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.NET Framework Class Library

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)
<ExtensionAttribute> _
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 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 in descending order 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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).
Exception Condition
ArgumentNullException

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.

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
            output.AppendLine(fruit)
        Next
        MsgBox(output.ToString())
    End Sub 

    ' This code produces the following output: 

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

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.