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)

static member DefaultIfEmpty<'TSource> : 
        source:IQueryable<'TSource> *
        defaultValue:'TSource -> IQueryable<'TSource>


Type: System.Linq.IQueryable<'TSource>

The IQueryable<'T> to return the specified value for if empty.

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


The type of the elements of source.

Exception Condition

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
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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