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


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

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

Public Shared Function ThenByDescending(Of TSource, TKey) (
	source As IOrderedEnumerable(Of TSource),
	keySelector As Func(Of TSource, TKey),
	comparer As IComparer(Of TKey)
) As IOrderedEnumerable(Of TSource)


Type: System.Linq.IOrderedEnumerable<TSource>

An IOrderedEnumerable<TElement> that contains elements to sort.

Type: System.Func<TSource, TKey>

A function to extract a key from each element.

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 in descending order 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.

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.

System_CAPS_note 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 null, 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.

The following code example demonstrates how to use ThenByDescending<TSource, TKey>(IOrderedEnumerable<TSource>, Func<TSource, TKey>, IComparer<TKey>) to perform a secondary ordering of the elements in a sequence in descending order by using a custom comparer.

' This class provides a custom implementation of the Compare() method.
Class CaseInsensitiveComparer
    Implements IComparer(Of String)

    Function Compare(ByVal x As String, ByVal y As String) As Integer _
        Implements IComparer(Of String).Compare

        ' Compare values and ignore case.
        Return String.Compare(x, y, True)
    End Function
End Class

Sub ThenByDescendingEx1()
    Dim fruits() As String = _
        {"apPLe", "baNanA", "apple", "APple", "orange", "BAnana", "ORANGE", "apPLE"}

    ' Sort the strings first by their length and then 
    ' by using a custom "case insensitive" comparer.
    Dim query As IEnumerable(Of String) = _
        fruits _
        .OrderBy(Function(fruit) fruit.Length) _
        .ThenByDescending(Function(fruit) fruit, New CaseInsensitiveComparer())

    ' Display the results.
    Dim output As New System.Text.StringBuilder
    For Each fruit As String In query
End Sub

' This code produces the following output:

' apPLe
' apple
' APple
' apPLE
' orange
' baNanA
' BAnana

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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