How to Find Union of Two Sets in C# Easily

In C#, you can find the union of two sets using the HashSet<T> class with its UnionWith method or by using LINQ's Union() method. Both approaches combine elements from two collections without duplicates.

Syntax

There are two common ways to find the union of two sets in C#:

• Using HashSet<T>: Create a HashSet<T> from one collection, then call UnionWith() passing the second collection.
• Using LINQ: Call Union() on one collection and pass the second collection as an argument.

Both methods return a collection with unique elements from both sets.

csharp

HashSet<T> set1 = new HashSet<T>(collection1);
set1.UnionWith(collection2);

// or using LINQ
var union = collection1.Union(collection2);

Example

This example shows how to find the union of two integer sets using both HashSet<int> and LINQ's Union(). It prints the combined unique elements.

csharp

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        var set1 = new HashSet<int> { 1, 2, 3, 4 };
        var set2 = new List<int> { 3, 4, 5, 6 };

        // Using HashSet.UnionWith
        set1.UnionWith(set2);
        Console.WriteLine("Union using HashSet:");
        foreach (var item in set1)
        {
            Console.Write(item + " ");
        }
        Console.WriteLine();

        // Using LINQ Union
        var unionLinq = new List<int> { 1, 2, 3, 4 }.Union(new List<int> { 3, 4, 5, 6 });
        Console.WriteLine("Union using LINQ:");
        foreach (var item in unionLinq)
        {
            Console.Write(item + " ");
        }
        Console.WriteLine();
    }
}

Output

Union using HashSet:
1 2 3 4 5 6 
Union using LINQ:
1 2 3 4 5 6 

Common Pitfalls

Common mistakes when finding union of sets in C# include:

• Using Concat() instead of Union(), which does not remove duplicates.
• Modifying the original collection unintentionally when using UnionWith() on a HashSet.
• Assuming Union() returns a HashSet—it returns an IEnumerable, so you may need to convert it to a list or set.

csharp

/* Wrong: Concat keeps duplicates */
var list1 = new List<int> {1, 2, 3};
var list2 = new List<int> {3, 4, 5};
var wrongUnion = list1.Concat(list2); // duplicates remain

/* Right: Use Union to remove duplicates */
var rightUnion = list1.Union(list2);

Quick Reference

Summary tips for union of two sets in C#:

• Use HashSet<T> and UnionWith() to modify a set in place.
• Use LINQ Union() to get a new collection without duplicates.
• Remember Union() returns IEnumerable<T>, so convert if needed.
• Avoid Concat() if you want unique elements.

Key Takeaways

Use HashSet.UnionWith() to combine sets and update the original set.

LINQ's Union() returns a new collection with unique elements from both sets.

Avoid Concat() when you want to remove duplicates in the union.

Union() returns IEnumerable, so convert it to a list or set if needed.

HashSet ensures fast lookups and uniqueness when working with sets.