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.
Assembly: System.Core (in System.Core.dll)
public static IQueryable<TSource> DefaultIfEmpty<TSource>( this IQueryable<TSource> source, TSource defaultValue )
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.
Type Parameters
- TSource
The type of the elements of source.
| Exception | Condition |
|---|---|
| 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. */
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