?. and ?() null-conditional operators (Visual Basic)
Tests the value of the left-hand operand for null (Nothing) before performing a member access (?.) or index (?()) operation; returns Nothing if the left-hand operand evaluates to Nothing. Note that in expressions that ordinarily return value types, the null-conditional operator returns a Nullable<T>.
These operators help you write less code to handle null checks, especially when descending into data structures. For example:
' Nothing if customers is Nothing
Dim length As Integer? = customers?.Length
' Nothing if customers is Nothing
Dim first As Customer = customers?(0)
' Nothing if customers, the first customer, or Orders is Nothing
Dim count As Integer? = customers?(0)?.Orders?.Count()
For comparison, the alternative code for the first of these expressions without a null-conditional operator is:
Dim length As Integer?
If customers IsNot Nothing Then
length = customers.Length
Else
length = Nothing
End If
Sometimes you need to take an action on an object that may be null, based on the value of a Boolean member on that object (like the Boolean property IsAllowedFreeShipping in the following example):
Dim customer = FindCustomerByID(123) 'customer will be Nothing if not found.
If customer IsNot Nothing AndAlso customer.IsAllowedFreeShipping Then
ApplyFreeShippingToOrders(customer)
End If
You can shorten your code and avoid manually checking for null by using the null-conditional operator as follows:
Dim customer = FindCustomerByID(123) 'customer will be Nothing if not found.
If customer?.IsAllowedFreeShipping Then ApplyFreeShippingToOrders(customer)
The null-conditional operators are short-circuiting. If one operation in a chain of conditional member access and index operations returns Nothing, the rest of the chain’s execution stops. In the following example, C(E) isn't evaluated if A, B, or C evaluates to Nothing.
A?.B?.C?(E)
Note that if Not someStr?.Contains("some string") or any other value that evaluates as Boolean? has the value of nothing or HasValue=false, the else block is run. The evaluation follows the SQL evaluation where null/nothing doesn't equal anything, not even another null/nothing.
Another use for null-conditional member access is to invoke delegates in a thread-safe way with much less code. The following example defines two types, a NewsBroadcaster and a NewsReceiver. News items are sent to the receiver by the NewsBroadcaster.SendNews delegate.
Public Module NewsBroadcaster
Dim SendNews As Action(Of String)
Public Sub Main()
Dim rec As New NewsReceiver()
Dim rec2 As New NewsReceiver()
SendNews?.Invoke("Just in: A newsworthy item...")
End Sub
Public Sub Register(client As Action(Of String))
SendNews = SendNews.Combine({SendNews, client})
End Sub
End Module
Public Class NewsReceiver
Public Sub New()
NewsBroadcaster.Register(AddressOf Me.DisplayNews)
End Sub
Public Sub DisplayNews(newsItem As String)
Console.WriteLine(newsItem)
End Sub
End Class
If there are no elements in the SendNews invocation list, the SendNews delegate throws a NullReferenceException. Before
null conditional operators, code like the following ensured that the delegate invocation list was not Nothing:
SendNews = SendNews.Combine({SendNews, client})
If SendNews IsNot Nothing Then
SendNews("Just in...")
End If
The new way is much simpler:
SendNews = SendNews.Combine({SendNews, client})
SendNews?.Invoke("Just in...")
The new way is thread-safe because the compiler generates code to evaluate SendNews one time only, keeping the result in a temporary variable. You need to explicitly call the Invoke method because there is no null-conditional delegate invocation syntax SendNews?(String).
