DefaultIfEmpty(TSource) Method (IQueryable(TSource), TSource)

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

Public Shared Function DefaultIfEmpty(Of TSource) (
	source As IQueryable(Of TSource),
	defaultValue As TSource
) As IQueryable(Of TSource)


Type: System.Linq.IQueryable(Of TSource)

The IQueryable(Of T) to return the specified value for if empty.

Type: TSource

The value to return if the sequence is empty.

Return Value

Type: System.Linq.IQueryable(Of TSource)

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

Type Parameters


The type of the elements of source.

Exception Condition

source is null.

The DefaultIfEmpty(Of TSource)(IQueryable(Of TSource), TSource) method generates a MethodCallExpression that represents calling DefaultIfEmpty(Of TSource)(IQueryable(Of TSource), 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), 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 defaultValue.

The following code example shows a situation in which it is useful to call DefaultIfEmpty(Of TSource)(IQueryable(Of TSource), TSource) in a LINQ query. A default value is passed to DefaultIfEmpty(Of TSource)(IQueryable(Of TSource), TSource) in this example.

' 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}})

' This query returns pets that are 10 or older. In case there are no pets 
' that meet that criteria, call DefaultIfEmpty(). This code passes an (optional) 
' default value to DefaultIfEmpty().
Dim oldPets() As String = pets.AsQueryable() _
    .Where(Function(Pet) Pet.Age >= 10) _
    .Select(Function(Pet) Pet.Name) _
    .DefaultIfEmpty("[EMPTY]") _
    MsgBox("First query: " + oldPets(0))
Catch ex As Exception
    Console.WriteLine("First query: An exception was thrown: " + ex.Message)
End Try

' This query selects only those pets that are 10 or older.
' This code does not call DefaultIfEmpty().
Dim oldPets2() As String = _
    pets.AsQueryable() _
    .Where(Function(Pet) Pet.Age >= 10) _
    .Select(Function(Pet) Pet.Name) _

' There may be no elements in the array, so directly
' accessing element 0 may throw an exception.
    MsgBox("Second query: " + oldPets2(0))
Catch ex As Exception
    MsgBox("Second query: An exception was thrown: " + ex.Message)
End Try

' This code produces the following output:
' First(query) : [EMPTY]
' Second query: An exception was thrown: Index was outside the bounds of the array.

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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