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

Syntax

'Declaration
<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)
public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey>(
    this IEnumerable<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 an 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 IEnumerable<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.

Two methods are defined to extend the type IOrderedEnumerable<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<TElement>, which means any number of consecutive calls to ThenBy or ThenByDescending can be made.

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 OrderByDescending<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>, IComparer<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.
      outputBlock.Text &= output.ToString() & vbCrLf
   End Sub

   ' This code produces the following output:
   '
   ' 9.7
   ' 0.5
   ' 8.3
   ' 6.3
   ' 1.3
   ' 6.2

      /// <summary>
      /// This IComparer class sorts by the fractional part of the decimal number.
      /// </summary>
      public class SpecialComparer : IComparer<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>
         public int Compare(decimal d1, decimal d2)
         {
            decimal fractional1, fractional2;

            // Get the fractional part of the first number.
            try
            {
               fractional1 = decimal.Remainder(d1, decimal.Floor(d1));
            }
            catch (DivideByZeroException)
            {
               fractional1 = d1;
            }
            // Get the fractional part of the second number.
            try
            {
               fractional2 = decimal.Remainder(d2, decimal.Floor(d2));
            }
            catch (DivideByZeroException)
            {
               fractional2 = d2;
            }

            if (fractional1 == fractional2)
               return Decimal.Compare(d1, d2);
            else if (fractional1 > fractional2)
               return 1;
            else
               return -1;
         }
      }

      public static void OrderByDescendingEx1()
      {
         List<decimal> decimals =
             new List<decimal> { 6.2m, 8.3m, 0.5m, 1.3m, 6.3m, 9.7m };

         IEnumerable<decimal> query =
             decimals.OrderByDescending(num =>
                                            num, new SpecialComparer());

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

      /*
       This code produces the following output:

       9.7
       0.5
       8.3
       6.3
       1.3
       6.2
      */

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.