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

 

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)

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

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.

Type Parameters

TSource

The type of the elements of source.

Exception Condition
ArgumentNullException

source is null.

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 null.

This method can be used to produce a left outer join when it is combined with the GroupJoin<TOuter, TInner, TKey, TResult>) method.

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.
    MsgBox(output.ToString())
End Sub

' 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.
MsgBox(output.ToString())

' This code produces the following output:
'
' 0

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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