C# | Remove all elements in a collection from a HashSet

In C#, you can use the ExceptWith() method to remove all the elements in a collection from a HashSet. The ExceptWith() method removes all the elements in the specified collection from the current HashSet.

Here is an example code that demonstrates how to use the ExceptWith() method to remove all the elements in a collection from a HashSet:

C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        HashSet<int> set1 = new HashSet<int> { 1, 2, 3, 4, 5 };
        HashSet<int> set2 = new HashSet<int> { 2, 4 };

        Console.WriteLine("Before removing elements from set1:");
        foreach (int num in set1)
        {
            Console.Write(num + " ");
        }
        Console.WriteLine();

        set1.ExceptWith(set2);

        Console.WriteLine("After removing elements from set1:");
        foreach (int num in set1)
        {
            Console.Write(num + " ");
        }
        Console.WriteLine();
    }
}

Output

Before removing elements from set1:
1 2 3 4 5 
After removing elements from set1:
1 3 5 

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.ExceptWith(IEnumerable) Method is used to remove all elements in the specified collection from the current HashSet object. Syntax:

mySet2.ExceptWith(mySet1)

Here, mySet1 and mySet2 are the two HashSets and the function returns the elements of mySet2 which are not in mySet1. 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 remove all elements
// in a collection from a HashSet
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>();

        // Inserting elements into HashSet mySet1
        for (int i = 0; i < 7; i++) {
            mySet1.Add(i);
        }

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

        // Inserting elements into HashSet mySet2
        for (int i = 4; i < 11; i++) {
            mySet2.Add(i);
        }

        // Removing all elements in a collection from a HashSet
        mySet2.ExceptWith(mySet1);

        // Printing the elements in mySet2
        // It only prints the elements which are
        // in mySet2 and not in mySet1
        foreach(int i in mySet2)
        {
            Console.WriteLine(i);
        }
    }
}

Example 2:

CSHARP

// C# code to remove all elements
// in a collection from a HashSet
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>();

        // Inserting elements into HashSet mySet1
        mySet1.Add("Punjab");
        mySet1.Add("Haryana");
        mySet1.Add("Jammu");
        mySet1.Add("Himachal");
        mySet1.Add("Delhi");

        // Displaying all elements in mySet1
        Console.WriteLine("The elements in mySet1 are : ");

        foreach(string i in mySet1)
        {
            Console.WriteLine(i);
        }

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

        // Inserting elements into HashSet mySet2
        mySet2.Add("Bangalore");
        mySet2.Add("Kerala");
        mySet2.Add("Uttar Pradesh");
        mySet2.Add("Himachal");
        mySet2.Add("Delhi");

        // Displaying all elements in mySet2
        Console.WriteLine("The elements in mySet2 are : ");

        foreach(string i in mySet2)
        {
            Console.WriteLine(i);
        }

        // Removing all elements in a collection from a HashSet
        mySet2.ExceptWith(mySet1);

        // Printing the elements in mySet2
        // It only prints the elements which are
        // in mySet2 and not in mySet1

        Console.WriteLine("The elements in mySet2 are : ");

        foreach(string i in mySet2)
        {
            Console.WriteLine(i);
        }
    }
}

Reference:

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