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C# | Intersection of two HashSets

C# | Intersection of two HashSets

Last Updated : 11 Jul, 2025

A

HashSet

is an unordered collection of the unique elements. It is found in

System.Collections.Generic

namespace. It is used in a situation where we want to prevent duplicates from being inserted in the collection. As far as performance is concerned, it is better in comparison to the list.

HashSet .IntersectWith(IEnumerable ) Method

is used to modify the current

HashSet

object to contain only elements that are present in that object and in the specified collection.

Syntax: 
mySet1.IntersectWith(mySet2)

Here

mySet1

and

mySet2

are the two HashSets.

Exception:

This method will give

ArgumentNullException

if the HashSet is

null

. Below given are some examples to understand the implementation in a better way:

Example 1: CSHARP 
// C# code to find Intersection 
//of two HashSets
using System;
using System.Collections.Generic;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a HashSet of integers
        HashSet<int> mySet1 = new HashSet<int>();

        // Creating a HashSet of integers
        HashSet<int> mySet2 = new HashSet<int>();

        // Inserting even numbers less than
        // equal to 10 in HashSet mySet1
        Console.WriteLine("Elements in Set 1 :");
        
        for (int i = 0; i < 5; i++) {
            mySet1.Add(i * 2);
            Console.WriteLine(i * 2);
        }

        // Inserting odd numbers less than
        // equal to 10 in HashSet mySet2
        Console.WriteLine("Elements in Set 2 : ");
        for (int i = 0; i < 5; i++) {
            mySet1.Add(i * 2 + 1);
            Console.WriteLine(i *2 + 1);
        }

        // Creating a new HashSet that contains
        // the Intersection of both the HashSet mySet1 & mySet2
        HashSet<int> ans = new HashSet<int>(mySet1);

        ans.IntersectWith(mySet2);

        // Printing the Intersection of both the HashSets
        // It should show no element in the output
        // as there is no element common in both
        // the HashSets
        foreach(int i in ans)
        {
            Console.WriteLine(i);
        }
    }
}
Output: 
Elements in Set 1 :
0
2
4
6
8
Elements in Set 2 : 
1
3
5
7
9
Example 2: CSHARP 
// C# code to find Intersection
// of two HashSets
using System;
using System.Collections.Generic;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a HashSet of strings
        HashSet<string> mySet1 = new HashSet<string>();

        // Creating a HashSet of strings
        HashSet<string> mySet2 = new HashSet<string>();

        // Inserting elements in mySet1
        mySet1.Add("Hello");
        mySet1.Add("Geeks");
        mySet1.Add("GeeksforGeeks");

        // Inserting elements in mySet2
        mySet2.Add("Geeks");
        mySet2.Add("and");
        mySet2.Add("GeeksforGeeks");
        mySet2.Add("are");
        mySet2.Add("the");
        mySet2.Add("best");

        // Creating a new HashSet that contains
        // the Intersection of both the HashSet mySet1 & mySet2
        HashSet<string> ans = new HashSet<string>(mySet1);

        ans.IntersectWith(mySet2);

        // Printing the Intersection of both the HashSet
        foreach(string i in ans)
        {
            Console.WriteLine(i);
        }
    }
}
Output: 
Geeks
GeeksforGeeks
Reference:

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