C# | Intersection of two HashSets

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:

// 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:

// 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:

• https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.hashset-1.intersectwith?view=netframework-4.7.2