ThenBy(TSource, TKey) Method (IOrderedEnumerable(TSource), Func(TSource, TKey))

Enumerable.ThenBy<TSource, TKey> Method (IOrderedEnumerable<TSource>, Func<TSource, TKey>)

 

Performs a subsequent ordering of the elements in a sequence in ascending order according to a key.

Namespace:   System.Linq
Assembly:  System.Core (in System.Core.dll)

public static IOrderedEnumerable<TSource> ThenBy<TSource, TKey>(
	this IOrderedEnumerable<TSource> source,
	Func<TSource, TKey> keySelector
)

Parameters

source
Type: System.Linq.IOrderedEnumerable<TSource>

An IOrderedEnumerable<TElement> that contains elements to sort.

keySelector
Type: System.Func<TSource, TKey>

A function to extract a key from each element.

Return Value

Type: System.Linq.IOrderedEnumerable<TSource>

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

Type Parameters

TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

Exception Condition
ArgumentNullException

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.

ThenBy and ThenByDescending are defined to extend the type IOrderedEnumerable<TElement>, which is also the return type of these methods. This design enables you to specify multiple sort criteria by applying any number of ThenBy or ThenByDescending methods.

System_CAPS_noteNote

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 [first criterion], [second criterion] (Visual C#) or Order By [first criterion], [second criterion] (Visual Basic) clause translates to an invocation of ThenBy<TSource, TKey>.

The following code example demonstrates how to use ThenBy<TSource, TKey>(IOrderedEnumerable<TSource>, Func<TSource, TKey>) to perform a secondary ordering of the elements in a sequence.

string[] fruits = { "grape", "passionfruit", "banana", "mango", 
                      "orange", "raspberry", "apple", "blueberry" };

// Sort the strings first by their length and then 
//alphabetically by passing the identity selector function.
IEnumerable<string> query =
    fruits.OrderBy(fruit => fruit.Length).ThenBy(fruit => fruit);

foreach (string fruit in query)
{
    Console.WriteLine(fruit);
}

/*
    This code produces the following output:

    apple
    grape
    mango
    banana
    orange
    blueberry
    raspberry
    passionfruit
*/

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 7.0
Windows Phone
Available since 8.1
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