GroupBy(TSource, TKey, TElement, TResult) Method (IEnumerable(TSource), Func(TSource, TKey), Func(TSource, TElement), Func(TKey, IEnumerable(TElement), TResult))

Enumerable.GroupBy<TSource, TKey, TElement, TResult> Method (IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>, Func<TKey, IEnumerable<TElement>, TResult>)


Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The elements of each group are projected by using a specified function.

Namespace:   System.Linq
Assembly:  System.Core (in System.Core.dll)

public static IEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(
	this IEnumerable<TSource> source,
	Func<TSource, TKey> keySelector,
	Func<TSource, TElement> elementSelector,
	Func<TKey, IEnumerable<TElement>, TResult> resultSelector


Type: System.Collections.Generic.IEnumerable<TSource>

An IEnumerable<T> whose elements to group.

Type: System.Func<TSource, TKey>

A function to extract the key for each element.

Type: System.Func<TSource, TElement>

A function to map each source element to an element in an IGrouping<TKey, TElement>.

Type: System.Func<TKey, IEnumerable<TElement>, TResult>

A function to create a result value from each group.

Return Value

Type: System.Collections.Generic.IEnumerable<TResult>

A collection of elements of type TResult where each element represents a projection over a group and its key.

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


The type of the elements in each IGrouping<TKey, TElement>.


The type of the result value returned by resultSelector.

In query expression syntax, a group by (Visual C#) or Group By Into (Visual Basic) clause translates to an invocation of GroupBy.

The following code example demonstrates how to use GroupBy<TSource, TKey, TElement, TResult>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>, Func<TKey, IEnumerable<TElement>, TResult>) to group the projected elements of a sequence and then project a sequence of results of type TResult.

class Pet
    public string Name { get; set; }
    public double Age { get; set; }

public static void GroupByEx4()
    // Create a list of pets.
    List<Pet> petsList =
        new List<Pet>{ new Pet { Name="Barley", Age=8.3 },
                       new Pet { Name="Boots", Age=4.9 },
                       new Pet { Name="Whiskers", Age=1.5 },
                       new Pet { Name="Daisy", Age=4.3 } };

    // Group Pet.Age values by the Math.Floor of the age.
    // Then project an anonymous type from each group
    // that consists of the key, the count of the group's
    // elements, and the minimum and maximum age in the group.
    var query = petsList.GroupBy(
        pet => Math.Floor(pet.Age),
        pet => pet.Age,
        (baseAge, ages) => new
            Key = baseAge,
            Count = ages.Count(),
            Min = ages.Min(),
            Max = ages.Max()

    // Iterate over each anonymous type.
    foreach (var result in query)
        Console.WriteLine("\nAge group: " + result.Key);
        Console.WriteLine("Number of pets in this age group: " + result.Count);
        Console.WriteLine("Minimum age: " + result.Min);
        Console.WriteLine("Maximum age: " + result.Max);

    /*  This code produces the following output:

        Age group: 8
        Number of pets in this age group: 1
        Minimum age: 8.3
        Maximum age: 8.3

        Age group: 4
        Number of pets in this age group: 2
        Minimum age: 4.3
        Maximum age: 4.9

        Age group: 1
        Number of pets in this age group: 1
        Minimum age: 1.5
        Maximum age: 1.5

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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