Enumerable.DefaultIfEmpty Method
Definition
Important
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Returns the elements of an IEnumerable<T>, or a default valued singleton collection if the sequence is empty.
Overloads
| DefaultIfEmpty<TSource>(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. |
| DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) |
Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty. |
DefaultIfEmpty<TSource>(IEnumerable<TSource>)
- Source:
- DefaultIfEmpty.cs
- Source:
- DefaultIfEmpty.cs
- Source:
- DefaultIfEmpty.cs
Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty.
public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
static System::Collections::Generic::IEnumerable<TSource> ^ DefaultIfEmpty(System::Collections::Generic::IEnumerable<TSource> ^ source);public static System.Collections.Generic.IEnumerable<TSource> DefaultIfEmpty<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);public static System.Collections.Generic.IEnumerable<TSource?> DefaultIfEmpty<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);static member DefaultIfEmpty : seq<'Source> -> seq<'Source><Extension()>
Public Function DefaultIfEmpty(Of TSource) (source As IEnumerable(Of TSource)) As IEnumerable(Of TSource)Type Parameters
- TSource
The type of the elements of source.
Parameters
- source
- IEnumerable<TSource>
The sequence to return a default value for if it is empty.
Returns
An IEnumerable<T> object that contains the default value for the TSource type if source is empty; otherwise, source.
Exceptions
source is null.
Examples
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.
class Pet
{
public string Name { get; set; }
public int Age { get; set; }
}
public static void DefaultIfEmptyEx1()
{
List<Pet> pets =
new List<Pet>{ new Pet { Name="Barley", Age=8 },
new Pet { Name="Boots", Age=4 },
new Pet { Name="Whiskers", Age=1 } };
foreach (Pet pet in pets.DefaultIfEmpty())
{
Console.WriteLine(pet.Name);
}
}
/*
This code produces the following output:
Barley
Boots
Whiskers
*/
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.
Console.WriteLine(output.ToString())
End Sub
' This code produces the following output:
'
' Barley
' Boots
' Whiskers
This example uses an empty sequence.
List<int> numbers = new List<int>();
foreach (int number in numbers.DefaultIfEmpty())
{
Console.WriteLine(number);
}
/*
This code produces the following output:
0
*/
' 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.
Console.WriteLine(output.ToString())
' This code produces the following output:
'
' 0
Remarks
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 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 method.
See also
Applies to
DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)
- Source:
- DefaultIfEmpty.cs
- Source:
- DefaultIfEmpty.cs
- Source:
- DefaultIfEmpty.cs
Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty.
public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
static System::Collections::Generic::IEnumerable<TSource> ^ DefaultIfEmpty(System::Collections::Generic::IEnumerable<TSource> ^ source, TSource defaultValue);public static System.Collections.Generic.IEnumerable<TSource> DefaultIfEmpty<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, TSource defaultValue);static member DefaultIfEmpty : seq<'Source> * 'Source -> seq<'Source><Extension()>
Public 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
- IEnumerable<TSource>
The sequence to return the specified value for if it is empty.
- defaultValue
- TSource
The value to return if the sequence is empty.
Returns
An IEnumerable<T> that contains defaultValue if source is empty; otherwise, source.
Examples
The following code example demonstrates how to use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method and specify a default value. The first sequence is not empty and the second sequence is empty.
class Pet
{
public string Name { get; set; }
public int Age { get; set; }
}
public static void DefaultIfEmptyEx2()
{
Pet defaultPet = new Pet { Name = "Default Pet", Age = 0 };
List<Pet> pets1 =
new List<Pet>{ new Pet { Name="Barley", Age=8 },
new Pet { Name="Boots", Age=4 },
new Pet { Name="Whiskers", Age=1 } };
foreach (Pet pet in pets1.DefaultIfEmpty(defaultPet))
{
Console.WriteLine("Name: {0}", pet.Name);
}
List<Pet> pets2 = new List<Pet>();
foreach (Pet pet in pets2.DefaultIfEmpty(defaultPet))
{
Console.WriteLine("\nName: {0}", pet.Name);
}
}
/*
This code produces the following output:
Name: Barley
Name: Boots
Name: Whiskers
Name: Default Pet
*/
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.
Console.WriteLine(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.
Console.WriteLine(output2.ToString())
End Sub
' This code produces the following output:
'
' Name: Barley
' Name: Boots
' Name: Whiskers
'
' Name: Default Pet
Remarks
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 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.
