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
Assembly: 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).| 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.
*/
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.
