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Queryable.ThenByDescending<TSource, TKey> Method (IOrderedQueryable<TSource>, Expression<Func<TSource, TKey>>, IComparer<TKey>)

 

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)

public static IOrderedQueryable<TSource> ThenByDescending<TSource, TKey>(
	this IOrderedQueryable<TSource> source,
	Expression<Func<TSource, TKey>> keySelector,
	IComparer<TKey> comparer
)

Parameters

source
Type: System.Linq.IOrderedQueryable<TSource>

An IOrderedQueryable<T> that contains elements to sort.

keySelector
Type: System.Linq.Expressions.Expression<Func<TSource, TKey>>

A function to extract a key from each element.

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

An IComparer<T> to compare keys.

Return Value

Type: System.Linq.IOrderedQueryable<TSource>

A collection whose elements are sorted in descending order according to a key.

Type Parameters

TSource

The type of the elements of source.

TKey

The type of the key that is returned by the keySelector function.

Exception Condition
ArgumentNullException

source or keySelector or comparer is null.

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.

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.

public class CaseInsensitiveComparer : IComparer<string>
{
    public int Compare(string x, string y)
    {
        return string.Compare(x, y, true);
    }
}

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)
        Console.WriteLine(fruit);
}

/*
    This code produces the following output:

    apPLe
    apple
    APple
    apPLE
    orange
    ORANGE
    baNanA
    BAnana
*/

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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