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)

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

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.

ExceptionCondition
ArgumentNullException

source is nullptr.

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.

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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