Article
11/18/2011

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

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Sorts the elements of a sequence in ascending order according to a key.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function OrderBy(Of TSource, TKey) ( _
    source As IEnumerable(Of TSource), _
    keySelector As Func(Of TSource, TKey) _
) As IOrderedEnumerable(Of TSource)

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector
)

Type Parameters

TSource
The type of the elements of source.

TKey
The type of the key returned by keySelector.

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>
A sequence of values to order.

keySelector
Type: System.Func<TSource, TKey>
A function to extract a key from an element.

Return Value

Type: System.Linq.IOrderedEnumerable<TSource>
An IOrderedEnumerable<TElement> whose elements are sorted according to a 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.

Exceptions

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

Remarks

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

To order a sequence by the values of the elements themselves, specify the identity function (x => x in Visual C# or Function(x) x in Visual Basic) for keySelector.

Two methods are defined to extend the type IOrderedEnumerable<TElement>, which is the return type of this method. These two methods, namely ThenBy and ThenByDescending, enable you to specify additional sort criteria to sort a sequence. ThenBy and ThenByDescending also return an IOrderedEnumerable<TElement>, which means any number of consecutive calls to ThenBy or ThenByDescending can be made.

Note:
Because IOrderedEnumerable<TElement> inherits from IEnumerable<T>, you can call OrderBy or OrderByDescending on the results of a call to OrderBy, OrderByDescending, ThenBy or ThenByDescending. Doing this introduces a new primary ordering that ignores the previously established ordering.

This method compares keys by using the default comparer Default.

This method performs a stable sort; that is, if the keys of two elements are equal, the order of the elements is preserved. In contrast, an unstable sort does not preserve the order of elements that have the same key.

In query expression syntax, an orderby (Visual C#) or Order By (Visual Basic) clause translates to an invocation of OrderBy.

Examples

The following code example demonstrates how to use OrderBy<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>) to sort the elements of a sequence.

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

   Sub OrderByEx1()
      ' Create an array of Pet objects.
      Dim pets() As Pet = {New Pet With {.Name = "Barley", .Age = 8}, _
                           New Pet With {.Name = "Boots", .Age = 4}, _
                           New Pet With {.Name = "Whiskers", .Age = 1}}

      ' Order the Pet objects by their Age property.
      Dim query As IEnumerable(Of Pet) = _
          pets.OrderBy(Function(pet) pet.Age)

      Dim output As New System.Text.StringBuilder
      For Each pt As Pet In query
         output.AppendLine(pt.Name & " - " & pt.Age)
      Next

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

   ' This code produces the following output:
   '
   ' Whiskers - 1
   ' Boots - 4
   ' Barley - 8

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

      public static void OrderByEx1()
      {
         Pet[] pets = { new Pet { Name="Barley", Age=8 },
                            new Pet { Name="Boots", Age=4 },
                            new Pet { Name="Whiskers", Age=1 } };

         IEnumerable<Pet> query = pets.OrderBy(pet => pet.Age);

         foreach (Pet pet in query)
         {
            outputBlock.Text += String.Format("{0} - {1}", pet.Name, pet.Age) + "\n";
         }
      }

      /*
       This code produces the following output:

       Whiskers - 1
       Boots - 4
       Barley - 8
      */

Version Information

Silverlight

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

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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