Runtime error using Xamarin Android with F# and ReactiveUI

Question

I have created a Xamarin Android project that is using F# and ReactiveUI.

When loading my Dashboard, I encounter a runtime exception (of type MissingMethodException) on the inherit line of this code snippet:

type DashboardViewModel(?host: IScreen) =
    inherit ReactiveViewModel()
    let host = LocateIfNone host
    member __.Title with get() = "Dashboard"
    interface IRoutableViewModel with
        member __.HostScreen = host
        member __.UrlPathSegment = "Dashboard"

The error message reads

Method 'Microsoft.FSharp.Quotations.FSharpExpr.Deserialize40' not found.

The ReactiveViewModel type is a thin wrapper around ReactiveObject:

type ReactiveViewModel() as this =
    inherit ReactiveObject()
    let mutable message = noMessage
    let uiContext = SynchronizationContext.Current
    member __.SyncContext with get() = uiContext
    member this.Message 
        with get() = message 
        and set(value) = 
            this.RaiseAndSetIfChanged(&message, value, "Message") |> ignore
            if message <> noMessage then this.RaiseAndSetIfChanged(&message, noMessage, "Message") |> ignore
    member val MessageSent = 
        this.WhenAnyValue(toLinq <@ fun vm -> vm.Message @>).ObserveOn(RxApp.MainThreadScheduler).Where(fun m -> m <> noMessage) with get

The project is open source: at the moment, it contains very little content. It can be found at https://github.com/SpiegelSoft/Astrid.

I have submitted a bug on the Xamarin bug tracker: https://bugzilla.xamarin.com/show_bug.cgi?id=51000

Are there any known fixes I can implement myself, so that I can close the bug of my own accord?

UPDATE 1

I've been investigating this issue this weekend.

The FSharp.Core version that is loaded is stuck on the obsolete version 2.3.98.1. This corresponds to the FSharp.Core.dll file in

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\MonoAndroid\v1.0

I have tried to remove this version and load the NuGet package FSharp.Core; however, when I build the Android project, the path always reverts to the obsolete file in the Reference Assemblies path.

Is there a way to override this behaviour?

UPDATE 2

Replacing the FSharp.Core.dll file in the Reference Assemblies path fixes the issue, but this is a very unsatisfactory sticking plaster, which I can't ask my users to apply. Ideally, I would like to find a way to prevent the .Droid project from loading FSharp.Core from the GAC rather than the NuGet package.

Answer 1

I just ran into a very similar issue the other day. My android app had a reference to a Profile7 F# PCL library, which made use of List.unfold, which I believe was introduced in F# 4. When I used the library in my app, I saw a MissingMethodException similar to what you are seeing. The version of FSharp.Core that Xamarin references by default when creating a new Android app didn't have this newer method. I got around it by editing the .fsproj file for the app to remove the original reference to FSharp.Core, and replaced it with a reference to the newer version ( I copy/pasted the tag from the PCL .fsproj file). It looks something like this:

<Reference Include="FSharp.Core">
   <Name>FSharp.Core</Name>
   <Private>True</Private>
   <AssemblyName>FSharp.Core.dll</AssemblyName>
   <HintPath>$(MSBuildExtensionsPath32)\..\Reference Assemblies\Microsoft\FSharp\.NETCore\$(TargetFSharpCoreVersion)\FSharp.Core.dll</HintPath>
</Reference> 

I was suprised to find that this seems to have fixed the problem for me. I'm not sure if I'll run into other issues down the line, but it may be worth trying this if you havent already.

UPDATE If this doesn't work immediately, follow the sequence of steps in Rob Lyndon's answer.

Answer 2

It appears that this has been fixed by the GitHub commit

https://github.com/xamarin/xamarin-android/commit/df41af046000556ed82f638e8041b7f718966a92

which removes FSharp.Core from the list of framework assemblies, and allows the project to be built without the NuGet FSharp.Core assembly being replaced.

Until this fix is released into the SDK, there is a workaround. The answer submitted by user3850711 works, but you need to apply the changes in a specific sequence, because otherwise the reference will be overwritten during the build.

1. Delete the existing reference to FSharp.Core.
2. Install or reinstall the FSharp.Core NuGet package.
3. Unload the project and add <HintPath>packages\FSharp.Core.4.0.0.1\lib\portable-net45+monoandroid10+monotouch10+xamarinios10\FSharp.Core.dll</HintPath> to the FSharp.Core project reference.