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Queryable.OrderByDescending(Of TSource, TKey) Method (IQueryable(Of TSource), Expression(Of 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 IQueryable(Of TSource), _
	keySelector As Expression(Of Func(Of TSource, TKey)), _
	comparer As IComparer(Of TKey) _
) As IOrderedQueryable(Of TSource)

Type Parameters

TSource

The type of the elements of source.

TKey

The type of the key returned by the function that is represented by keySelector.

Parameters

source
Type: System.Linq.IQueryable(Of TSource)

A sequence of values to order.

keySelector
Type: System.Linq.Expressions.Expression(Of 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.IOrderedQueryable(Of TSource)
An IOrderedQueryable(Of T) 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 IQueryable(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 or comparer is Nothing.

This method has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).

The OrderByDescending(Of TSource, TKey)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), IComparer(Of TKey)) method generates a MethodCallExpression that represents calling OrderByDescending(Of TSource, TKey)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), IComparer(Of TKey)) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Of TElement)(Expression) method of the IQueryProvider represented by the Provider property of the source parameter. The result of calling CreateQuery(Of TElement)(Expression) is cast to type IOrderedQueryable(Of T) and returned.

The query behavior that occurs as a result of executing an expression tree that represents calling OrderByDescending(Of TSource, TKey)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), IComparer(Of TKey)) depends on the implementation of the type of the source parameter. The expected behavior is that it sorts the elements of source in descending order, based on the key obtained by invoking keySelector on each element of source. The comparer parameter is used to compare keys.

The following code example demonstrates how to use OrderByDescending(Of TSource, TKey)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), IComparer(Of TKey)) to sort the elements of a sequence in descending order by using a custom comparer.

    ''' <summary> 
    ''' This class provides a custom implementation of the 
    ''' IComparer.Compare() method. It sorts by the fractional part of the decimal number. 
    ''' </summary> 
    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.AsQueryable(). _
            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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