Enumerable.OrderByDescending<TSource, TKey> Method (IEnumerable<TSource>, Func<TSource, TKey>)
Sorts the elements of a sequence in descending order according to a key.
Assembly: System.Core (in System.Core.dll)
public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey>( this IEnumerable<TSource> source, Func<TSource, TKey> keySelector )
The type of the elements of source.
The type of the key returned by keySelector.
- Type: System.Collections.Generic.IEnumerable<>
A sequence of values to order.
- Type: System.Func<, >
A function to extract a key from an element.
Return ValueType: System.Linq.IOrderedEnumerable<>
An IOrderedEnumerable<TElement> whose elements are sorted in descending order according to a key.
Usage NoteIn Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).
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.
For an example of this method, see OrderByDescending<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>, IComparer<TKey>).
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.
This method compares keys by using the default comparer Default.
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.
In query expression syntax, an orderby descending (Visual C#) or Order By Descending (Visual Basic) clause translates to an invocation of OrderByDescending.