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Queryable.DefaultIfEmpty<TSource> Method (IQueryable<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.Queryable (in System.Linq.Queryable.dll)
  System.Core (in System.Core.dll)

public static IQueryable<TSource> DefaultIfEmpty<TSource>(
	this IQueryable<TSource> source,
	TSource defaultValue
)

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable<TSource>

The IQueryable<T> to return the specified value for if empty.

defaultValue
Type: TSource

The value to return if the sequence is empty.

Return Value

Type: System.Linq.IQueryable<TSource>
An IQueryable<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 IQueryable<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.

The DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method generates a MethodCallExpression that represents calling DefaultIfEmpty<TSource>(IQueryable<TSource>, 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>, 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 defaultValue.

The following code example shows a situation in which it is useful to call DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) in a LINQ query. A default value is passed to DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) in this example.

                // 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 } };

                // This query selects only those pets that are 10 or older. 
                // In case there are no pets that meet that criteria, call 
                // DefaultIfEmpty(). This code passes an (optional) default 
                // value to DefaultIfEmpty(). 
                string[] oldPets =
                    pets.AsQueryable()
                    .Where(pet => pet.Age >= 10)
                    .Select(pet => pet.Name)
                    .DefaultIfEmpty("[EMPTY]")
                    .ToArray();

                Console.WriteLine("First query: " + oldPets[0]);

                // This query selects only those pets that are 10 or older. 
                // This code does not call DefaultIfEmpty(). 
                string[] oldPets2 =
                    pets.AsQueryable()
                    .Where(pet => pet.Age >= 10)
                    .Select(pet => pet.Name)
                    .ToArray();

                // There may be no elements in the array, so directly 
                // accessing element 0 may throw an exception. 
                try
                {
                    Console.WriteLine("Second query: " + oldPets2[0]);
                }
                catch (Exception e)
                {
                    Console.WriteLine("Second query: An exception was thrown: " + e.Message);
                }

                /*
                    This code produces the following output:

                    First query: [EMPTY]
                    Second query: An exception was thrown: Index was outside the bounds of the array.
                */

.NET Framework

Supported in: 4.6, 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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