What is the SetEquals() method in C#?

Syntax

public bool SetEquals (System.Collections.Generic.IEnumerable<T> other);

Parameters

This method takes a collection IEnumerable<T> as input and compares its elements to the HashSet elements.

Return value

SetEquals() returns true if the HashSet and input collection contain the same elements, and returns false otherwise.

Things to note

The SetEquals() method ignores duplicate items when comparing set equality.
The order of elements is also ignored.
SetEquals() is an $O(n+m)$ operation, where $n$ is the count of elements in the passed collection of elements and $m$ is the number of elements in the HashSet<T>.
In a special case where the passed input collection is also a HashSet<T> with the same equality comparer as the current HashSet<T> object, the method becomes an $O(n)$ operation, where $n$ is the number of elements in the passed input collection.
SetEquals() throws an exception if the input collection is null.
This method does not change the state of the HashSet<T>.

using System;
using System.Collections.Generic;
class HashSetEquals
{
    static void Main()
    {
        HashSet<int> numSet = new HashSet<int>();
        numSet.Add(1);
        numSet.Add(5);
        numSet.Add(9);
        numSet.Add(11);
        numSet.Add(15);
        numSet.Add(6);
        Console.WriteLine("HashSet Elements : ");
        PrintSet(numSet);
        List<int> otherList = new List<int>{9,11,15,6,5,1};
        Console.WriteLine("Input Collection 1 : {0}", string.Join(" ", otherList.ToArray()));
        bool isSetEquals = numSet.SetEquals(otherList);
        Console.WriteLine($"Is HashSet equal to Input Collection 1 ?: {isSetEquals}");
        
        otherList = new List<int>{9,11,23,6,5,1};
        Console.WriteLine("Input Collection 2 : {0}", string.Join(" ", otherList.ToArray()));
        isSetEquals = numSet.SetEquals(otherList);
        Console.WriteLine($"Is HashSet equal to Input Collection 2 ? : {isSetEquals}");
    }
    private static void PrintSet(HashSet<int> set)
    {
      Console.Write("{");
      foreach (int i in set)
      {
          Console.Write(" {0}", i);
      }
      Console.WriteLine(" }");
    }
}

Explanation

In the example above, we create a HashSet of integers and add a few numbers to it.
We also create two lists of integers, one with the same elements as the HashSet and another with a few different elements.
We call SetEquals() with the first list of integers as input. Since the list has the same elements as the HashSet, SetEquals() returns True.
We call SetEquals() again with the second list of integers as input. Since the second list has a few different elements from the HashSet, SetEquals() returns false.
We print the elements of the HashSet and both lists.

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