For the sample program:
type public MyClass(reasonForLiving:string) =
member x.ReasonForLiving with get() = reasonForLiving
let classFactory () = MyClass("up to you")
let live () =
let instance = classFactory()
if instance = null then raise(System.Exception("null is not living... that's why OO languages die from bugs"))
instance
I get the error "The type 'MyClass' does not have null as a proper value" when I go to use this class as a return value of implicitly typed functions and compare it to null (b/c of compatibility requirements with C# dependency injection I cannot rely on F# option types).
I can easily fix this by changing the null check to:
if instance :> obj = null then
However, I know ("feel") this is completely "wrong". Especially when I consider how MyClass is a reference type that shouldn't need to be boxed (speaking from a C# background).
I've read about "F# Value Restriction" and how it impacts type inference, but I can't seem to gleam how it applies to this scenario.
Q: Is there another way to do this?
Aside #1: I found a simpler method of getting the error…
type public MyClass(reasonForLiving:string) =
member x.ReasonForLiving with get() = reasonForLiving
let nullMyClass : MyClass = null
Aside #2: I did try System.Nullable without thinking… MyClass is a reference type and not a value type (struct) which Nullable<_> requires. So, just reassures me that I REALLY am dealing with a reference type and leaves me wondering why an object cast suddenly makes this work.
Update: For anyone interested, I used this as one solution for Common Service Locator with the three functions below. Each service requested must support null, so if the service class is defined in F#, you need to add the [<AllowNullLiteral>]
:
let private getServiceLocator () =
try Some(Microsoft.Practices.ServiceLocation.ServiceLocator.Current)
with | _ -> None
let private getService serviceFactory =
let serviceLocator = getServiceLocator()
let service = match serviceLocator with
| None -> serviceFactory()
| _ ->
match serviceLocator.Value.GetInstance<'a>() with
| null -> serviceFactory()
| svc -> svc
match service with
| null -> None
| _ -> Some(service)
let private getRequiredService serviceFactory =
let service = getService serviceFactory
match service with
| None -> raise(MissingServiceException(""))
| _ -> service.Value
Best Answer
Use the
[<AllowNullLiteral>]
attribute:By default, F# types do not allow null (thank heavens!). This attribute is useful for interop with other .NET languages and allows assignment/comparison with null.