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


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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: System.Collections.Generic.IEnumerable<TSource>

The sequence to return the specified value for if it is empty.

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.

Type Parameters


The type of the elements of source.

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<TOuter, TInner, TKey, TResult>) 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))
        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

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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