Enumerable.ThenBy Method

Definition

Namespace:

System.Linq

Assembly:

System.Linq.dll

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

Overloads

ThenBy<TSource,TKey>(IOrderedEnumerable<TSource>, Func<TSource,TKey>)	Performs a subsequent ordering of the elements in a sequence in ascending order according to a key.
ThenBy<TSource,TKey>(IOrderedEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>)	Performs a subsequent ordering of the elements in a sequence in ascending order by using a specified comparer.

ThenBy<TSource,TKey>(IOrderedEnumerable<TSource>, Func<TSource,TKey>)

Source:

OrderBy.cs

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

public:
generic <typename TSource, typename TKey>
[System::Runtime::CompilerServices::Extension]
 static System::Linq::IOrderedEnumerable<TSource> ^ ThenBy(System::Linq::IOrderedEnumerable<TSource> ^ source, Func<TSource, TKey> ^ keySelector);

public static System.Linq.IOrderedEnumerable<TSource> ThenBy<TSource,TKey> (this System.Linq.IOrderedEnumerable<TSource> source, Func<TSource,TKey> keySelector);

static member ThenBy : System.Linq.IOrderedEnumerable<'Source> * Func<'Source, 'Key> -> System.Linq.IOrderedEnumerable<'Source>

<Extension()>
Public Function ThenBy(Of TSource, TKey) (source As IOrderedEnumerable(Of TSource), keySelector As Func(Of TSource, TKey)) As IOrderedEnumerable(Of TSource)

Type Parameters

TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

Parameters

source

IOrderedEnumerable<TSource>

An IOrderedEnumerable<TElement> that contains elements to sort.

keySelector

Func<TSource,TKey>

A function to extract a key from each element.

Returns

IOrderedEnumerable<TSource>

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

Exceptions

ArgumentNullException

source or keySelector is null.

Examples

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
*/

' Create an array of strings.
Dim fruits() As String =
{"grape", "passionfruit", "banana", "mango",
 "orange", "raspberry", "apple", "blueberry"}

' Sort the strings first by their length and then
' alphabetically by passing the identity function.
Dim query As IEnumerable(Of String) =
fruits _
.OrderBy(Function(fruit) fruit.Length) _
.ThenBy(Function(fruit) fruit)

' Display the results.
Dim output As New System.Text.StringBuilder
For Each fruit As String In query
    output.AppendLine(fruit)
Next
Console.WriteLine(output.ToString())

' This code produces the following output:
'
' apple
' grape
' mango
' banana
' orange
' blueberry
' raspberry
' passionfruit

Remarks

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 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 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.

Note

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

See also

• orderby clause (C# Reference)
• Order By Clause (Visual Basic)

ThenBy<TSource,TKey>(IOrderedEnumerable<TSource>, Func<TSource,TKey>, IComparer<TKey>)

Source:

OrderBy.cs

Performs a subsequent ordering of the elements in a sequence in ascending order by using a specified comparer.

public:
generic <typename TSource, typename TKey>
[System::Runtime::CompilerServices::Extension]
 static System::Linq::IOrderedEnumerable<TSource> ^ ThenBy(System::Linq::IOrderedEnumerable<TSource> ^ source, Func<TSource, TKey> ^ keySelector, System::Collections::Generic::IComparer<TKey> ^ comparer);

public static System.Linq.IOrderedEnumerable<TSource> ThenBy<TSource,TKey> (this System.Linq.IOrderedEnumerable<TSource> source, Func<TSource,TKey> keySelector, System.Collections.Generic.IComparer<TKey>? comparer);

static member ThenBy : System.Linq.IOrderedEnumerable<'Source> * Func<'Source, 'Key> * System.Collections.Generic.IComparer<'Key> -> System.Linq.IOrderedEnumerable<'Source>

<Extension()>
Public Function ThenBy(Of TSource, TKey) (source As IOrderedEnumerable(Of TSource), keySelector As Func(Of TSource, TKey), comparer As IComparer(Of TKey)) As IOrderedEnumerable(Of TSource)

Type Parameters

TSource

The type of the elements of source.

TKey

The type of the key returned by keySelector.

Parameters

source

IOrderedEnumerable<TSource>

An IOrderedEnumerable<TElement> that contains elements to sort.

keySelector

Func<TSource,TKey>

A function to extract a key from each element.

comparer

IComparer<TKey>

An IComparer<T> to compare keys.

Returns

IOrderedEnumerable<TSource>

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

Exceptions

ArgumentNullException

source or keySelector is null.

Remarks

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 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 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.

Note

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.