Artikel
11/18/2011

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

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

Returns the elements of the specified sequence or the specified 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), _
    defaultValue As TSource _
) As IEnumerable(Of TSource)

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

Type Parameters

TSource
The type of the elements of source.

Parameters

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

defaultValue
Type: TSource
The value to return if the sequence is empty.

Return Value

Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> that contains defaultValue 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.

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.

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

Examples

The following code example demonstrates how to use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method and specify a default value. The first sequence is not empty and the second sequence is empty.

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

   Sub DefaultIfEmptyEx2()
      ' Create a Pet object to use as the default value.
      Dim defaultPet As New Pet With {.Name = "Default Pet", .Age = 0}

      ' Create a List of Pet objects.
      Dim pets1 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 output1 As New System.Text.StringBuilder
      ' Enumerate the items in the list, calling DefaultIfEmpty() 
      ' with a default value.
      For Each pet As Pet In pets1.DefaultIfEmpty(defaultPet)
         output1.AppendLine("Name: " & pet.Name)
      Next

      ' Display the output.
      outputBlock.Text &= output1.ToString() & vbCrLf

      ' Create an empty List.
      Dim pets2 As New List(Of Pet)

      Dim output2 As New System.Text.StringBuilder
      ' Enumerate the items in the list, calling DefaultIfEmpty() 
      ' with a default value.
      For Each pet As Pet In pets2.DefaultIfEmpty(defaultPet)
         output2.AppendLine("Name: " & pet.Name)
      Next

      ' Display the output.
      outputBlock.Text &= output2.ToString() & vbCrLf
   End Sub

   ' This code produces the following output:
   '
   ' Name: Barley
   ' Name: Boots
   ' Name: Whiskers
   '
   ' Name: Default Pet

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

      public static void DefaultIfEmptyEx2()
      {
         Pet defaultPet = new Pet { Name = "Default Pet", Age = 0 };

         List<Pet> pets1 =
             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 pets1.DefaultIfEmpty(defaultPet))
         {
            outputBlock.Text += String.Format("Name: {0}", pet.Name) + "\n";
         }

         List<Pet> pets2 = new List<Pet>();

         foreach (Pet pet in pets2.DefaultIfEmpty(defaultPet))
         {
            outputBlock.Text += String.Format("\nName: {0}", pet.Name) + "\n";
         }
      }

      /*
       This code produces the following output:

       Name: Barley
       Name: Boots
       Name: Whiskers

       Name: Default Pet
      */

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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