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Table of contents
1.
Introduction
2.
What are hashmaps? 
3.
What are HashMaps in C#
3.1.
Implementation of Hashtables in C#
4.
Implementing a HashMap in C#
5.
Features of HashMap in C#
6.
Alternative of Hashmap in C#
7.
Advantages and Disadvantages of Hashmap in C#
8.
Difference between HashMap and Hashtable in C#
9.
Frequently Asked Questions
9.1.
What is the equivalent of Java HashMap in C#?
9.2.
What is the internal working of HashMap in C#?
9.3.
Is HashMap thread safe C#?
9.4.
Is HashMap faster than Hashtable?
9.5.
Is HashMap better than Hashtable?
9.6.
Which is better Hashtable or Dictionary C#?
10.
Conclusion
Last Updated: Mar 27, 2024
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HashMap in C#

Author Nagendra
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Data structures & algorithms (Beginner to Intermediate)
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Introduction

Hey Ninjas!! Welcome to another article on C#. Today we will learn about C# hashmap. C# is a high-level object-oriented programming language. It allows developers to create a wide variety of .Net compatible apps. These apps are secure and reliable.

Hashmap in C#

The blog explains the details of C# hashmap, HashTable, the alternative of C# Hashmap and the difference between HashTable and Dictionary in C#.

What are hashmaps? 

A HashMap is a data structure used for storing data in key-value pairs. The hashmap data structure uses HashTable for storing data. HashMap provides fast lookups using keys. HashMap insertion and retrieval take O(1) time complexity. The space complexity of using HashMap is O(n).

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What are HashMaps in C#

Unlike Java, C# does not have the HashMap class. The Hashtable class is used for implementing the C# hashmap. A Hashtable is a non-generic collection that stores key-value pairs. It is also implemented using a hash table. It is defined in the System.Collections namespace. HashTable allows us to store different data types. The key and value can have different data types.

Syntax:

public class Hashtable : IDictionary, ICollection, IEnumerable

 

The Hashtable class implements the IDictionary, ICollection, IEnumerable, IDeserializationCallback, ISerializable, and ICloneable interfaces. The Hashtable requires a unique key. The key cannot contain a null value.

Implementation of Hashtables in C#

Let's understand the implementation of HashTables in C#. 

Code:

using System;
using System.Collections;

namespace Coding Ninjas Studio
{
    class CodingNinjas
    {
        static void Main(string[] args)
        {
            // Creating a new hash table
            Hashtable hashtable = new Hashtable();

            // Add key-value pairs
            hashtable.Add("Jharkhand", "Ranchi");
            hashtable.Add("Bihar", "Patna");
            hashtable.Add("Rajasthan", "Jaipur");
            hashtable.Add("Gujrat", "Gandhinagar");
            hashtable.Add("Maharashtra", "Mumbai");   
                     
            //printing the key and values
            foreach(DictionaryEntry element in hashtable)
            {
                Console.WriteLine("The value at key {0} is {1}", element.Key, element.Value);
            }
        }
    }
}


Output:

The value at key Maharashtra is Mumbai
The value at key Bihar is Patna
The value at key Gujrat is Gandhinagar
The value at key Rajasthan is Jaipur
The value at key Jharkhand is Ranchi


Explanation:

In the above program, we had created a HashTable and added values to it. The DictionaryEntry is used for accessing the elements of the HashTable. It accesses the elements of the HashTable using the element.Key and values using element.Value.

Must Read IEnumerable vs IQueryablehash function in data structure.

Implementing a HashMap in C#

Implementing a HashMap in C# involves creating a data structure that allows for efficient storage and retrieval of key-value pairs. Here's a step-by-step explanation of how you can implement a basic HashMap in C#:

The step-by-step implementation of HashMap in C# is:

  1. Define the HashMap Class: Start by creating a class to represent the HashMap. This class will contain methods for adding, retrieving, and removing key-value pairs, as well as managing the underlying data structure.
  2. Choose a Data Structure: In C#, you can implement a HashMap using an array of linked lists or an array of arrays (buckets). Each element in the array corresponds to a bucket, and each bucket holds a linked list of key-value pairs that hash to the same index.
  3. Hash Function: Implement a hash function that maps keys to indices in the array. This function should distribute keys evenly across the array to minimize collisions.
  4. Collision Handling: Handle collisions by using a chaining technique. If multiple keys hash to the same index, store them in a linked list within the corresponding bucket.
  5. Add Method: Implement a method to add key-value pairs to the HashMap. Compute the hash value of the key, determine the index in the array, and add the key-value pair to the linked list at that index.
  6. Get Method: Implement a method to retrieve the value associated with a given key. Compute the hash value of the key, determine the index in the array, and search the linked list at that index for the key.
  7. Remove Method: Implement a method to remove key-value pairs from the HashMap. Compute the hash value of the key, determine the index in the array, and remove the key-value pair from the linked list at that index.
  8. Handle Resizing: Implement logic to resize the array when the load factor (the ratio of the number of elements to the size of the array) exceeds a certain threshold. Resizing involves creating a new array with a larger size, rehashing existing elements, and transferring them to the new array.

Features of HashMap in C#

The features of HashMap in C# are:

  • Fast Access: HashMaps provide fast access to elements based on their keys. The time complexity for insertion, retrieval, and deletion operations is O(1) on average, assuming a good hash function and minimal collisions.
  • Dynamic Sizing: HashMaps automatically resize themselves to accommodate a varying number of elements. This ensures optimal space utilization and prevents performance degradation due to a large number of collisions.
  • Flexible Key Types: HashMaps in C# allow for keys of various types, including primitive types, custom objects, and strings.
  • Key-Value Pair Storage: HashMaps store data in the form of key-value pairs, allowing efficient association between keys and their corresponding values.
  • Collision Resolution: HashMaps handle collisions gracefully by using chaining or open addressing techniques. Chaining involves storing multiple key-value pairs at the same index in the array, while open addressing involves probing for alternative locations within the array.
  • High Performance: HashMaps offer high performance for associative array operations, making them suitable for a wide range of applications, including caching, indexing, and data retrieval.

Alternative of Hashmap in C#

C# comes with an alternative of hashmaps. C# Hashmaps are also used using Dictionary. The Dictionary is a generic collection that stores key-value pairs. The Dictionary class is part of the System.Collections.Generic namespace. Unlike HashTables, you cannot store different data types in a dictionary. The key and value must be of the same data type. 

Alternative of Hashmap in C# image

Syntax:

public class Dictionary<TKey, TValue> : IDictionary<TKey, TValue>, IReadOnlyDictionary<TKey, TValue>, ICollection<KeyValuePair<TKey, TValue>>, IReadOnlyCollection<KeyValuePair<TKey, TValue>>, IEnumerable<KeyValuePair<TKey, TValue>>, IEnumerable


Let's understand the implementation of Dictionary in C#.

Code:

using System;
using System.Collections.Generic;


namespace Coding Ninjas Studio
{
    class CodingNinjas
    {
        static void Main(string[] args)
        {
            // Creating a new dictionary
            Dictionary<string, string> dictionary = new Dictionary<string, string>();

            // Add key-value pairs
            dictionary.Add("Jharkhand", "Ranchi");
            dictionary.Add("Bihar", "Patna");
            dictionary.Add("Rajasthan", "Jaipur");
            dictionary.Add("Gujrat", "Gandhinagar");
            dictionary.Add("Maharashtra", "Mumbai");
            
            //printing the key and values
            foreach(KeyValuePair<string, string> item in dictionary)
            {
                Console.WriteLine("The value at key {0} is {1}", item.Key, item.Value);
            }
        }
    }
}


Output:

The value at key Jharkhand is Ranchi
The value at key Bihar is Patna
The value at key Rajasthan is Jaipur
The value at key Gujrat is Gandhinagar
The value at key Maharashtra is Mumbai

 

Explanation:

In the above program, we had created a dictionary and added values to it. The KeyValuePair is used for accessing the elements of the Dictionary. It accesses the elements of the Dictionary using item.Key and values using item.Value.

Advantages and Disadvantages of Hashmap in C#

The advantages of C# hashmaps are as follows:

dvantages of Hashmap image
  • Hashmaps allow you to quickly look up and retrieve values based on a key. The average time complexity is O(1).
     
  •  Hashmaps also allows you to insert key-value pairs quickly. The average time complexity is O(1).
     
  • Hashmaps allow you to use any data object as a key.
     
  • Unlike arrays, you do not need to specify the size of a hashmap.
     

The disadvantages of C# hashmaps are as follows:

disadvantages of C# hashmaps image
  • Hashmaps do not preserve the order in which elements are inserted.
     
  • Hashmaps are not thread-safe. It means that if multiple threads try to access and modify the hashmap simultaneously, it can lead to undefined behaviour.

Difference between HashMap and Hashtable in C#

Here's a comparison of HashMap and Hashtable in C#:

Feature HashMap Hashtable
Implementation Introduced in .NET Framework 2.0 as part of the Collections.Generic namespace. Available since early versions of .NET Framework. Part of the System.Collections namespace.
Thread Safety Not inherently thread-safe. Requires synchronization for concurrent access. Hashtable is thread-safe for read operations but not for write operations. Concurrent access can lead to exceptions or data corruption.
Null Values Allows one null key and multiple null values. Does not allow null keys or values.
Iteration Order Does not guarantee any specific iteration order. Does not guarantee any specific iteration order.
Performance Slightly faster due to lack of synchronization overhead. Slower due to built-in synchronization mechanisms.
Use of Enumerator Can use the generic enumerator provided by the IEnumerable interface. Uses the non-generic enumerator provided by the IDictionary interface.
Type Safety Provides type safety through generics. Does not provide type safety; relies on object references.
Extensibility More extensible and flexible, supports LINQ queries and lambda expressions. Limited extensibility compared to HashMap.
Availability Available in .NET Core and .NET Framework. Available in .NET Framework but marked as deprecated.

Frequently Asked Questions

What is the equivalent of Java HashMap in C#?

The equivalent of Java HashMap in C# is Dictionary<TKey, TValue>.

What is the internal working of HashMap in C#?

HashMap in C# uses hashing to store and retrieve key-value pairs efficiently in an array of buckets.

Is HashMap thread safe C#?

HashMap in C# is not inherently thread-safe and requires synchronization for concurrent access.

Is HashMap faster than Hashtable?

HashMap is generally faster than Hashtable due to reduced synchronization overhead.

Is HashMap better than Hashtable?

HashMap is preferred over Hashtable due to its improved performance and modern features.

Which is better Hashtable or Dictionary C#?

Dictionary<TKey, TValue> is better than Hashtable due to its type safety, extensibility, and compatibility with modern C# features.

Also Read About - singleton design pattern in c#

Conclusion

In this article, we have discussed the details of C# hashmap, HashTable, the alternative of C# Hashmap and the difference between HashTable and Dictionary in C#.

We hope that the blog has helped you enhance your knowledge regarding C# hashmaps and if you would like to learn more, check out our articles on HashMap. You can refer to our guided paths on the Coding Ninjas Studio platform to learn more about DSADBMSCompetitive ProgrammingPythonJavaJavaScript, etc. To practice and improve yourself in the interview, you can also check out Top 100 SQL problemsInterview experienceCoding interview questions, and the Ultimate guide path for interviews. Do upvote our blogs to help other ninjas grow. Happy Coding!!

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