Article
11/18/2011

Queryable.GroupBy<TSource, TKey> Method (IQueryable<TSource>, Expression<Func<TSource, TKey>>)

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Groups the elements of a sequence according to a specified key selector function.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function GroupBy(Of TSource, TKey) ( _
    source As IQueryable(Of TSource), _
    keySelector As Expression(Of Func(Of TSource, TKey)) _
) As IQueryable(Of IGrouping(Of TKey, TSource))

public static IQueryable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, TKey>> keySelector
)

Type Parameters

TSource
The type of the elements of source.

TKey
The type of the key returned by the function represented in keySelector.

Parameters

source
Type: System.Linq.IQueryable<TSource>
An IQueryable<T> whose elements to group.

keySelector
Type: System.Linq.Expressions.Expression<Func<TSource, TKey>>
A function to extract the key for each element.

Return Value

Type: System.Linq.IQueryable<IGrouping<TKey, TSource>>
An IQueryable<IGrouping<TKey, TSource>> in C# or IQueryable(Of IGrouping(Of TKey, TSource)) in Visual Basic where each IGrouping<TKey, TElement> object contains a sequence of objects and a key.

Usage Note

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

Exceptions

Exception	Condition
ArgumentNullException	source or keySelector is nulla null reference (Nothing in Visual Basic).

Remarks

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The GroupBy<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>) method generates a MethodCallExpression that represents calling GroupBy<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery<TElement>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling GroupBy<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>) depends on the implementation of the type of the source parameter. The expected behavior is that it groups the elements of source by a key value that is obtained by invoking keySelector on each element.

Examples

The following code example demonstrates how to use GroupBy<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>) to group the elements of a sequence.

   Structure Pet
      Public Name As String
      Public Age As Integer
   End Structure

   Shared Sub GroupByEx1()
      ' Create a list of Pet objects.
      Dim pets As New List(Of Pet)(New Pet() { _
                      New Pet With {.Name = "Barley", .Age = 8}, _
                      New Pet With {.Name = "Boots", .Age = 4}, _
                      New Pet With {.Name = "Whiskers", .Age = 1}, _
                      New Pet With {.Name = "Daisy", .Age = 4}})

      ' Group the pets using Pet.Age as the key.
      ' Use Pet.Name as the value for each entry.
      Dim query = pets.AsQueryable().GroupBy(Function(pet) pet.Age)

      Dim output As New System.Text.StringBuilder
      ' Iterate over each IGrouping in the collection.
      For Each ageGroup In query
         output.AppendFormat("Age group: {0}   Number of pets: {1}{2}", ageGroup.Key, ageGroup.Count(), vbCrLf)
      Next

      ' Display the output.
      outputBlock.Text &= output.ToString() & vbCrLf
   End Sub

   ' This code produces the following output:
   '
   ' Age group: 8   Number of pets: 1
   ' Age group: 4   Number of pets: 2
   ' Age group: 1   Number of pets: 1

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

         public static void GroupByEx1()
         {
            // Create a list of Pet objects.
            List<Pet> pets =
                new List<Pet>{ new Pet { Name="Barley", Age=8 },
                                   new Pet { Name="Boots", Age=4 },
                                   new Pet { Name="Whiskers", Age=1 },
                                   new Pet { Name="Daisy", Age=4 } };

            // Group the pets using Pet.Age as the key.
            // Use Pet.Name as the value for each entry.
            var query = pets.AsQueryable().GroupBy(pet => pet.Age);

            // Iterate over each IGrouping in the collection.
            foreach (var ageGroup in query)
            {
               outputBlock.Text += String.Format("Age group: {0}  Number of pets: {1}", ageGroup.Key, ageGroup.Count()) + "\n";
            }
         }

         /*
             This code produces the following output:

             Age group: 8  Number of pets: 1
             Age group: 4  Number of pets: 2
             Age group: 1  Number of pets: 1

         */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

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

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