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: generic<typename TSource> [ExtensionAttribute] static IQueryable<TSource>^ DefaultIfEmpty( 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.
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