HashMap in C#

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 IQueryable, hash 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. 

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.

HashMaps in Real-World Applications

Hashmaps have a major advantage. For insert, delete, and search the time complexity is O(1). Because of this hashmaps is used in various applications such as finding duplicates, searching for distinct elements, and finding the frequency of elements. 

1. File Systems: To efficiently index and retrieve file metadata, file systems use hash maps. Every file has a unique key or identifier that makes it possible to quickly retrieve information about it, including its name, size, rights, and disk location. The efficiency of file system operations is improved by hash maps, which allow for quick file lookup and administration activities.
    
2. Password Verification: Hash Maps are used in authentication systems to safely store user credentials. Hashed passwords are kept as matching values, and keys are either usernames or email addresses. The system hashes the password entered at login and compares it to the hash that is kept in the Hash Map. This method preserves user privacy while guaranteeing quick and safe password verification. 
    
3. Compiler Operation: Hash Maps are essential to the administration of symbol tables in compilers. Identifiers found in the source code, including variable names, function names, and type names, are saved in a symbol table throughout the compilation process. Hash Maps are a useful tool for effectively mapping identifiers to associated properties, including memory addresses, data types, and scope details. 
    
4. Graph Algorithms: Dictionaries are used to store adjacency lists and node properties efficiently in graph algorithms such as Dijkstra's algorithm and A* search, which facilitates graph traversal and manipulation.

Best Practices for Implementing a HashMap in C#

Gaining knowledge about how to avoid typical mistakes is half the fight won. Nonetheless, you can get the resources you need for C# HashMap implementation by following best practices. Some of the best practices are described below: 

• Make Use of Proper Types as Keys: Select key types that offer effective equality comparison and hashing. In order to ensure consistent behavior and performance, immutable types or types with reliable hash codes are preferred. Mutable types and types with mutable key properties should not be used since they can cause unpredictable behavior and performance problems.
   
• Avoid unnecessary resizing: Reduce the number of times the HashMap needs to be resized by choosing a suitable load factor and starting capacity. Optimizing them can enhance performance because resizing operations are costly because they require rehashing every part. Keep an eye on the HashMap's load factor and adjust it proactively as necessary to strike a balance between performance and memory utilization.
   
• Effectively handle null values: If your hash map permits it, provide appropriate handling for null values. Make sure that the absence of a key and null values can be distinguished by your hash map implementation. To express null, for instance, use boolean values or nullable value types (Nullable).

Advantages and Disadvantages of Hashmap in C#

The advantages of C# hashmaps are as follows:

• 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:

• 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#:

Frequently Asked Questions

Does C# have HashMap?

Yes, C# offers a data structure called Dictionary that is comparable to Java's HashMap in that it makes key-value pair storing and retrieval easier.

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#, Regular Expressions in C# , HashSet 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 DSA, DBMS, Competitive Programming, Python, Java, JavaScript, etc. To practice and improve yourself in the interview, you can also check out Top 100 SQL problems, Interview experience, Coding interview questions, and the Ultimate guide path for interviews. Do upvote our blogs to help other ninjas grow. Happy Coding!!