Queryable.OrderByDescending<TSource, TKey> Method (IQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<TKey>)
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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
- source
Type: System.Linq.IQueryable<TSource>
A sequence of values to order.
- keySelector
Type: System.Linq.Expressions.Expression<Func<TSource, TKey>>
A function to extract a key from an element.
- comparer
Type: System.Collections.Generic.IComparer<TKey>
An IComparer<T> to compare keys.
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.