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Queryable.DefaultIfEmpty(TSource) Method (IQueryable(TSource))

Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

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

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable(TSource)

The IQueryable(T) to return a default value for if empty.

Return Value

Type: System.Linq.IQueryable(TSource)
An IQueryable(T) that contains default(TSource) 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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

source is a null reference (Nothing in Visual Basic).

The DefaultIfEmpty(TSource)(IQueryable(TSource)) method generates a MethodCallExpression that represents calling DefaultIfEmpty(TSource)(IQueryable(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)) 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 default(TSource).

The following code examples demonstrate how to use DefaultIfEmpty(TSource)(IQueryable(TSource)) to provide a default value in case the source sequence is empty.

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

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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