Remove duplicates from a List in C#

This post will discuss how to remove duplicates from a list in C# without destroying the original order of the elements.

1. Using `HashSet`

We know that HashSet<T> does not permit any duplicate elements. Therefore, if we convert the given list (with duplicates) to HashSet<T> and then convert it back to the list, we’ll get a list with all distinct elements.

The following code example demonstrates how to use the HashSet<T> collection for removing duplicates from the list.

using System;

using System.Linq;

using System.Collections.Generic;

public class Example

{

public static List<T> removeDuplicates<T>(List<T> list) {

return new HashSet<T>(list).ToList();

}

public static void Main()

{

List<int> list = new List<int> { 2, 3, 3, 4, 1, 2, 5 };

List<int> distinct = removeDuplicates(list);

Console.WriteLine(String.Join(",", distinct));

}

Output: 2,3,4,1,5

Download Run Code

Please note that the above solution creates a new list and destroys the original ordering of the elements. The following code demonstrates how to use HashSet<T> with List’s RemoveAll() method to in-place remove duplicates from the list and maintain the order of elements.

using System;

using System.Collections.Generic;

public static class Extentions

{

public static void removeDuplicates<T>(this List<T> list)

{

HashSet<T> hashset = new HashSet<T>();

list.RemoveAll(x => !hashset.Add(x));

}

public class Example

{

public static void Main()

{

List<int> list = new List<int> { 2, 3, 3, 4, 1, 2, 5 };

list.removeDuplicates();

Console.WriteLine(String.Join(",", list));

}

Output: 2,3,4,1,5

Download Run Code

2. Using `Enumerable.Distinct()` method (`System.Linq`)

To preserve the original order, we can also use LINQ’s Distinct() method. The following code example demonstrates how to use the Distinct() to return distinct elements from a list of integers.

using System;

using System.Collections.Generic;

using System.Linq;

public class Example

{

public static void Main()

{

List<int> list = new List<int> { 2, 3, 3, 4, 1, 2, 5 };

List<int> distinct = list.Distinct().ToList();

Console.WriteLine(String.Join(",", distinct));

}

Output: 2,3,4,1,5

Download Run Code

3. Using `Enumerable.Union()` method (`System.Linq`)

Another solution is to use LINQ’s Union() method, which gives an IEnumerable<T> which contains the elements from two sequences, excluding duplicates. To convert the resultant sequence to a list, call the ToList() method.

using System;

using System.Collections.Generic;

using System.Linq;

public class Example

{

public static void Main()

{

List<int> list = new List<int>() { 1, 2, 2, 3, 2, 4, 1, 4, 5 };

List<int> distinct = list.Union(list).ToList();

Console.WriteLine(String.Join(',', distinct));

}

Output: 1,2,3,4,5

Download Run Code

That’s all about removing duplicates from a List in C#.

Remove duplicates from a List in C#

1. Using HashSet

2. Using Enumerable.Distinct() method (System.Linq)

3. Using Enumerable.Union() method (System.Linq)

Share this:

1. Using `HashSet`

2. Using `Enumerable.Distinct()` method (`System.Linq`)

3. Using `Enumerable.Union()` method (`System.Linq`)