C# - Hashtable Class

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Hashtable Class

The Hashtable class represents a collection of key-and-value pairs that are organized based on the hash code of the key. It uses the key to access the elements in the collection.

A hash table is used when you need to access elements by using key, and you can identify a useful key value. Each item in the hash table has a key/value pair. The key is used to access the items in the collection.

Properties of Hashtable Class

The following table lists some of the commonly used properties of the Hashtable class −

Methods Hashtable Class

The following table lists some of the commonly used methods of the Hashtable class −

Creating a Hashtable

You can create an empty Hashtable by declaring it using the Hashtable class constructor as follows:p>

Hashtable hashtable = new Hashtable();

Initializing a Hashtable

Once Hashtable is created, you can initialize a it by adding key-value pairs as follows:

hashtable.Add("001", "Rahul Verma");
hashtable.Add("002", "Priya Desai");
hashtable.Add("003", "Vikram Singh");

Adding Elements to a Hashtable

You can add elements to a Hashtable using the Add() method or the indexer. The Add() method accepts the key and its value and pushes it into the Hashtable. In the case of the indexer, use the key as an index in the Hashtable and assign the values directly.

Using Add() Method

using System;
using System.Collections;

class Program {
    static void Main(string[] args) {
        Hashtable hashtable = new Hashtable();
        hashtable.Add("A", "Amit Sharma");
        hashtable.Add("B", "Bhavna Mehta");
        hashtable.Add("C", "Chirag Patel");

        foreach (DictionaryEntry entry in hashtable) {
            Console.WriteLine($"Key: {entry.Key}, Value: {entry.Value}");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

Key: C, Value: Chirag Patel
Key: B, Value: Bhavna Mehta
Key: A, Value: Amit Sharma

Using Indexer

using System;
using System.Collections;

class Program {
    static void Main(string[] args) {
        Hashtable hashtable = new Hashtable();
        hashtable["D"] = "Deepika Rao";
        hashtable["E"] = "Eshan Verma";

        foreach (DictionaryEntry entry in hashtable) {
            Console.WriteLine($"Key: {entry.Key}, Value: {entry.Value}");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

Key: D, Value: Deepika Rao
Key: E, Value: Eshan Verma

Accessing Elements from a Hashtable

You can access values from a Hashtable using their corresponding keys by placing the key, enclosed in double quotes, inside square brackets after the Hashtable name.

Example

The following example demonstrates how you can access an element from a Hashtable:

using System;
using System.Collections;

class Program {
    static void Main(string[] args) {
        Hashtable hashtable = new Hashtable();
        hashtable.Add("X", "Xenia Gupta");

        Console.WriteLine("Value at key 'X': " + hashtable["X"]);
    }
}

When the above code is compiled and executed, it produces the following result −

Value at key 'X': Xenia Gupta

Iterating Through a Hashtable

You can iterate through a Hashtable using a foreach loop. Keys and values can be printed using the .Key and .Value properties.

Example

The following example demonstrates how you can iterate over a Hashtable's elements:

using System;
using System.Collections;

class Program {
    static void Main(string[] args) {
        Hashtable hashtable = new Hashtable();
        hashtable.Add("101", "Rahul Mishra");
        hashtable.Add("102", "Sneha Kapoor");
        hashtable.Add("103", "Irfan Khan");

        foreach (DictionaryEntry entry in hashtable) {
            Console.WriteLine($"Key: {entry.Key}, Value: {entry.Value}");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

Key: 101, Value: Rahul Mishra
Key: 102, Value: Sneha Kapoor
Key: 103, Value: Irfan Khan

Checking for Existence of Keys or Values

You can check whether a specific key or value exists in a Hashtable. Use the .ContainsKey() method to check for a key, and the .ContainsValue() method to check for a value.

Example: Check if a key exists

using System;
using System.Collections;

class Program {
    static void Main(string[] args) {
        Hashtable hashtable = new Hashtable();
        hashtable.Add("P", "Pooja Rani");

        if (hashtable.ContainsKey("P")) {
            Console.WriteLine("Key 'P' exists in the hashtable.");
        } else {
            Console.WriteLine("Key 'P' does not exist.");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

Key 'P' exists in the hashtable.

Example: Check if a value exists

using System;
using System.Collections;

class Program {
    static void Main(string[] args) {
        Hashtable hashtable = new Hashtable();
        hashtable.Add("Q", "Qasim Ali");

        if (hashtable.ContainsValue("Qasim Ali")) {
            Console.WriteLine("Value 'Qasim Ali' exists in the hashtable.");
        } else {
            Console.WriteLine("Value does not exist.");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

Value 'Qasim Ali' exists in the hashtable.

Removing Elements from a Hashtable

You can remove elements from a Hashtable using the Remove() method. The method accepts the key and remove the element (i.e., key and value both).

Example

The following example demonstrates how you can remove specific element from a Hashtable:

using System;
using System.Collections;

class Program {
    static void Main(string[] args) {
        Hashtable hashtable = new Hashtable();
        hashtable.Add("K", "Kiran Joshi");
        hashtable.Add("L", "Lakshmi Iyer");

        hashtable.Remove("K");

        foreach (DictionaryEntry entry in hashtable) {
            Console.WriteLine($"Key: {entry.Key}, Value: {entry.Value}");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

Key: L, Value: Lakshmi Iyer

Clearing a Hashtable

To remove all elements from a Hashtable, use the Clear() method.

Example

The following example demonstrating to clear a Hashtable:

using System;
using System.Collections;

class Program {
    static void Main(string[] args) {
        Hashtable hashtable = new Hashtable();
        hashtable.Add("U", "Usha Devi");
        hashtable.Add("V", "Vikram Yadav");

        hashtable.Clear();

        Console.WriteLine("Total items after clearing: " + hashtable.Count);
    }
}

When the above code is compiled and executed, it produces the following result −

Total items after clearing: 0

Example: Hashtable Operations

The following example demonstrates the concept −

using System;
using System.Collections;

namespace CollectionsApplication {
   class Program {
      static void Main(string[] args) {
         Hashtable ht = new Hashtable();
         
         ht.Add("001", "Zara Ali");
         ht.Add("002", "Abida Rehman");
         ht.Add("003", "Joe Holzner");
         ht.Add("004", "Mausam Benazir Nur");
         ht.Add("005", "M. Amlan");
         ht.Add("006", "M. Arif");
         ht.Add("007", "Ritesh Saikia");
         
         if (ht.ContainsValue("Nuha Ali")) {
            Console.WriteLine("This student name is already in the list");
         } else {
            ht.Add("008", "Nuha Ali");
         }
         
         // Get a collection of the keys.
         ICollection key = ht.Keys;
         
         foreach (string k in key) {
            Console.WriteLine(k + ": " + ht[k]);
         }
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

001: Zara Ali
002: Abida Rehman
003: Joe Holzner
004: Mausam Benazir Nur
005: M. Amlan
006: M. Arif
007: Ritesh Saikia
008: Nuha Ali