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)

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

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.

Type Parameters

TSource

The type of the elements of source.

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(Of TOuter, TInner, TKey, TResult)) 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

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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