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

'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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

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.

ThenBy and 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 ThenBy or ThenByDescending methods.

NoteNote

Because IOrderedEnumerable(Of TElement) inherits from IEnumerable(Of 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.

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
        MsgBox(output.ToString())

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

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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