C# Remove Duplicates from List

How to remove duplicates from a list in C#

This tutorial explains how to remove duplicates from a list in C#.

We will introduce three different ways how to remove duplicates from a list.

When using the List <T> class, you may want to manage data without duplicate elements. However, with the current List<T> class, duplicate elements can be added.

Learn more:

Determining with List<T>.Contains(T) before adding elements

When using List <T>, you must declare the following in the using statement.

using System.Collections.Generic;

As a standard practice, List<T>.Contains(T) can be used to prevent duplication of elements in a list.

When adding an element to a list, the List<T>.Contains(T) method can be used to determine if the element already exists in the list.

The following sample program creates a new list named new_list, loops through the list named list, and adds each element to new_list if it does not exist.

using System;
using System.Collections.Generic;
namespace CSharpExample
{
    public class Program
    {
        public static void Main(string[] args)
        {
            List<string> list = new List<string>
            {
                "A1", "A2", "A2", "A1", "A2", "A3", "A4", "A3"
            };
            List<string> new_list = new List<string>();
            foreach (string item in list)
            {
                //If the item does not exist, add it to "new_list"
                if (!new_list.Contains(item))
                {
                    new_list.Add(item);
                }
            }
            //Display items in the list
            foreach (string item in new_list)
            {
                Console.WriteLine("Item: {0}", item);
            }
        }
    }
}

Output:

Item: A1
Item: A2
Item: A3
Item: A4

Removing duplicates from a list using C#'s HashSet class

In C#, the HashSet class is used to create sets. A set is a collection of individual objects that have no specific order. That is, the elements of a set are not repeated and unordered. You can eliminate repeated elements from a list by putting the list into a HashSet and then using LINQ's ToList() function to return that HashSet to the list. The following code shows how to remove duplicate elements from a list using the class HashSet:

using System;
using System.Collections.Generic;
using System.Linq;
namespace CSharpExample
{
    public class Program
    {
        public static void Main(string[] args)
        {
            List<string> list = new List<string>
            {
                "A4", "A1", "A2", "A2", "A1",
                "A2", "A3", "A4", "A3", "A5",
                "A6"
            };
            //Initialize a HashSet instance
            HashSet<string> hashset = new HashSet<string>(list);
            //Convert it to a list using ToList() method
            List<string> new_list = hashset.ToList();
            //Display items in the list
            foreach (string item in new_list)
            {
                Console.WriteLine("Item: {0}", item);
            }
 
        }
    }
}

Output:

Item: A4
Item: A1
Item: A2
Item: A3
Item: A5
Item: A6

Removing duplicates from a list using C#'s LINQ Distinct() method

LINQ's Distinct() method is used to select unique values from a C# data structure, and the ToList() function converts a collection of elements into a C# list. The following code sample illustrates how to remove duplicate values from a list using C# LINQ techniques:

using System;
using System.Linq;
using System.Collections.Generic;
namespace CSharpExample
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Initialize an empty list named "list"
            List<string> list = new List<string>();
            //Add elements to the list
            list.Add("A1");
            list.Add("A1");
            list.Add("A2");
            list.Add("A2");
            list.Add("A2");
            list.Add("A3");
            list.Add("A3");
            list.Add("A4");
            list.Add("A4");
            list.Add("A5");
            list.Add("A6");
            //Remove duplicates and return a new list named "new_list"
            List<string> new_list = list.Distinct().ToList();
            //Display items in the list
            foreach (string item in new_list)
            {
                Console.WriteLine("Item: {0}", item);
            }
        }
    }
}

Output:

Item: A1
Item: A2
Item: A3
Item: A4
Item: A5
Item: A6

Summary

This C# tutorial introduced how to remove duplicates from a list using the C# LINQ Distinct() method, HashSet class, and List<T>.Contains(T) method.

Reference: Enumerable.Distinct Method