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Queryable.DefaultIfEmpty<TSource> Method (IQueryable<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 IQueryable<TSource> DefaultIfEmpty<TSource>(
	this IQueryable<TSource> source
)

Parameters

source
Type: System.Linq.IQueryable<TSource>

The IQueryable<T> to return a default value for if empty.

Return Value

Type: System.Linq.IQueryable<TSource>

An IQueryable<T> that contains default(TSource) if source is empty; otherwise, source.

Type Parameters

TSource

The type of the elements of source.

Exception Condition
ArgumentNullException

source is null.

The DefaultIfEmpty<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling DefaultIfEmpty<TSource>(IQueryable<TSource>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery<TElement>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling DefaultIfEmpty<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns source if it is not empty. Otherwise, it returns an IQueryable<T> that contains default(TSource).

The following code examples demonstrate how to use DefaultIfEmpty<TSource>(IQueryable<TSource>) to provide a default value in case the source sequence is empty.

class Pet
{
    public string Name { get; set; }
    public int Age { get; set; }
}

public static void DefaultIfEmptyEx1()
{
    // Create a list of Pet objects.
    List<Pet> pets =
        new List<Pet>{ new Pet { Name="Barley", Age=8 },
                       new Pet { Name="Boots", Age=4 },
                       new Pet { Name="Whiskers", Age=1 } };

    // Call DefaultIfEmtpy() on the collection that Select()
    // returns, so that if the initial list is empty, there
    // will always be at least one item in the returned array.
    string[] names =
        pets.AsQueryable()
        .Select(pet => pet.Name)
        .DefaultIfEmpty()
        .ToArray();

    string first = names[0];
    Console.WriteLine(first);
}

/*
    This code produces the following output:

    Barley
*/

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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