Queryable.DefaultIfEmpty Method

Definition

Namespace:

System.Linq

Assemblies:

netstandard.dll, System.Linq.Queryable.dll

Returns the elements in a sequence or a default valued singleton collection if the sequence is empty.

Overloads

DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)	Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty.
DefaultIfEmpty<TSource>(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.

DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)

Source:

Queryable.cs

Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty.

C#

public static System.Linq.IQueryable<TSource> DefaultIfEmpty<TSource>(this System.Linq.IQueryable<TSource> source, TSource defaultValue);

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

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

defaultValue

TSource

The value to return if the sequence is empty.

Returns

IQueryable<TSource>

An IQueryable<T> that contains defaultValue if source is empty; otherwise, source.

Exceptions

ArgumentNullException

source is null.

Examples

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.

C#

// 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.
*/

Remarks

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.

DefaultIfEmpty<TSource>(IQueryable<TSource>)

Source:

Queryable.cs

Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty.

C#

public static System.Linq.IQueryable<TSource?> DefaultIfEmpty<TSource>(this System.Linq.IQueryable<TSource> source);

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

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

Returns

IQueryable<TSource>

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

Exceptions

ArgumentNullException

source is null.

Examples

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

C#

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
*/

Remarks

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).