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Enumerable.ThenByDescending<TSource, TKey> Method (IOrderedEnumerable<TSource>, Func<TSource, TKey>, IComparer<TKey>)

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Performs a subsequent ordering of the elements in a sequence in descending order by using a specified comparer.

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

Syntax

'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)

public static IOrderedEnumerable<TSource> ThenByDescending<TSource, TKey>(
    this IOrderedEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    IComparer<TKey> comparer
)

Type Parameters

  • TSource
    The type of the elements of source.
  • TKey
    The type of the key returned by keySelector.

Parameters

  • keySelector
    Type: System.Func<TSource, TKey>
    A function to extract a key from each element.

Return Value

Type: System.Linq.IOrderedEnumerable<TSource>
An IOrderedEnumerable<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<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

source or keySelector is nulla null reference (Nothing in Visual Basic).

Remarks

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.

ThenBy and ThenByDescending are defined to extend the type IOrderedEnumerable<TElement>, which is also the return type of these methods. This design enables you to specify multiple sort criteria by applying any number of ThenBy or ThenByDescending methods.

NoteNote:

Because IOrderedEnumerable<TElement> inherits from IEnumerable<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 nulla null reference (Nothing in Visual Basic), 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.

Examples

The following code example demonstrates how to use ThenByDescending<TSource, TKey>(IOrderedEnumerable<TSource>, Func<TSource, TKey>, IComparer<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


      public class CaseInsensitiveComparer : IComparer<string>
      {
         public int Compare(string x, string y)
         {
            return string.Compare(x, y, StringComparison.CurrentCultureIgnoreCase);
         }
      }

      public static void ThenByDescendingEx1()
      {
         string[] fruits = { "apPLe", "baNanA", "apple", "APple", "orange", "BAnana", "ORANGE", "apPLE" };

         // Sort the strings first ascending by their length and 
         // then descending using a custom case insensitive comparer.
         IEnumerable<string> query =
             fruits
             .OrderBy(fruit => fruit.Length)
             .ThenByDescending(fruit => fruit, new CaseInsensitiveComparer());

         foreach (string fruit in query)
         {
            outputBlock.Text += fruit + "\n";
         }
      }

      /*
          This code produces the following output:

          apPLe
          apple
          APple
          apPLE
          orange
          ORANGE
          baNanA
          BAnana
      */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.