Flatten a list of lists in C#

This post will discuss how to flatten a list of lists in C#.

1. Using `Enumerable.SelectMany()` method (`System.Linq`)

We can use LINQ’s SelectMany() method to map each element of a list to an IEnumerable<T> and flattens the resulting sequences into a single list. The following code example demonstrates how to use the SelectMany to flatten a list.

using System;

using System.Linq;

using System.Collections.Generic;

public class Example

{

public static void Main()

{

List<List<int>> listOfLists = new List<List<int>>()

{

new List<int>() { 1, 2, 3 },

new List<int>() { 4, 5 },

new List<int>() { 6, 7, 8, 9 }

};

List<int> flattenedList = listOfLists.SelectMany(x => x).ToList();

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

}

Output: 1,2,3,4,5,6,7,8,9

Download Run Code

2. Using LINQ Query Expressions

The following code example shows how to use LINQ query expressions to flatten a list: