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

 
System_CAPS_noteNote

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

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

<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)

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.

Type Parameters

TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

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.

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.

System_CAPS_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 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 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

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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