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Queryable.ThenByDescending<TSource, TKey> Method (IOrderedQueryable<TSource>, Expression<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 IOrderedQueryable(Of TSource), _
    keySelector As Expression(Of Func(Of TSource, TKey)), _
    comparer As IComparer(Of TKey) _
) As IOrderedQueryable(Of TSource)

public static IOrderedQueryable<TSource> ThenByDescending<TSource, TKey>(
    this IOrderedQueryable<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 that is returned by the keySelector function.

Parameters

Return Value

Type: System.Linq.IOrderedQueryable<TSource>
A collection 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 IOrderedQueryable<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 ThenBy<TSource, TKey>(IOrderedQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<TKey>) method generates a MethodCallExpression that represents calling ThenBy<TSource, TKey>(IOrderedQueryable<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 ThenBy<TSource, TKey>(IOrderedQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<TKey>) depends on the implementation of the type of the source parameter. The expected behavior is that it performs a secondary sort of the elements of source in descending order, based on the key obtained by invoking keySelector on each element of source. All previously established sort orders are preserved. The comparer parameter is used to compare key values.

Examples

The following code example demonstrates how to use ThenByDescending<TSource, TKey>(IOrderedQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<TKey>) to perform a secondary ordering of the elements in a sequence in descending order by using a custom comparer.

   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 ascending by their length and 
      ' then descending by using a custom case insensitive comparer.
      Dim query = fruits.AsQueryable() _
          .OrderBy(Function(fruit) fruit.Length) _
          .ThenByDescending(Function(fruit) fruit, New CaseInsensitiveComparer())

      Dim output As New System.Text.StringBuilder
      For Each fruit As String In query
         output.AppendLine(fruit)
      Next

      ' Display the results.
      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.AsQueryable()
                .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

Platforms

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