Enumerable.DefaultIfEmpty Method (System.Linq)

Type Parameters

TSource
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)
    ' 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:
     
      ' 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)
' Display the output.
Console.WriteLine(output.ToString())
' This code produces the following output:
      
       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 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
       
       ) method.
      
      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
     
      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)
    ' 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)
    ' 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 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
        
        ) method.