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Enumerable.DefaultIfEmpty<TSource> Method (IEnumerable<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 static IEnumerable<TSource> DefaultIfEmpty<TSource>(
	this IEnumerable<TSource> source,
	TSource defaultValue
)

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.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

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.

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))
         {
            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
      */



Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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