Mono Bug: Error message when attempting to observe a newly assigned value after mutating an object member

Question

I'm observing an error message on a data-tip when attempting to observe a newly assigned value after mutating an object member.

Viewmodel:

I'm trying to mutate the header member of a viewmodel object.

let mutable header : NavBarItem option = None

member x.Header      with get()  = header
                     and  set(v) = header       <- v
                                   base.NotifyPropertyChanged(<@ x.Header @>)
...
//----------------------------------
// THE FOLLOWING ERRORS
//----------------------------------
member x.LoadAsync(panelId:PanelId) =

   let items = seq [NavBarItem() :> UITemplate]

   let result = items  |> Seq.map (fun v -> v :?> NavBarItem)
                       |> Seq.tryHead // Error recovering value

   x.Header <- result

Error:

Here's the error after I hover my cursor over x.Header:

Incorrect number or types of arguments Parameter name: arguments

Here's a bug report that I observed after researching the error message.

UPDATE:

I attempted to serialize the sequence of objects that I did have access to so that I could deserialize the json and access an individual item.

That also failed:

   let temp = seq [NavBarItem() :> UITemplate] |> filterToNavBarItems 
                                               |> Seq.cast<NavBarItem>

   let json  = JsonConvert.SerializeObject(temp)
   let items = JsonConvert.DeserializeObject<NavBarItem seq>(json)
   let item  = items |> Seq.tryHead

   x.Header <- item // Error recovering value

Json:

[{"MeasureId":"some_navbar_id","Header":"Dashboard","Subheader":"Trauma Alerting","EmblemLabel":"","IsEmblemAttached":false,"EmblemBGColor":"","EmblemFGColor":""}]

Appendix:

type UITemplate =
    abstract member ToString   : unit -> string
    abstract member OrderIndex : int with get, set

type NavBarItem() =

    inherit ViewModelBase()

    let mutable header = ""

    interface UITemplate with 
        member x.ToString() = "NavBarItem"
        member val OrderIndex = -1 with get,set

    member x.Header  with get()  = header
                     and  set(v) = header <- v
                                   base.NotifyPropertyChanged(<@ x.Header @>)


type ViewModelBase () =

    let propertyChanged = Event<PropertyChangedEventHandler,PropertyChangedEventArgs>()
        
    let getPropertyName = function 
        | PropertyGet(_,pi,_) -> pi.Name
        | _ -> invalidOp "Expecting property getter expression"
        
    interface INotifyPropertyChanged with

        [<CLIEvent>]
        member this.PropertyChanged = propertyChanged.Publish
        
    member this.NotifyPropertyChanged propertyName = 
        propertyChanged.Trigger(this,PropertyChangedEventArgs(propertyName))

    member this.NotifyPropertyChanged quotation = 
        quotation |> getPropertyName |> this.NotifyPropertyChanged

Answer 1

It is hard to answer your question, because the code cannot really be compiled as-is. This makes it hard to see where the error is, because it is likely related to something that is not shown in your snippet.

I tried getting the code to compile by removing as much as possible. I ended up with the following, but this works fine:

type ViewModelBase() = 
  member x.NotifyPropertyChanged(a:Microsoft.FSharp.Quotations.Expr<'a>) = ()

type NavBarItem() =
    inherit ViewModelBase()
    let mutable header : NavBarItem option = None
    member x.Header      
      with get()  = header
      and  set(v) = 
        header <- v
        base.NotifyPropertyChanged(<@ x.Header @>)
    member x.LoadAsync(panelId:int) = async {
      let result = None
      x.Header  <- result }

If you can extend this minimal sample so that it exhibits the error, then I'm sure people will be able to help.