WebApi Default Args Binder

CustomModelBinder.fs

type CustomBinder() =
    interface IModelBinder with
        member this.BindModel(actionContext:HttpActionContext, bindingContext :ModelBindingContext) =
            let qs = HttpUtility.ParseQueryString(actionContext.Request.RequestUri.Query)
            bindingContext.Model <- 
                if (qs.AllKeys |> Seq.exists(fun q -> q.ToLower() = bindingContext.ModelName.ToLower())) then    
                    qs.[bindingContext.ModelName] |> Some
                else
                    None
            true

type CustomBinderProvider() =
    inherit ModelBinderProvider()
    override this.GetBinder(configuration: System.Web.Http.HttpConfiguration , modelType: System.Type) =
        CustomBinder() :> IModelBinder
                 

type TestController() =
    inherit ApiController()

    [<HttpGetAttribute>]
    member this.Test([<ModelBinderAttribute(typeof<CustomBinderProvider>)>]?argument: string) =
        let arg = defaultArg argument "Hello"
        arg