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:
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.

No code example is currently available or this language may not be supported.

Universal Windows Platform
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
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