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Enumerable.DefaultIfEmpty<TSource> Method (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.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

public static IEnumerable<TSource> DefaultIfEmpty<TSource>(
	this IEnumerable<TSource> source
)

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>

The sequence to return a default value for if it is empty.

Return Value

Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> object that contains the default value for the TSource type 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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

source is null.

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.

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
            */

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
            */

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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