open System

open System.Collections

open System.Collections.Generic

open System.Linq

open System.Text

open System.Runtime.Serialization.Formatters

open System.IO

open System.Data

open System.ComponentModel.DataAnnotations

open System.Globalization

open System.Reflection

open Microsoft.FSharp.Reflection

​

open Newtonsoft.Json

open Newtonsoft.Json.Linq

open Newtonsoft.Json.Converters

open Newtonsoft.Json.Serialization

​

// https://stackoverflow.com/questions/59284404/why-does-an-f-discriminated-union-fails-to-have-its-typeconverter-respected-by

​

type NoConverter<'a> () =

    inherit JsonConverter()

    override this.CanConvert(t) = (t = typedefof<'a>)

    override this.CanRead = false

    override this.CanWrite = false

    override this.WriteJson(_, _, _) =  raise (NotImplementedException());

    override this.ReadJson(_, _, _, _) =  raise (NotImplementedException());

​

type [<System.ComponentModel.TypeConverterAttribute(typeof<CC>)>] C = A of string

and CC() = 

    inherit System.ComponentModel.TypeConverter()

    override this.CanConvertFrom (_, t) = (t = typeof<string>)

    override this.ConvertFrom(_, _, s) = s :?> string |> A |> box<C>

    override this.CanConvertTo (_, t) = t = typeof<string>

    override this.ConvertTo(_, _, s, _) = s :?> C |> fun (A s) -> s |> box<string>

let settings = JsonSerializerSettings(Converters = [|NoConverter<C>()|])

printfn "%s" (Newtonsoft.Json.JsonConvert.SerializeObject(A "123", settings))

​