C# program to find common elements in three arrays using sets

In C#, you can find common elements in three arrays efficiently using HashSet collections. A HashSet provides fast lookups and set operations, making it ideal for finding intersections between collections of data.

The HashSet<T> class offers built-in methods like IntersectWith() that can perform set operations directly, eliminating the need for complex loops and comparisons.

Syntax

Following is the syntax for creating a HashSet from an array −

var hashSet = new HashSet<int>(array);

Following is the syntax for finding intersection using IntersectWith() −

hashSet1.IntersectWith(hashSet2);
hashSet1.IntersectWith(hashSet3);

Using HashSet IntersectWith Method

The most efficient approach uses the IntersectWith() method to find common elements −

using System;
using System.Collections.Generic;

public class Program {
    public static void Main() {
        int[] arr1 = { 99, 57, 63, 98 };
        int[] arr2 = { 43, 99, 33, 57 };
        int[] arr3 = { 99, 57, 42 };

        // Create HashSets from arrays
        var h1 = new HashSet<int>(arr1);
        var h2 = new HashSet<int>(arr2);
        var h3 = new HashSet<int>(arr3);

        Console.WriteLine("Array 1: " + string.Join(", ", arr1));
        Console.WriteLine("Array 2: " + string.Join(", ", arr2));
        Console.WriteLine("Array 3: " + string.Join(", ", arr3));

        // Find intersection of all three sets
        h1.IntersectWith(h2);
        h1.IntersectWith(h3);

        Console.WriteLine("Common elements: " + string.Join(", ", h1));
    }
}

The output of the above code is −

Array 1: 99, 57, 63, 98
Array 2: 43, 99, 33, 57
Array 3: 99, 57, 42
Common elements: 99, 57

Using LINQ Intersect Method

Another approach uses LINQ's Intersect() method for a more functional programming style −

using System;
using System.Linq;

public class Program {
    public static void Main() {
        int[] arr1 = { 99, 57, 63, 98 };
        int[] arr2 = { 43, 99, 33, 57 };
        int[] arr3 = { 99, 57, 42 };

        Console.WriteLine("Array 1: " + string.Join(", ", arr1));
        Console.WriteLine("Array 2: " + string.Join(", ", arr2));
        Console.WriteLine("Array 3: " + string.Join(", ", arr3));

        // Find common elements using LINQ
        var commonElements = arr1.Intersect(arr2).Intersect(arr3);

        Console.WriteLine("Common elements: " + string.Join(", ", commonElements));
    }
}

The output of the above code is −

Array 1: 99, 57, 63, 98
Array 2: 43, 99, 33, 57
Array 3: 99, 57, 42
Common elements: 99, 57

Comparison of Approaches

Conclusion

Finding common elements in three arrays using HashSet collections is efficient and straightforward in C#. The HashSet.IntersectWith() method provides optimal performance, while LINQ's Intersect() offers excellent readability for functional programming approaches.