Queryable.OrderByDescending(Of TSource, TKey) Method (IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)), IComparer(Of TKey))

 
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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 IQueryable(Of TSource),
	keySelector As Expression(Of Func(Of TSource, TKey)),
	comparer As IComparer(Of TKey)
) As IOrderedQueryable(Of TSource)

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.

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.

Exception Condition
ArgumentNullException

source or keySelector or comparer is null.

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

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 7.1
Windows Phone
Available since 8.1
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