Articolo
11/18/2011

Enumerable.DefaultIfEmpty<TSource> Method (IEnumerable<TSource>)

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function DefaultIfEmpty(Of TSource) ( _
    source As IEnumerable(Of TSource) _
) As IEnumerable(Of TSource)

public static IEnumerable<TSource> DefaultIfEmpty<TSource>(
    this IEnumerable<TSource> source
)

Type Parameters

TSource
The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>
The sequence to return a default value for if it is empty.

Return Value

Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> object that contains the default value for the TSource type if source is empty; otherwise, source.

Usage Note

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

Exceptions

Exception	Condition
ArgumentNullException	source 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.

The default value for reference and nullable types is nulla null reference (Nothing in Visual Basic).

This method can be used to produce a left outer join when it is combined with the GroupJoin) method.

Examples

The following code examples demonstrate how to use DefaultIfEmpty<TSource>(IEnumerable<TSource>) to provide a default value in case the source sequence is empty.

This example uses a non-empty sequence.

   Structure Pet
      Public Name As String
      Public Age As Integer
   End Structure

   Sub DefaultIfEmptyEx1()
      ' Create a List of Pet objects.
      Dim pets As New List(Of Pet)(New Pet() _
                                   {New Pet With {.Name = "Barley", .Age = 8}, _
                                    New Pet With {.Name = "Boots", .Age = 4}, _
                                    New Pet With {.Name = "Whiskers", .Age = 1}})

      Dim output As New System.Text.StringBuilder
      ' Iterate through the items in the List, calling DefaultIfEmpty().
      For Each pet As Pet In pets.DefaultIfEmpty()
         output.AppendLine(pet.Name)
      Next

      ' Display the output.
      outputBlock.Text &= output.ToString() & vbCrLf
   End Sub

   ' This code produces the following output:
   '
   ' Barley
   ' Boots
   ' Whiskers

      class Pet
      {
         public string Name { get; set; }
         public int Age { get; set; }
      }

      public static void DefaultIfEmptyEx1()
      {
         List<Pet> pets =
             new List<Pet>{ new Pet { Name="Barley", Age=8 },
                                new Pet { Name="Boots", Age=4 },
                                new Pet { Name="Whiskers", Age=1 } };

         foreach (Pet pet in pets.DefaultIfEmpty())
         {
            outputBlock.Text += pet.Name + "\n";
         }
      }

      /*
       This code produces the following output:

       Barley
       Boots
       Whiskers
      */

This example uses an empty sequence.

      ' Create an empty List.
      Dim numbers As New List(Of Integer)()

      Dim output As New System.Text.StringBuilder
      ' Iterate through the items in the List, calling DefaultIfEmpty().
      For Each number As Integer In numbers.DefaultIfEmpty()
         output.AppendLine(number)
      Next

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

      ' This code produces the following output:
      '
      ' 0

      List<int> numbers = new List<int>();

      foreach (int number in numbers.DefaultIfEmpty())
      {
         outputBlock.Text += number + "\n";
      }

      /*
       This code produces the following output:

       0
      */

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.

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