What is the best way to add two HashSets together in C#?

In C#, the best way to add two HashSet objects together depends on whether you want to modify an existing HashSet or create a new one containing the elements of both.

Here's a breakdown of the methods and their use cases:

1. Modifying an Existing HashSet (UnionWith):

If you want to add all the elements from one HashSet to another, modifying the original, the UnionWith method is the most efficient approach. This method directly modifies the target HashSet.

Example:

HashSet<int> set1 = new HashSet<int>() { 1, 2, 3 };
HashSet<int> set2 = new HashSet<int>() { 3, 4, 5 };

set1.UnionWith(set2);

// set1 now contains { 1, 2, 3, 4, 5 }

This approach is beneficial when you need to update an existing HashSet and don't need to preserve the original state of the second HashSet.

2. Creating a New HashSet (Using LINQ):

If you want to create a new HashSet that contains the combined elements of both HashSet objects without modifying the originals, you can use LINQ's Concat and then create a new HashSet.

Example:

using System.Linq;

HashSet<int> set1 = new HashSet<int>() { 1, 2, 3 };
HashSet<int> set2 = new HashSet<int>() { 3, 4, 5 };

HashSet<int> combinedSet = new HashSet<int>(set1.Concat(set2));

// combinedSet contains { 1, 2, 3, 4, 5 }
// set1 and set2 remain unchanged.

This method is advantageous when you want to maintain the original HashSet objects while creating a new one with the combined elements. Note that Concat will include duplicates in the resulting IEnumerable, but the HashSet constructor will automatically remove them.

3. Manual Iteration (Less Efficient):

While possible, manually iterating through one HashSet and adding elements to another is generally less efficient than using UnionWith or LINQ.

Example:

HashSet<int> set1 = new HashSet<int>() { 1, 2, 3 };
HashSet<int> set2 = new HashSet<int>() { 3, 4, 5 };

foreach (var item in set2)
{
    set1.Add(item);
}

// set1 now contains { 1, 2, 3, 4, 5 }

This approach is less efficient because it involves iterating and calling Add for each element, which can be slower than the optimized UnionWith method. It's generally not recommended unless you have specific reasons to iterate manually.

Summary:

- Use UnionWith if you want to modify an existing HashSet.

- Use LINQ (Concat) if you want to create a new HashSet without modifying the originals.

- Avoid manual iteration unless absolutely necessary.

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