C# | Check if a HashSet is a proper subset of the specified collection

Last Updated : 11 Jul, 2025
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. HashSet<T>.IsProperSubsetOf(IEnumerable<T>) method is used to check whether a HashSet<T> object is a proper subset of the specified collection. Syntax: 
mySet1.IsProperSubsetOf(mySet2);
Here, mySet1 and mySet2 are two HashSets. Return Value: This method returns True if mySet1 is proper subset of mySet2 otherwise it returns false. Exception: This method will give ArgumentNullException if the HashSets is null. Below given are some examples to understand the implementation in a better way: Example 1: CSHARP 
// C# code to Check if a HashSet is a proper
// subset of the specified collection
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 in HashSet
        mySet1.Add("Geeks");
        mySet1.Add("GeeksQuiz");

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

        // Inserting elements in HashSet
        mySet2.Add("DS");
        mySet2.Add("C++");
        mySet2.Add("Java");
        mySet2.Add("JavaScript");
        mySet2.Add("GeeksQuiz");
        mySet2.Add("Geeks");

        // Check if a HashSet is a proper
        // subset of the specified collection
        Console.WriteLine(mySet1.IsProperSubsetOf(mySet2));
    }
}
Output: 
True
Example 2: CSHARP 
// C# code to Check if a HashSet is a proper
// subset of the specified collection
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 in HashSet
        mySet1.Add(4);
        mySet1.Add(8);
        mySet1.Add(12);
        mySet1.Add(16);

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

        // Inserting elements in HashSet
        mySet2.Add(4);
        mySet2.Add(8);
        mySet2.Add(15);
        mySet2.Add(20);

        // Check if a HashSet is a proper
        // subset of the specified collection
        Console.WriteLine(mySet1.IsProperSubsetOf(mySet2));
    }
}
Output: 
False
Reference:
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