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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
Assemblies:   System.Linq (in System.Linq.dll)
  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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

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))
                {
                    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
            */

.NET Framework

Supported in: 4.6, 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

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

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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