Enumerable.DefaultIfEmpty(TSource) Method (IEnumerable(TSource))
Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty.
Assembly: System.Core (in System.Core.dll)
The type of the elements of source.
- Type: System.Collections.Generic.IEnumerable()
The sequence to return a default value for if it is empty.
Return ValueType: System.Collections.Generic.IEnumerable()
An IEnumerable(T) object that contains the default value for the TSource type if source is empty; otherwise, source.
Usage NoteIn Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable(). 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).
source is a null reference (Nothing in Visual Basic).
This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.
The default value for reference and nullable types is a null reference (Nothing in Visual Basic).
This method can be used to produce a left outer join when it is combined with the GroupJoin) method.
The following code examples demonstrate how to use to provide a default value in case the source sequence is empty.
This example uses a non-empty sequence.
This example uses an empty sequence.
Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.