Queryable.DefaultIfEmpty(Of TSource) Method (IQueryable(Of 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 IQueryable(Of TSource)
) As IQueryable(Of TSource)

Parameters

source
Type: System.Linq.IQueryable(Of TSource)

The IQueryable(Of T) to return a default value for if empty.

Return Value

Type: System.Linq.IQueryable(Of TSource)

An IQueryable(Of T) that contains default(TSource) if source is empty; otherwise, source.

Type Parameters

TSource

The type of the elements of source.

Exception Condition
ArgumentNullException

source is null.

The DefaultIfEmpty(Of TSource)(IQueryable(Of TSource)) method generates a MethodCallExpression that represents calling DefaultIfEmpty(Of TSource)(IQueryable(Of TSource)) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Of TElement)(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling DefaultIfEmpty(Of TSource)(IQueryable(Of TSource)) depends on the implementation of the type of the source parameter. The expected behavior is that it returns source if it is not empty. Otherwise, it returns an IQueryable(Of T) that contains default(TSource).

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

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

Shared 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}})

    ' Call DefaultIfEmtpy() on the collection that Select()
    ' returns, so that if the initial list is empty, there
    ' will always be at least one item in the returned array.
    Dim names() As String = pets.AsQueryable() _
        .Select(Function(Pet) Pet.Name) _
        .DefaultIfEmpty() _
        .ToArray()

    Dim first As String = names(0)
    MsgBox(first)

    ' This code produces the following output:
    '
    ' Barley

End Sub

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 7.1
Windows Phone
Available since 8.1
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