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
)

Parameters

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

An IEnumerable<T> whose elements to group.

keySelector
Type: System.Func<TSource, TKey>

A function to extract the key for each element.

elementSelector
Type: System.Func<TSource, TElement>

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

resultSelector
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

TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

TElement

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

TResult

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 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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