Enumerable.GroupBy Method

Groups the elements of a sequence according to a specified key selector function.


public static IEnumerable<TSource> GroupBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector
)

Description

The GroupBy method projects each element of a sequence into an element of type TResult. This overloaded version of GroupBy groups the elements of an IEnumerable<TSource> based on a key derived from each element. The grouping is performed by a specified key selector function.

This method performs deferred execution.

Parameters

Name	Type	Description
`source`	`IEnumerable<TSource>`	An `IEnumerable<TSource>` whose elements to group.
`keySelector`	`Func<TSource, TKey>`	A function to extract the key from each element.

Returns

Type	Description
`IEnumerable<IGrouping<TKey, TSource>>`	An `IEnumerable<IGrouping<TKey, TSource>>` where each grouping contains elements that share the same key.

Example

The following code example demonstrates how to use the GroupBy method to group a list of numbers by whether they are even or odd.


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

public class Example
{
    public static void Main()
    {
        List<int> numbers = new List<int> { 5, 4, 2, 3, 1, 1, 6, 8, 7, 9, 10 };

        // Group numbers by whether they are even or odd
        var groupedNumbers = numbers.GroupBy(n => n % 2 == 0 ? "Even" : "Odd");

        // Print the groups
        foreach (var group in groupedNumbers)
        {
            Console.WriteLine($"Group: {group.Key}");
            foreach (var number in group)
            {
                Console.Write($"{number} ");
            }
            Console.WriteLine();
        }
    }
}

Output:


Group: Odd
5 3 1 1 7 9
Group: Even
4 2 6 8 10