Enumerable.ThenByDescending(Of TSource, TKey) Method (IOrderedEnumerable(Of TSource), Func(Of TSource, TKey), IComparer(Of TKey))
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
Performs a subsequent ordering of the elements in a sequence in descending order by using a specified comparer.
Assembly: System.Core (in System.Core.dll)
'Declaration <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(Of TSource)
An IOrderedEnumerable(Of TElement) that contains elements to sort.
- keySelector
- Type: System.Func(Of TSource, TKey)
A function to extract a key from each 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 IOrderedEnumerable(Of TSource). When you use instance method syntax to call this method, omit the first parameter.| Exception | Condition |
|---|---|
| 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.
Enumerable.ThenBy and Enumerable.ThenByDescending are defined to extend the type IOrderedEnumerable(Of TElement), which is also the return type of these methods. This design enables you to specify multiple sort criteria by applying any number of Enumerable.ThenBy or Enumerable.ThenByDescending methods.
Note: |
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Because IOrderedEnumerable(Of TElement) inherits from IEnumerable(Of T), you can call Enumerable.OrderBy or Enumerable.OrderByDescending on the results of a call to Enumerable.OrderBy, Enumerable.OrderByDescending, Enumerable.ThenBy or Enumerable.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 ThenByDescending(Of TSource, TKey)(IOrderedEnumerable(Of TSource), Func(Of TSource, TKey), IComparer(Of 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 outputBlock.Text &= output.ToString() & vbCrLf End Sub ' This code produces the following output: ' apPLe ' apple ' APple ' apPLE ' orange ' ORANGE ' baNanA ' BAnana
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