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.