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

May 11, 2014

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
Assembly:  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

TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

TResult

The type of the result value returned by 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.
resultSelector
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.

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



Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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