Enumerable.DefaultIfEmpty<TSource> Method (IEnumerable<TSource>, TSource)

Type Parameters

• TSource
  The type of the elements of source.

Parameters

• source
  Type: System.Collections.Generic.IEnumerable<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<TSource>
An IEnumerable<T> that contains defaultValue if source is empty; otherwise, source.

   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.
      outputBlock.Text &= output1.ToString() & vbCrLf

      ' 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.
      outputBlock.Text &= output2.ToString() & vbCrLf
   End Sub

   ' This code produces the following output:
   '
   ' Name: Barley
   ' Name: Boots
   ' Name: Whiskers
   '
   ' Name: Default Pet

      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))
         {
            outputBlock.Text += String.Format("Name: {0}", pet.Name) + "\n";
         }

         List<Pet> pets2 = new List<Pet>();

         foreach (Pet pet in pets2.DefaultIfEmpty(defaultPet))
         {
            outputBlock.Text += String.Format("\nName: {0}", pet.Name) + "\n";
         }
      }

      /*
       This code produces the following output:

       Name: Barley
       Name: Boots
       Name: Whiskers

       Name: Default Pet
      */