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

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

Usage Note

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
            */

.NET Framework

.NET Framework Client Profile

Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference