How to flatten a list using LINQ C#?

Flattening a list means converting a List<List<T>> to List<T>. For example, converting a List<List<int>> containing multiple integer lists into a single List<int> with all elements combined.

The SelectMany operator in LINQ is used to project each element of a sequence to an IEnumerable<T> and then flatten the resulting sequences into one sequence. It combines records from a sequence of results and converts them into a single flattened result.

Syntax

The basic syntax for flattening using SelectMany −

var flattened = nestedList.SelectMany(innerList => innerList);

Using LINQ query syntax −

var flattened = from outerList in nestedLists
                from item in outerList
                select item;

Using SelectMany Method

The SelectMany method provides a direct way to flatten nested collections −

using System;
using System.Collections.Generic;
using System.Linq;

namespace DemoApplication {
   public class Program {
      static void Main(string[] args) {
         List<List<int>> listOfNumLists = new List<List<int>> {
            new List<int> { 1, 2 },
            new List<int> { 3, 4 },
            new List<int> { 5, 6 }
         };
         
         var numList = listOfNumLists.SelectMany(i => i);
         
         Console.WriteLine("Numbers in the flattened list:");
         foreach(var num in numList) {
            Console.WriteLine(num);
         }
      }
   }
}

The output of the above code is −

Numbers in the flattened list:
1
2
3
4
5
6

Using LINQ Query Syntax

You can also use LINQ query syntax with multiple from clauses to achieve the same result −

using System;
using System.Collections.Generic;
using System.Linq;

namespace DemoApplication {
   public class Program {
      static void Main(string[] args) {
         List<List<int>> listOfNumLists = new List<List<int>> {
            new List<int> { 1, 2 },
            new List<int> { 3, 4 },
            new List<int> { 5, 6 }
         };
         
         var numList = from listOfNumList in listOfNumLists
                       from value in listOfNumList
                       select value;
         
         Console.WriteLine("Numbers in the flattened list:");
         foreach(var num in numList) {
            Console.WriteLine(num);
         }
      }
   }
}

The output of the above code is −

Numbers in the flattened list:
1
2
3
4
5
6

Flattening with Transformation

You can combine flattening with data transformation by providing a selector function −

using System;
using System.Collections.Generic;
using System.Linq;

namespace DemoApplication {
   public class Program {
      static void Main(string[] args) {
         List<List<string>> wordLists = new List<List<string>> {
            new List<string> { "hello", "world" },
            new List<string> { "linq", "rocks" }
         };
         
         var upperCaseWords = wordLists.SelectMany(list => list.Select(word => word.ToUpper()));
         
         Console.WriteLine("Flattened and uppercase words:");
         foreach(var word in upperCaseWords) {
            Console.WriteLine(word);
         }
      }
   }
}

The output of the above code is −

Flattened and uppercase words:
HELLO
WORLD
LINQ
ROCKS

Comparison of Approaches

Conclusion

LINQ's SelectMany operator is the primary method for flattening nested collections in C#. It can be used with both method syntax and query syntax, and supports transformation during the flattening process. Choose the approach that best fits your coding style and requirements.