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HashSet Collection in C#

Updated on Sep 3, 2025
 
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An unordered collection of unique items is known as a HashSet. It is found in System.Collections.Generic namespace. Some of the features of HashSet Collection are as follows:

  1. The HashSet Collection provides operations with high performance.
  2. There are no duplicate elements in the HashSet Collection.
  3. There is no maximum capacity for the elements in the HashSet. The capacity increases as the number of number of elements increase.
  4. There is no particular order of the elements in the HashSet Collection.

Constructors in HashSet Collection

The different constructors and their description is given as follows:

Constructors

Description

HashSet <T>()

This constructor initializes a new instance of HashSet <T> class. This instance is empty and it uses the default equality comparer.

HashSet<T>(IEnumerable<T>)

This constructor initializes a new instance of HashSet <T> class. It contains elements copied from the specified collection with a capacity to accomodate all elements copied.

IEqualityComparer<T>)

HashSet <T> class. It uses the specified equality comparer that contains elements from the specified collection.

HashSet<T>(IEqualityComparer<T>)

This constructor initializes a new instance of HashSet <T> class. This instance is empty and it uses the specified equality comparer.

HashSet<T>(IEnumerable<T>,

HashSet<T>(Int32)

This constructor initializes a new instance of HashSet <T> class. This instance is empty but it has a reserved space for capacity items. It uses the default equality comparer.

HashSet<T>(Int32, IEqualityComparer<T>)

This constructor initializes a new instance of HashSet <T> class. It can accomodate the capacity elements and uses the specified quality compare.

HashSet<T>(SerializationInfo, StreamingContext)

This constructor initializes a new instance of HashSet <T> class using serialized data.

Table: Constructors in HashSet Collection in C#
Source: MSDN

Properties in HashSet Collection

The different properties and their description is given as follows:

Properties

Description

Comparer

This property gets the IEqualityComparer<T> object. This object is used to determine equality for the values in the set.

Count

This property gets the number of elements that are contained in a set.

Table: Properties in HashSet Collection in C#
Source: MSDN

Methods in HashSet Collection

The different methods and their description is given as follows:

Methods

Description

Add(T)

This method adds the specified element to the set.

Clear()

This method removes all the elements from a HashSet<T> object.

Contains(T)

This method determines whether a HashSet<T> object contains the specified element.

CopyTo(T[])

This method copies the elements of a HashSet<T> object to an array.

CreateSetComparer()

This method returns an IEqualityComparer object. this object can be used for the equality testing of a HashSet<T> object.

Equals(Object)

This method determines if the specified object is equal to the current object or not.

GetEnumerator()

This method returns an enumerator that iterates through a HashSet<T> object.

GetHashCode()

This method is the default hash function.

GetType()

This method gets the Type of the current instance.

IntersectWith(IEnumerable<T>)

This method modifies the current HashSet<T> object to contain only those elements that are present in that object and in the specified collection.

MemberwiseClone()

This method creates a shallow copy of the current Object.

Remove(T)

This method removes the specified element from the HashSet<T> object.

RemoveWhere(Predicate<T>)

This method removes all the elements that match the conditions that are defined by a specified predicate from a HashSet<T> collection.

SetEquals(IEnumerable<T>)

This method determines if a HashSet<T> object and the specified collection contain the same elements.

SymmetricExceptWith(IEnumerable<T>)

This method modifies the current HashSet<T> object to contain only those elements that are present either in that object or in the specified collection, but not in both.

ToString()

This method returns a string that represents the current object.

TrimExcess()

This method sets the capacity of a HashSet<T> object to the actual number of elements that it contains, rounded up to a nearby, implementation-specific value.

TryGetValue(T, T)

This method searches the set for a given value and returns the equal value if it finds it.

UnionWith(IEnumerable<T>)

This method modifies the current HashSet<T> object to contain all the elements that are present in itself, the specified collection, or both.

Table: Methods in HashSet Collection in C#
Source: MSDN

HashSet Operations

Some of the operations in a HashSet are given as follows:

Adding Elements in HashSet

The Add() method can be used to add elements into a HashSet. The program that demonstrates this is given as follows:

using System;
using System.Collections.Generic;
namespace HashSetDemo
{   
    class Example
     {
        static void Main(string[] args)
       {
           HashSet<int> set1 = new HashSet<int>();

           for (int i = 1; i <= 10; i++)
           {
               set1.Add(i);
           }

           Console.WriteLine("The first 10 natural numbers are:");
           foreach (int i in set1)
           {
               Console.WriteLine(i);
           }
       }
   }
}

Source Code: Program to add elements in HashSet in C#

The output of the above program is given below:

The first 10 natural numbers are:
1
2
3
4
5
6
7
8
9
10

Printing Elements of HashSet

The elements of the Hashset can be printed using the foreach loop. A program for this is given below:

using System;
using System.Collections.Generic;
namespace HashSetDemo
{   class Example
   { static void Main(string[] args)
       {
           HashSet<int> set1 = new HashSet<int>();
           for (int i = 1; i <= 10; i++)
           {
               set1.Add(i);
           }
           Console.WriteLine("The even numbers in the HashSet are:");
           foreach (int i in set1)
           {
               if(i%2 == 0)
               {
                   Console.WriteLine(i);
               }
           }
       }
   }
}

Source Code: Program to print elements in HashSet in C#

The output of the above program is given below:

The even numbers in the HashSet are:
2
4
6
8
10

Removing Elements from HashSet

The Remove() method can be used to remove elements from a HashSet. The program that demonstrates this is given as follows:

using System;
using System.Collections.Generic;
namespace HashSetDemo
{   class Example
   { static void Main(string[] args)
       {
           HashSet<int> set1 = new HashSet<int>();

           for (int i = 1; i <= 10; i++)
           {
               set1.Add(i);
           }
           Console.WriteLine("The elements in the HashSet are:");
           foreach (int i in set1)
           {
               Console.WriteLine(i);
           }
           for (int i = 1; i <= 10; i++)
           {
               if(i%2 == 0)
               {
                   set1.Remove(i);
               }
           }
           Console.WriteLine("The odd elements in the HashSet are:");
           foreach (int i in set1)
           {
               Console.WriteLine(i);
           }
       }
   }
}

Source Code: Program to remove elements in HashSet in C#

The output of the above program is given below:

The elements in the HashSet are:
1
2
3
4
5
6
7
8
9
10
The odd elements in the HashSet are:
1
3
5
7
9

Finding an element in HashSet

The Contains method finds if an element is present in HashSet or not. If it is present, Contains() returns TRUE and otherwise it returns FALSE.

The program that demonstrates this is given as follows:

using System;
using System.Collections.Generic;
namespace HashSetDemo
{   class Example
   { static void Main(string[] args)
       {
           HashSet<int> set1 = new HashSet<int>();
           for (int i = 1; i <= 10; i++)
           {
               set1.Add(i);
           }
           Console.WriteLine("The elements in the HashSet are:");
           foreach (int i in set1)
           {
               Console.WriteLine(i);
           }
           Console.WriteLine("5 is present in HashSet: " + set1.Contains(5));
           Console.WriteLine("50 is present in HashSet: " + set1.Contains(50));
       }
   }
}

The output of the above program is given below:

The elements in the HashSet are:
1
2
3
4
5
6
7
8
9
10

5 is present in HashSet: True
50 is present in HashSet: False
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