Queryable.DefaultIfEmpty(Of TSource) Method (IQueryable(Of TSource))
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
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)
'Declaration <ExtensionAttribute> _ Public Shared Function DefaultIfEmpty(Of TSource) ( _ source As IQueryable(Of 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 a default value for if empty.
Return Value
Type: System.Linq.IQueryable(Of TSource)An IQueryable(Of 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(Of TSource). When you use instance method syntax to call this method, omit the first parameter.| Exception | Condition |
|---|---|
| ArgumentNullException | source is Nothing. |
The DefaultIfEmpty(Of TSource)(IQueryable(Of TSource)) method generates a MethodCallExpression that represents calling DefaultIfEmpty(Of TSource)(IQueryable(Of 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)) 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 default(TSource).
The following code examples demonstrate how to use DefaultIfEmpty(Of TSource)(IQueryable(Of TSource)) to provide a default value in case the source sequence is empty.
Structure Pet Public Name As String Public Age As Integer End Structure Shared Sub DefaultIfEmptyEx1() ' 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}}) ' Call DefaultIfEmtpy() on the collection that Select() ' returns, so that if the initial list is empty, there ' will always be at least one item in the returned array. Dim names() As String = pets.AsQueryable() _ .Select(Function(Pet) Pet.Name) _ .DefaultIfEmpty() _ .ToArray() Dim first As String = names(0) outputBlock.Text &= first & vbCrLf ' This code produces the following output: ' ' Barley End Sub