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

public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey>(
	this IEnumerable<TSource> source,
	Func<TSource, TKey> keySelector,
	IComparer<TKey> comparer


Type: System.Collections.Generic.IEnumerable<TSource>

A sequence of values to order.

Type: System.Func<TSource, TKey>

A function to extract a key from an element.

Type: System.Collections.Generic.IComparer<TKey>

An IComparer<T> to compare keys.

Return Value

Type: System.Linq.IOrderedEnumerable<TSource>

An IOrderedEnumerable<TElement> whose elements are sorted in descending order according to a key.

Type Parameters


The type of the elements of source.


The type of the key returned by keySelector.

Exception Condition

source or keySelector is null.

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.


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 null, 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.

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.

/// <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.
            fractional1 = decimal.Remainder(d1, decimal.Floor(d1));
        catch (DivideByZeroException)
            fractional1 = d1;
        // Get the fractional part of the second number.
            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;
            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)

 This code produces the following output:


Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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