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
*/
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