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

'Declaration
<ExtensionAttribute> _
Public Shared Function DefaultIfEmpty(Of TSource) ( _
	source As IQueryable(Of TSource), _
	defaultValue As TSource _
) As IQueryable(Of TSource)

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable(Of TSource)
The IQueryable(Of 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(Of TSource)
An IQueryable(Of 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(Of TSource). When you use instance method syntax to call this method, omit the first parameter.

ExceptionCondition
ArgumentNullException

source is Nothing.

The DefaultIfEmpty(Of TSource)(IQueryable(Of TSource), TSource) method generates a MethodCallExpression that represents calling DefaultIfEmpty(Of TSource)(IQueryable(Of TSource), TSource) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Of 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(Of TSource)(IQueryable(Of 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(Of T) that contains defaultValue.

The following code example shows a situation in which it is useful to call DefaultIfEmpty(Of TSource)(IQueryable(Of TSource), TSource) in a LINQ query. A default value is passed to DefaultIfEmpty(Of TSource)(IQueryable(Of TSource), TSource) in this example.


      ' Create a list of Pet objects.
      Dim pets As New List(Of Pet)(New Pet() { _
                         New Pet With {.Name = "Barley", .Age = 8}, _
                         New Pet With {.Name = "Boots", .Age = 4}, _
                         New Pet With {.Name = "Whiskers", .Age = 1}})

      ' This query returns pets that are 10 or older. In case there are no pets 
      ' that meet that criteria, call DefaultIfEmpty(). This code passes an (optional) 
      ' default value to DefaultIfEmpty().
      Dim oldPets() As String = pets.AsQueryable() _
          .Where(Function(Pet) Pet.Age >= 10) _
          .Select(Function(Pet) Pet.Name) _
          .DefaultIfEmpty("[EMPTY]") _
          .ToArray()
      Try
         outputBlock.Text &= "First query: " + oldPets(0) & vbCrLf
      Catch ex As Exception
         outputBlock.Text &= "First query: An exception was thrown: " & _
                             ex.Message & vbCrLf
      End Try

      ' This query selects only those pets that are 10 or older.
      ' This code does not call DefaultIfEmpty().
      Dim oldPets2() As String = _
          pets.AsQueryable() _
          .Where(Function(Pet) Pet.Age >= 10) _
          .Select(Function(Pet) Pet.Name) _
          .ToArray()

      ' There may be no elements in the array, so directly
      ' accessing element 0 may throw an exception.
      Try
         outputBlock.Text &= "Second query: " & oldPets2(0) & vbCrLf
      Catch ex As Exception
         outputBlock.Text &= "Second query: An exception was thrown: " & _
                              ex.Message & vbCrLf
      End Try

      ' This code produces the following output:
      '
      ' First(query) : [EMPTY]
      ' Second query: An exception was thrown: Index was outside the bounds of the array.



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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