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


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

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

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


Type: System.Collections.Generic.IEnumerable<TSource>

A sequence of values to order.

Type: System.Func<TSourceTKey>

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.

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.

Exception Condition

source or keySelector is null.

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.


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.

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

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)
        Console.WriteLine("{0} - {1}", pet.Name, pet.Age);

 This code produces the following output:

 Whiskers - 1
 Boots - 4
 Barley - 8

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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