Artikel
11/18/2011

Enumerable.ThenBy<TSource, TKey> Method (IOrderedEnumerable<TSource>, Func<TSource, TKey>, IComparer<TKey>)

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Performs a subsequent ordering of the elements in a sequence in ascending order by using a specified comparer.

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

Syntax

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

public static IOrderedEnumerable<TSource> ThenBy<TSource, TKey>(
    this IOrderedEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    IComparer<TKey> comparer
)

Type Parameters

TSource
The type of the elements of source.

TKey
The type of the key returned by keySelector.

Parameters

source
Type: System.Linq.IOrderedEnumerable<TSource>
An IOrderedEnumerable<TElement> that contains elements to sort.

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

comparer
Type: System.Collections.Generic.IComparer<TKey>
An IComparer<T> to compare keys.

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 IOrderedEnumerable<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.

ThenBy and ThenByDescending are defined to extend the type IOrderedEnumerable<TElement>, which is also the return type of these methods. This design enables you to specify multiple sort criteria by applying any number of ThenBy or ThenByDescending methods.

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.

If comparer is nulla null reference (Nothing in Visual Basic), the default comparer Default is used to compare keys.

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.

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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