Queryable.OrderByDescending<TSource, TKey> Method (IQueryable<TSource>, Expression<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 IQueryable(Of TSource), _
    keySelector As Expression(Of Func(Of TSource, TKey)), _
    comparer As IComparer(Of TKey) _
) As IOrderedQueryable(Of TSource)
public static IOrderedQueryable<TSource> OrderByDescending<TSource, TKey>(
    this IQueryable<TSource> source,
    Expression<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 the function that is represented by keySelector.

Parameters

Return Value

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

Exceptions

Exception Condition
ArgumentNullException

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

Remarks

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

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

The query behavior that occurs as a result of executing an expression tree that represents calling OrderByDescending<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<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.

Examples

The following code example demonstrates how to use OrderByDescending<TSource, TKey>(IQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<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.
      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 };

            // Sort the decimal values in descending order
            // by using a custom comparer.
            IEnumerable<decimal> query =
                decimals.AsQueryable()
                .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

Platforms

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