C# Hashtable with Examples

A Hashtable is a collection of key/value pairs that are arranged based on the hash code of the key. In other words, a Hashtable is used to create a collection that uses a hash table for storage. It generally optimizes the lookup by calculating the hash code of every key and storing it into another basket automatically and when you access the value from the hashtable at that time it matches the hashcode with the specified key. It is the non-generic type of collection that is defined in the System. Collections namespace. 

Important Points:

• In Hashtable, the key cannot be null, but the value can be.
• In Hashtable, key objects must be immutable as long as they are used as keys in the Hashtable.
• The capacity of a Hashtable is the number of elements that Hashtable can hold.
• A hash function is provided by each key object in the Hashtable.
• The Hashtable class implements the IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback, and ICloneable interfaces.
• In the hashtable, you can store elements of the same type and of the different types.
• The elements of the hashtable that is a key/value pair are stored in DictionaryEntry, so you can also cast the key/value pairs to a DictionaryEntry.
• that key that must be unique. Duplicate keys are not allowed.

How to create a Hashtable?

The Hashtable class provides 16 different types of constructors that are used to create a hashtable, here we only use Hashtable() constructor. To read more about Hashtable’s constructors you can refer to C# | Hashtable Class This constructor is used to create an instance of the Hashtable class which is empty and has the default initial capacity, load factor, hash code provider, and comparer. Now, let’s see how to create a hashtable using Hashtable() constructor:

Step 1: Include System. Collections namespace in your program with the help of using keyword: 

using System.Collections;

Step 2: Create a hashtable using Hashtable class as shown below: 

Hashtable hashtable_name = new Hashtable();

Step 3: If you want to add a key/value pair in your hashtable, then use Add() method to add elements in your hashtable. And you can also store a key/value pair in your hashtable without using Add() method. 

Example:

C#

// C# program to illustrate how
// to create a hashtable
using System;
using System.Collections;

class GFG {

    // Main Method
    static public void Main()
    {

        // Create a hashtable
        // Using Hashtable class
        Hashtable my_hashtable1 = new Hashtable();

        // Adding key/value pair
        // in the hashtable
        // Using Add() method
        my_hashtable1.Add("A1", "Welcome");
        my_hashtable1.Add("A2", "to");
        my_hashtable1.Add("A3", "GeeksforGeeks");

        Console.WriteLine("Key and Value pairs from my_hashtable1:");

        foreach(DictionaryEntry ele1 in my_hashtable1)
        {
            Console.WriteLine("{0} and {1} ", ele1.Key, ele1.Value);
        }

        // Create another hashtable
        // Using Hashtable class
        // and adding key/value pairs
        // without using Add method
        Hashtable my_hashtable2 = new Hashtable() {
                                      {1, "hello"},
                                          {2, 234},
                                        {3, 230.45},
                                         {4, null}};

        Console.WriteLine("Key and Value pairs from my_hashtable2:");

        foreach(var ele2 in my_hashtable2.Keys)
        {
            Console.WriteLine("{0}and {1}", ele2,
                            my_hashtable2[ele2]);
        }
    }
}

Output

Key and Value pairs from my_hashtable1:
A3 and GeeksforGeeks 
A2 and to 
A1 and Welcome 
Key and Value pairs from my_hashtable2:
4and 
3and 230.45
2and 234
1and hello

How to remove elements from the hashtable?

In Hashtable, you are allowed to remove elements from the hashtable. The Hashtable class provides two different methods to remove elements and the methods are:

• Clear : This method is used to remove all the objects from the hashtable.
• Remove : This method is used to remove the element with the specified key from the hashtable.

Example:

C#

// C# program to illustrate how
// remove elements from the hashtable
using System;
using System.Collections;

class GFG {

    // Main Method
    static public void Main()
    {

        // Create a hashtable
        // Using Hashtable class
        Hashtable my_hashtable = new Hashtable();

        // Adding key/value pair
        // in the hashtable
        // Using Add() method
        my_hashtable.Add("A1", "Welcome");
        my_hashtable.Add("A2", "to");
        my_hashtable.Add("A3", "GeeksforGeeks");

        // Using remove method
        // remove A2 key/value pair
        my_hashtable.Remove("A2");

        Console.WriteLine("Key and Value pairs :");

        foreach(DictionaryEntry ele1 in my_hashtable)
        {
            Console.WriteLine("{0} and {1} ", ele1.Key, ele1.Value);
        }

        // Before using Clear method
        Console.WriteLine("Total number of elements present"+
                 " in my_hashtable:{0}", my_hashtable.Count);

        my_hashtable.Clear();

        // After using Clear method
        Console.WriteLine("Total number of elements present in"+
                       " my_hashtable:{0}", my_hashtable.Count);
    }
}

Output

Key and Value pairs :
A3 and GeeksforGeeks 
A1 and Welcome 
Total number of elements present in my_hashtable:2
Total number of elements present in my_hashtable:0

How to check the availability of key/value pair in hashtable?

In hashtable, you can check whether the given pair is present or not using the following methods:

• Contains: This method is used to check whether the Hashtable contains a specific key.
• ContainsKey: This method is also used to check whether the Hashtable contains a specific key.
• ContainsValue: This method is used to check whether the Hashtable contains a specific value.

Example:

C#

// C# program to illustrate how
// to check key/value present
// in the hashtable or not
using System;
using System.Collections;

class GFG {

    // Main Method
    static public void Main()
    {

        // Create a hashtable
        // Using Hashtable class
        Hashtable my_hashtable = new Hashtable();

        // Adding key/value pair in the hashtable
        // Using Add() method
        my_hashtable.Add("A1", "Welcome");
        my_hashtable.Add("A2", "to");
        my_hashtable.Add("A3", "GeeksforGeeks");

        // Determine whether the given
        // key present or not
        // using Contains method
        Console.WriteLine(my_hashtable.Contains("A3"));
        Console.WriteLine(my_hashtable.Contains(12));
        Console.WriteLine();

        // Determine whether the given
        // key present or not
        // using ContainsKey method
        Console.WriteLine(my_hashtable.ContainsKey("A1"));
        Console.WriteLine(my_hashtable.ContainsKey(1));
        Console.WriteLine();

        // Determine whether the given
        // value present or not
        // using ContainsValue method
        Console.WriteLine(my_hashtable.ContainsValue("geeks"));
        Console.WriteLine(my_hashtable.ContainsValue("to"));
    }
}

Output

True
False

True
False

False
True

How to update an Hash Table?

In C#, the Hashtable class does not provide a direct method to update the value of an existing key. However, you can achieve the update by following these steps:

1. Check if the key exists in the Hashtable using the ContainsKey method.
2. If the key exists, retrieve the current value using the key and store it in a variable.
3. Assign the new value to the key in the Hash table using the same key.
4. Optionally, remove the old key/value pair if needed.

Here’s an example that demonstrates how to update a value in a Hash table:

C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Create a new dictionary
        Dictionary<string, string> dictionary = new Dictionary<string, string>();

        // Add some key-value pairs
        dictionary.Add("key1", "value1");
        dictionary.Add("key2", "value2");

        // Updating the value of an existing key
        string keyToUpdate = "key1";
        if (dictionary.ContainsKey(keyToUpdate))
        {
            dictionary[keyToUpdate] = "updatedValue";
        }

        // Accessing the updated value
        string updatedValue;
        if (dictionary.TryGetValue(keyToUpdate, out updatedValue))
        {
            Console.WriteLine("Updated value: " + updatedValue);
        }

        // Print all key-value pairs in the dictionary
        foreach (KeyValuePair<string, string> kvp in dictionary)
        {
            Console.WriteLine("Key: " + kvp.Key + ", Value: " + kvp.Value);
        }
    }
}

Output

Updated value: updatedValue
Key: key1, Value: updatedValue
Key: key2, Value: value2