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Enumerable.OrderByDescending(Of TSource, TKey) Method (IEnumerable(Of TSource), Func(Of TSource, TKey), IComparer(Of TKey))

Sorts the elements of a sequence in descending order by using a specified comparer.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)
'Declaration
<ExtensionAttribute> _
Public Shared Function OrderByDescending(Of TSource, TKey) ( _
	source As IEnumerable(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.Collections.Generic.IEnumerable(Of TSource)

A sequence of values to order.

keySelector
Type: System.Func(Of TSource, TKey)

A function to extract a key from an element.

comparer
Type: System.Collections.Generic.IComparer(Of TKey)

An IComparer(Of T) to compare keys.

Return Value

Type: System.Linq.IOrderedEnumerable(Of TSource)
An IOrderedEnumerable(Of 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 IEnumerable(Of 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).
ExceptionCondition
ArgumentNullException

source or keySelector is Nothing.

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(Of 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(Of TElement), which means any number of consecutive calls to ThenBy or ThenByDescending can be made.

NoteNote

Because IOrderedEnumerable(Of TElement) inherits from IEnumerable(Of 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 Nothing, 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 OrderByDescending(Of TSource, TKey)(IEnumerable(Of TSource), Func(Of TSource, TKey), IComparer(Of TKey)) to sort the elements of a sequence in descending order by using a transform function and a custom comparer.

    ' This class provides a custom implementation  
    ' of the IComparer.Compare() method. 
    Class SpecialComparer
        Implements IComparer(Of Decimal)
        ''' <summary> 
        ''' Compare two decimal numbers by their fractional parts. 
        ''' </summary> 
        ''' <param name="d1">The first decimal to compare.</param> 
        ''' <param name="d2">The second decimal to compare.</param> 
        ''' <returns>1 if the first decimal's fractional part is greater than 
        ''' the second decimal's fractional part, 
        ''' -1 if the first decimal's fractional 
        ''' part is less than the second decimal's fractional part, 
        ''' or the result of calling Decimal.Compare() 
        ''' if the fractional parts are equal.</returns> 
        Function Compare(ByVal d1 As Decimal, ByVal d2 As Decimal) As Integer _
            Implements IComparer(Of Decimal).Compare

            Dim fractional1 As Decimal 
            Dim fractional2 As Decimal 

            ' Get the fractional part of the first number. 
            Try
                fractional1 = Decimal.Remainder(d1, Decimal.Floor(d1))
            Catch ex As DivideByZeroException
                fractional1 = d1
            End Try 

            ' Get the fractional part of the second number. 
            Try
                fractional2 = Decimal.Remainder(d2, Decimal.Floor(d2))
            Catch ex As DivideByZeroException
                fractional2 = d2
            End Try 

            If (fractional1 = fractional2) Then 
                ' The fractional parts are equal, so compare the entire numbers. 
                Return Decimal.Compare(d1, d2)
            ElseIf (fractional1 > fractional2) Then 
                Return 1
            Else 
                Return -1
            End If 
        End Function 
    End Class 

    Sub OrderByDescendingEx1()
        ' Create a list of decimal values. 
        Dim decimals As New List(Of Decimal)(New Decimal() _
                                             {6.2D, 8.3D, 0.5D, 1.3D, 6.3D, 9.7D})

        ' Order the elements of the list by passing 
        ' in the custom IComparer class. 
        Dim query As IEnumerable(Of Decimal) = _
            decimals.OrderByDescending(Function(num) num, _
                                       New SpecialComparer())

        Dim output As New System.Text.StringBuilder
        For Each num As Decimal In query
            output.AppendLine(num)
        Next 

        ' Display the output.
        MsgBox(output.ToString())
    End Sub 

    ' This code produces the following output: 
    
    ' 9.7 
    ' 0.5 
    ' 8.3 
    ' 6.3 
    ' 1.3 
    ' 6.2

.NET Framework

Supported in: 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.

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