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 )
Type Parameters
- TSource
The type of the elements of 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.
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).| 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 */
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.
