How to check if HashSet and collection share common items in C#

The HashSet<T> generic class in the System.Collections.Generic namespaceIn C-sharp, we use namespaces to organize too many classes and handle applications easily. provides the Overlaps() method, which is used to determine if a HashSet<T> and another IEnumerable<T> share common elements.

Syntax

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

  • The Overlaps() method takes an IEnumerable<T> collectionspecialized classes for data storage and retrieval classes. as input, and returns True if the HashSet<T> and the input collection have at least one element in common.

  • It returns False if there are no common elements between the HashSet<T> and the input collection.

Determining set Overlap in HashSet with Overlaps() method

Things to note

  • This method is an O(n) operation, where n counts elements in the HashSet<T>.

  • The Overlap() method throws an exception if the input collection is null.

  • This method does not change the state of the HashSet<T>.

The linear time complexity O(n) means that any particular algorithm takes proportionally more time to finish as the input grows.

Code

In the example below, we create a HashSet of integers and add a few numbers.

We also create two Lists of integers, one that has common elements with the HashSet, and another with no common elements.

using System;
using System.Collections.Generic;
class HashSetOverlap {
    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};
        Console.WriteLine("Input List 1: {0}",
        string.Join(" ", otherList.ToArray()));
        bool hasCommonElem = numSet.Overlaps(otherList);
        Console.WriteLine("Does HashSet share common elements with List 1? : {0}", hasCommonElem);
        otherList = new List<int>{29,34,90};
        Console.WriteLine("Input List 2: {0}", 
        string.Join(" ", otherList.ToArray()));
        
        hasCommonElem = numSet.Overlaps(otherList);
        Console.WriteLine("Does HashSet share common elements with List 2 ? : {0}", hasCommonElem);
    }
    private static void PrintSet(HashSet<int> set) {
      Console.Write("{");
      foreach (int i in set) {
          Console.Write(" {0}", i);
      }
      
      Console.WriteLine(" }");
    }
}

Explanation

In the code above, we call the Overlaps() method with the first list of integers as input. Since it has a few common elements with HashSet, Overlaps() returns True and it is displayed in the output.

We again call the Overlaps() method with the second list of integers as input. Since it has no common elements with HashSet, Overlaps() returns False, and this is also displayed in the output.

We print the elements of the HashSet and both lists. The program prints the output below and exits.

HashSet Elements : 
{ 1 5 9 11 15 6 }
Input List 1: 9 11
Does HashSet share common elements with List 1? : True
Input List 2: 29 34 90
Does HashSet share common elements with List 2 ? : False
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