more about the HashTable class in C#, read our article C#−HashTable Class.

Let’s see the steps we usually follow to create a HashTable collection in C#.

First, we include System. Collections namespace in our program

using System. Collections;

Next, we create a hashtable using the Hashtable class. For this, we use a default constructor.

Hashtable hashtable_name = new Hashtable();

Now we can use the “Add()” method to add elements to the HashTable.

So here, we can either initialize the entire HashTable while creating its instance or use Add() method to add elements to the HashTable one by one.

Example 1

The following program demonstrates the creation of HashTable in C#.

using System;
using System. Collections;
class MyHashTable {
   // Main Method
   static public void Main() {

      // Create hashtable using the default constructor
      Hashtable indianNumberSystem = new Hashtable();
      
      //add a key/value pair 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 ele1 in indianNumberSystem){
         Console.WriteLine("{0} ({1}) ", ele1.Key, ele1.Value);
      }      
   }
}

In the above program, we have defined a HashTable instance using the default constructor. Then we use the Add() method to add key/value pairs to the HashTable. Finally, the HashTable contents are printed one by one using a for-each loop.

Output

The above program generates the following output.

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)

The program displays a HashTable containing the place values of Indian Number System. Note that since this is a simple program to create a Hashtable and add contents to it, the output is not formatted.

Example 2

Let’s take another example of creating a HashTable in C#. The following program uses a different constructor for creating HashTable.

using System;
using System.Collections;
class MyHashTable {
   // Main Method
   static public void Main() {

      // Create hashtable without using Add method
      Hashtable my_hashtable1 = new Hashtable() {{"K1", "New York"}};

      // Adding key/value pair in the hashtable using Add() method
      my_hashtable1.Add("K2", "Paris");
      my_hashtable1.Add("K3", "London");
      my_hashtable1.Add("K4", "Mumbai");
      my_hashtable1.Add("K5", "Berlin");
      
      Console.WriteLine("Key, Value pairs from my_hashtable1:");
      foreach(DictionaryEntry ele1 in my_hashtable1){
         Console.WriteLine("{0} and {1} ", ele1.Key, ele1.Value);
      }      
   }
}

As we can see in the above code, first we create a HashTable object with one key-value pair. Then we use add() method of the HashTable class to add elements to the HashTable. Finally, using a for-each loop, we iterate through the HashTable object to print each hashTable element (key-value pair).

Output

The above program produces the following output.

Key, Value pairs from my_hashtable1:
K2 and Paris 
K1 and New York 
K3 and London 
K4 and Mumbai 
K5 and Berlin 

In the above output, the key-value pairs are displayed in reverse alphabetical order of values. This is the default output of the HashTable since we haven’t provided any code to format the output. HashTable class provides various methods to organize/format the output which we will learn in subsequent tutorials.

In this tutorial, we have discussed how to create a HashTable collection in C#. HashTable is a non-generic collection of key-value pairs. The keys in HashTable are unique, non-null values. Values can be null and duplicated. We can create the HashTable in C# using the HashTable Class provided by Systems. Collections interface and modify it using various methods provided by this class.