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

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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)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function ThenBy(Of TSource, TKey) ( _
    source As IOrderedEnumerable(Of TSource), _
    keySelector As Func(Of TSource, TKey) _
) As IOrderedEnumerable(Of TSource)

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

Type Parameters

  • TSource
    The type of the elements of source.
  • TKey
    The type of the key returned by keySelector.

Parameters

  • 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.

Usage Note

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

Exceptions

Exception Condition
ArgumentNullException

source or keySelector is nulla null reference (Nothing in Visual Basic).

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 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.

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.

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.

      ' 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


      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)
      {
         outputBlock.Text += fruit + "\n";
      }

      /*
          This code produces the following output:

          apple
          grape
          mango
          banana
          orange
          blueberry
          raspberry
          passionfruit
      */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.