Add elements of a List to a HashSet in C#

This post will discuss how to add elements of a List<T> to a HashSet<T> in C#.

1. Using `HashSet<T>.UnionWith()` Method

The HashSet<T>.UnionWith() method modifies the HashSet by merging all its elements with the specified collection. The following example demonstrates the usage of the UnionWith() method by adding elements of a List<T> to an existing HashSet<T>:

using System;

using System.Collections.Generic;

public class Example

{

public static void Main()

{

List<int> list = new List<int>() { 2, 3, 4 };

HashSet<int> set = new HashSet<int>() { 1, 2, 4, 7 };

set.UnionWith(list);

Console.WriteLine(String.Join(", ", set)); // 1, 2, 4, 7, 3

}

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2. Using `foreach` loop

Alternatively, you can create an extension method that act as AddRange() equivalent for a HashSet<T>. It adds elements of the specified collection to a HashSet<T> using a foreach loop. Note that a Set data structure doesn’t permit duplicates and maintains no particular order of its elements.

using System;

using System.Collections.Generic;

public static class Extensions

{

public static void AddRange<T>(this HashSet<T> source, IEnumerable<T> collection)

{

foreach (T item in collection) {

source.Add(item);

}

public class Example

{

public static void Main()

{

List<int> list = new List<int>() { 2, 3, 4 };

HashSet<int> set = new HashSet<int>() { 1, 2, 4, 7 };

set.AddRange(list);

Console.WriteLine(String.Join(", ", set)); // 1, 2, 4, 7, 3

}

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3. Using `Enumerable.Concat()` Method

Both above solutions add all elements of the List<T> to the HashSet<T>. You can use LINQ’s Enumerable.Concat() method to create a new collection containing elements of both the source set and the given list.

using System;

using System.Linq;

using System.Collections.Generic;

public class Example

{

public static void Main()

{

List<int> list = new List<int>() { 2, 3, 4 };

HashSet<int> set = new HashSet<int>() { 1, 2, 4, 7 };

HashSet<int> union = set.Concat(list).ToHashSet();

Console.WriteLine(String.Join(", ", union)); // 1, 2, 4, 7, 3

}

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That’s all about adding elements of a List<T> to a HashSet<T> in C#.

Add elements of a List to a HashSet in C#

1. Using HashSet<T>.UnionWith() Method

2. Using foreach loop

3. Using Enumerable.Concat() Method

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1. Using `HashSet<T>.UnionWith()` Method

2. Using `foreach` loop

3. Using `Enumerable.Concat()` Method