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ParallelEnumerable.DefaultIfEmpty(TSource) Method (ParallelQuery(TSource), TSource)

Returns the elements of the specified parallel 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 : 
        source:ParallelQuery<'TSource> * 
        defaultValue:'TSource -> ParallelQuery<'TSource> 

Type Parameters


The type of the elements of source.


Type: System.Linq.ParallelQuery(TSource)

The sequence to return the specified value for if it is empty.

Type: TSource

The value to return if the sequence is empty.

Return Value

Type: System.Linq.ParallelQuery(TSource)
A sequence 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 ParallelQuery(TSource). 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).


The query was canceled with the token passed in through WithCancellation.


One or more exceptions occurred during the evaluation of the query.

.NET Framework

Supported in: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1
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