Protobuf DU serialisation

gistfile1.fs

#r "System.Xml.dll"
#r "System.Runtime.Serialization.dll"
#r @"D:\Appdev\fsharp.actor\packages\protobuf-net.2.0.0.640\lib\net40\protobuf-net.dll"

#load "Library1.fs"
open ProtoBuf
open System.IO
open System.Runtime.Serialization
open Protobuf
open Microsoft.FSharp.Reflection


[<ProtoContract(SkipConstructor = true)>]
type Result = 
    | Success of string
    | Error of string


[<CLIMutable>]
type ResultRecord = {
    Id : int
    Name : string
}


ProtoBuf.Meta.RuntimeTypeModel.Default.Add(typeof<ResultRecord>, true).Add("Id", "Name")

let cases = FSharpType.GetUnionCases(typeof<Result>)
typeof<Result>.GetNestedTypes()
|> Array.filter (fun x -> not <| x.Name.EndsWith("Tags"))
|> Array.iteri (fun i x -> 
                printfn "Registered %A" x.Name 
                let factory = FSharpValue.PreComputeUnionConstructorInfo(cases.[i])
                printfn "Return type %A" factory.ReturnType
                ProtoBuf.Meta.RuntimeTypeModel.Default.Add(x, false).SetFactory(factory) |> ignore)

let serialise instance = 
    use ms = new MemoryStream()
    Serializer.Serialize(ms, instance)
    ms.ToArray()

let deserialise (bytes:byte[]) = 
    use ms = new MemoryStream(bytes)
    Serializer.Deserialize(ms)

let du : Result = 
    serialise (Success "Foo")
    |> deserialise 

//Unfortunately I get the following exception..
//System.ArgumentException: The factory-method must return object or FSI_0006+Result+Success
//Parameter name: factory
//   at ProtoBuf.Meta.RuntimeTypeModel.VerifyFactory(MethodInfo factory, Type type) in c:\Dev\protobuf-net\protobuf-net\Meta\RuntimeTypeModel.cs:line 1958
//   at [email protected](Int32 i, Type x) in D:\Appdev\playground\Protobuf\Protobuf\Script.fsx:line 27
//   at Microsoft.FSharp.Collections.ArrayModule.IterateIndexed[T](FSharpFunc`2 action, T[] array)
//   at <StartupCode$FSI_0006>.$FSI_0006.main@() in D:\Appdev\playground\Protobuf\Protobuf\Script.fsx:line 21
//Stopped due to error

//Which is clear enough I just can't see how to construct a MehtodInfo that returns the correct type.