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

Enumerable.ThenBy(Of TSource, TKey) Method (IOrderedEnumerable(Of TSource), Func(Of TSource, TKey))

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

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)

'Declaration
<ExtensionAttribute> _
Public Shared 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
Type: System.Linq.IOrderedEnumerable(Of TSource)
An IOrderedEnumerable(Of TElement) that contains elements to sort.
keySelector
Type: System.Func(Of TSource, TKey)
A function to extract a key from each element.

Return Value

Type: System.Linq.IOrderedEnumerable(Of TSource)
An IOrderedEnumerable(Of TElement) whose elements are sorted according to a key.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IOrderedEnumerable(Of TSource). When you use instance method syntax to call this method, omit the first parameter.

ExceptionCondition
ArgumentNullException

source or keySelector is Nothing.

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.

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

NoteNote:

Because IOrderedEnumerable(Of TElement) inherits from IEnumerable(Of T), you can call Enumerable.OrderBy or Enumerable.OrderByDescending on the results of a call to Enumerable.OrderBy, Enumerable.OrderByDescending, Enumerable.ThenBy or Enumerable.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.

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


      ' 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
      outputBlock.Text &= output.ToString() & vbCrLf

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



Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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