Json.NET Type Convertor for F# Option<_> support

OptionTypeConverter.fs

module OptionTypeConverter

open System
open Microsoft.FSharp.Reflection
open Newtonsoft.Json

// http://gorodinski.com/blog/2013/01/05/json-dot-net-type-converters-for-f-option-list-tuple/

type OptionTypeConverter() =
  inherit JsonConverter()

  override x.CanConvert(t) =
    t.IsGenericType && t.GetGenericTypeDefinition() = typedefof<option<_>>

  override x.WriteJson(writer, value, serializer) =
    let value =
      if value = null then null
      else
        let _,fields = FSharpValue.GetUnionFields(value, value.GetType())
        fields.[0]
    serializer.Serialize(writer, value)

  override x.ReadJson(reader, t, existingValue, serializer) =
    let innerType = t.GetGenericArguments().[0]
    let innerType =
      if innerType.IsValueType then (typedefof<Nullable<_>>).MakeGenericType([|innerType|])
      else innerType
    let value = serializer.Deserialize(reader, innerType)
    let cases = FSharpType.GetUnionCases(t)
    if value = null then FSharpValue.MakeUnion(cases.[0], [||])
    else FSharpValue.MakeUnion(cases.[1], [|value|])