Hashtable

In this article, We will learn about the concept of Hashtable in C#.

• Hashtable is the non-generic collection that stores data in key-value pair, same as generic Dictionary collection.
• In the Hashtable data, key-value pairs are organized based on the key’s hash code.
• A key is used to access value from the Hashtable.
• Basically, a Hashtable comes into the picture when we need to access an element by the key. 
• Each element in the Hashtable has a key-value pair.

Hashtable Characteristics

• Hashtable stores elements in the form of key-value pairs.
• Hashtable comes under the System.Collection namespace.
• Hashtable implements the IDictionary interface. 
• Hashtable key must be unique and cannot be null or empty.
• A value in a Hashtable can be null or empty or duplicate.
• A value in the Hashtable can be accessed by passing the associated key in the indexer.
• An element in the Hashtable is stored as DictionaryEntry objects.

There are a number of methods and properties available in Hashtable. So First, we will see the Method of Hashtable below,

1. public virtual void Add(object key, object value): 

 This method is used to add an element with the specified key-value pairs.

2. public virtual void Remove(object key):

 This method is used to remove an element with the specified key from the Hashtable.

3. public virtual void Clear():

This method is used when we want to remove all elements from the Hashtable.

4. public virtual bool ContainsKey(object key): 

 This method is used to determine whether Hashtable contains a specified key or not.

5. public virtual bool ContainsValue(object value):

This method is used to determine whether Hashtable contains a specified value or not.

Now, we will see the Properties of Hashtable as below,

1. Count:

Get the total number of elements available in Hashtable.

2. Keys:

Gets an ICollection containing the key in the Hashtable.

3. Values:

Gets an ICollection containing the value in the Hashtable.

4. Item:

Gets or sets the value associated with the specified key.

5. IsFixedSize:

This property will get a value indicating whether the Hashtable has a fixed size.

6. IsReadOnly:

This Property will get a value indicating whether the Hashtable is read-only.

Example of  Create Hashtable and Add element.

using System;
using System.Collections;

namespace HashtableExample
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("\n Hashtable Example");

            //Create Hashtable object
            Hashtable Phones = new Hashtable();

            //Add() method of Hashtable
            Phones.Add("Apple", "Iphone 11, Iphone 11 pro, Iphone 12, Iphone 12 mini");
            Phones.Add("Samsung", "Note 9, Note 10, Galaxy s20, Galaxy s20 ultra");
            Phones.Add("Realme", "Realme 6, Realme 6 pro, Realme 7, Realme 7i, Realme GT neo2");

            //Phones.Add("Realme","Realme 1"); it will allow only unique key, so it will give a compile time error

            Phones.Add("Vivo", ""); //Hashtable allow a null or empty or duplicate value

            //Display all Hashtable elements
            Console.WriteLine("\n Display Hastable element after Adding \n");
            foreach(DictionaryEntry Phone in Phones)
            {
                Console.WriteLine(" Key : {0}  ,  Value : {1}",Phone.Key,Phone.Value);
            }
        }
    }
}

The output of an above Example is as below,

In the above example, We can see that we are trying to add a duplicate key to Hashtable. But, As we know that the Hashtable will not allow adding a duplicate or null key, It allows only a unique key. But, we can add the null or empty, or duplicate value of an element in Hashtable.

Although, if we are trying to add a duplicate or null key. Then, it will give a compile-time error as below,

Example of Remove element from Hashtable,

//Remove Method of Hashtable
  if (Phones.ContainsKey("Vivo")) //It will chek whether it will contain an element of specified key.
  {
     Phones.Remove("Vivo"); //It will remove an element that contain a specified key "Vivo"
  }
  
//Display all Hashtable elements after Remove an element

  Console.WriteLine("\n Display Hastable element after Remove \n");
  foreach (DictionaryEntry Phone in Phones)
  {
    Console.WriteLine(" Key : {0}  ,  Value : {1}", Phone.Key, Phone.Value);
  }

The Output of the above example,

In the above example, we use the ContainsKey() method for checking the availability of a specified key in the Hashtable.

Example of how to update the element value,

//Update the value of specified key
  Phones["Apple"] = "Iphone 13 pro max";

 //Display all Hashtable elements after updating an element
 Console.WriteLine("\n Display Hastable element after Update \n");
 foreach (DictionaryEntry Phone in Phones)
 {
    Console.WriteLine(" Key : {0}  ,  Value : {1}", Phone.Key, Phone.Value);
 }

The output of the above example is as below,

Advantages 

• The main advantage of Hashtable is synchronization.
• It allows the execution time of lookup, retrieve, and set operations to remain nearly constant, even for a large number of sets.
• Hashtable has the ability to locate the element quickly even when the amount of data is huge.
• There is no requirement to scan through the entire data set to find an element or to sort the elements in order to perform a Binary search.
• The key can be hashed and the value’s location can be found directly if the key for the desired element value is known.

Disadvantages

• Hashtable collisions are practically unavoidable. when hashing a random subset of a large set of possible keys.
• Hashtable becomes quite inefficient when there are many collisions occur.

I hope this article will help you to understand the concept of HAshtable in C#.

Thank you.