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 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.


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.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

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)
            outputBlock.Text += "\nAge group: " + result.Key + "\n";
            outputBlock.Text += "Number of pets in this age group: " + result.Count + "\n";
            outputBlock.Text += "Minimum age: " + result.Min + "\n";
            outputBlock.Text += "Maximum age: " + result.Max + "\n";

         /*  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


Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.