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MSDN Library

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

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

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

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.


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<TKey, IEnumerable<TSource>, 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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

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, TResult>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TKey, IEnumerable<TSource>, TResult>) to group the elements of a sequence and project a sequence of results of type TResult.

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

            public static void GroupByEx3()
                // 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 objects by the Math.Floor of their 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),
                    (age, pets) => new
                        Key = age,
                        Count = pets.Count(),
                        Min = pets.Min(pet => pet.Age),
                        Max = pets.Max(pet => pet.Age)

                // 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

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
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