C# | Create HashSet from another collection

Last Updated : 22 Feb, 2023

In C#, you can create a HashSet<T> from another collection by using the HashSet<T> constructor that takes an IEnumerable<T> parameter. Here's an example:

C# 
using System;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        // create a List of integers
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

        // create a HashSet from the List
        HashSet<int> numberSet = new HashSet<int>(numbers);

        // print out the contents of the HashSet using a foreach loop
        Console.WriteLine("Contents of HashSet:");
        foreach (int number in numberSet)
        {
            Console.WriteLine(number);
        }
    }
}

Output
Contents of HashSet:
1
2
3
4
5

A HashSet is an unordered collection of the unique elements. It comes under 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. You can create a HashSet from another collection by passing the collection as an argument while creating the object of HashSet. Below given are some examples to understand the implementation in a better way: Example 1: 

CSHARP 
// C# code to Create HashSet
// from another collection
using System;
using System.Collections.Generic;

class GFG {

    // Driver code
    public static void Main()
    {

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

        // Inserting even numbers less than
        // equal to 10 in HashSet mySet
        for (int i = 0; i < 5; i++) {
            mySet.Add(i * 2);
        }

        // Creating new HashSet mySet_new from already
        // Created HashSet mySet
        HashSet<int> mySet_new = new HashSet<int>(mySet);

        Console.WriteLine("The elements in newly created HashSet are : ");

        // Displaying the elements of newly created HashSet
        foreach(int i in mySet_new)
        {
            Console.WriteLine(i);
        }
    }
}

Example 2: 

CSHARP 
// C# code to Create HashSet
// from another collection
using System;
using System.Collections.Generic;

class GFG {

    // Driver code
    public static void Main()
    {

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

        // Inserting elements into HashSet mySet
        mySet.Add("Delhi");
        mySet.Add("Noida");
        mySet.Add("Chandigarh");
        mySet.Add("New York");
        mySet.Add("Bangalore");

        // Creating new HashSet mySet_new from already
        // Created HashSet mySet
        HashSet<string> mySet_new = new HashSet<string>(mySet);

        Console.WriteLine("The elements in newly created HashSet are : ");

        // Displaying the elements of newly created HashSet
        foreach(string i in mySet_new)
        {
            Console.WriteLine(i);
        }
    }
}
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