IDependencyResolver interface implementation in F# Web API - dependency-injection

Question

I am re-writing a C# ASP.NET Web API application in F#. I have Models and Controllers done and I moved onto MyDependencyResolver that implements IDependencyResolver.
I am having a problem implementing the GetService method, whose signature in C# is:
object GetService(System.Type serviceType)
So I need to return an obj and take a System.Type as a parameter.
This is what I have so far in F#:
type MyDependencyResolver() =
interface System.Web.Http.Dependencies.IDependencyResolver with
member this.BeginScope() : IDependencyScope =
this :> IDependencyScope
member this.GetService(serviceType:Type) : obj =
if (serviceType = typeof<Controllers.HomeController>) then
let homeController = new Controllers.HomeController(arg1, arg2, arg3, arg4)
homeController :> obj
// ???
elif (serviceType = typeof<_>) then
null
member this.GetServices (serviceType: Type) :IEnumerable<obj> =
let x = new List<obj>()
x :> IEnumerable<obj>
member this.Dispose() =
()
So if serviceType is of type HomeController I want to return an instance of HomeController, and if it's of any other type I want to return null. How do I do that in F#?
Edit:
GetService method in C#:
public object GetService(Type serviceType)
{
if (serviceType == typeof(Controllers.HomeController)){
return new Controllers.HomeController(arg1, arg2, arg3, arg4);
}
return null;
}

Answer 1

You could just use Activator.CreateInstance(serviceType) but where do you get the constructor arguments from?
In my projects I use Unity, which is configured as follows:
let private ConfigureUnity (config : HttpConfiguration) =
let rec unityResolver (container : IUnityContainer) =
{ new IDependencyResolver with
member this.BeginScope() =
unityResolver(container.CreateChildContainer()) :> IDependencyScope
member this.GetService serviceType =
try container.Resolve(serviceType) with
| :? ResolutionFailedException -> null
member this.GetServices serviceType =
try container.ResolveAll(serviceType) with
| :? ResolutionFailedException -> Seq.empty
member this.Dispose() = container.Dispose()
}
config.DependencyResolver <- (new UnityContainer())
.RegisterType<IFoo, FooImplementation>(new HierarchicalLifetimeManager())
.RegisterType<IBar, BarImplementation>(new HierarchicalLifetimeManager())
|> unityResolver
Classes (such as your controllers) are then resolved automatically and Unity will create dependencies (the constructor arguments) for you. Using other dependency injection frameworks should be straightforward.