Enumerable.DefaultIfEmpty(Of TSource) Method (IEnumerable(Of TSource), TSource)

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)

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

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable(Of 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(Of TSource)
An IEnumerable(Of 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(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).

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.

The following code example demonstrates how to use the DefaultIfEmpty(Of TSource)(IEnumerable(Of 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.
        MsgBox(output1.ToString())

        ' 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.
        MsgBox(output2.ToString())
    End Sub

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



.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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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