gistfile1.fs

type UnionTypeConverter() =
    inherit JsonConverter()

    let doRead pos (reader: JsonReader) = 
        reader.Read() |> ignore //pop start obj type label
       // printfn "%sRead %s %A %A" ("".PadLeft(reader.Depth)) pos reader.Value reader.TokenType 

    override x.CanConvert(typ:Type) =
        let result = 
            ((typ.GetInterface(typeof<System.Collections.IEnumerable>.FullName) = null) 
            && FSharpType.IsUnion typ)
        result

    override x.WriteJson(writer: JsonWriter, value: obj, serializer: JsonSerializer) =
        let t = value.GetType()
        let write (name : string) (fields : obj []) = 
            writer.WriteStartObject()
            writer.WritePropertyName("case")
            writer.WriteValue(name)  
            writer.WritePropertyName("values")
            serializer.Serialize(writer, fields)
            writer.WriteEndObject()   

        let (info, fields) = FSharpValue.GetUnionFields(value, t)
        write info.Name fields

    override x.ReadJson(reader: JsonReader, objectType: Type, existingValue: obj, serializer: JsonSerializer) =      
         let cases = FSharpType.GetUnionCases(objectType)
         if reader.TokenType <> JsonToken.Null  
         then 
           // printfn "%A %A" objectType.FullName existingValue
            doRead "1" reader
            doRead "2" reader
            //printfn "Cases %A %A %A" cases reader.TokenType reader.Value
            let case = cases |> Array.find(fun x -> x.Name = if reader.Value = null then "None" else reader.Value.ToString())
            doRead "3" reader
            doRead "4" reader
            doRead "5" reader //pop item name
            let fields =  [| 
                   for field in case.GetFields() do
                       let result = serializer.Deserialize(reader, field.PropertyType)
                       reader.Read() |> ignore
                       yield result
             |] 
            let result = FSharpValue.MakeUnion(case, fields)
            while reader.TokenType <> JsonToken.EndObject do
                doRead "6" reader         
            result
         else
            FSharpValue.MakeUnion(cases.[0], [||]) 

//Example usage

open Newtonsoft.Json

type HtmlElement = 
    | Value of string option
    | Node of string * HtmlElement
    

let jsonSettings = 
    let js = new JsonSerializerSettings()
    js.Converters.Add(new UnionTypeConverter())
    js


let x = Node("div", Node("a", Value(None)))

let serialisedValue = JsonConvert.SerializeObject(x, Formatting.Indented, jsonSettings)

let y = JsonConvert.DeserializeObject<HtmlElement>(serialisedValue, jsonSettings)

(* Produces this json *)
"{
  "case": "Node",
  "values": [
    "div",
    {
      "case": "Node",
      "values": [
        "a",
        {
          "case": "Value",
          "values": [
            null
          ]
        }
      ]
    }
  ]
}"