open System

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/73551490/how-to-define-singleorarrayconverter-to-handle-json-property-in-f-port-from-c

​

type SingleOrArrayConverter<'T>() =

    inherit JsonConverter()

    override this.CanConvert(objectType) = objectType = typeof<ResizeArray<'T>> 

    override this.ReadJson(reader, objectType, existingValue, serializer) =  // Unlike in C# it's not necessary to declare the types of the arguments when there is no ambiguity

        let token = JToken.Load (reader)

        if token.Type = JTokenType.Array then

            // Upcast to obj as upcasting is not automatic for returned value

            // https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/casting-and-conversions#upcasting

            (token.ToObject<ResizeArray<'T>> ()) :> obj

        else

            ResizeArray [|(token.ToObject<'T> ())|] :> obj  // Convert array to List<T> then upcast to object

    override this.CanWrite = false                       // Simplified syntax

    override this.WriteJson(writer, value, serializer) = raise (new NotImplementedException())

type Locality = {

    category: string

    id: int

    location: string

    postcode: int

    state: string

    latitude: decimal

    longitude: decimal

}

​

type Localities = {

    [<JsonProperty("locality")>]

    [<JsonConverter(typeof<SingleOrArrayConverter<Locality>>)>]  // Fix syntax: typeof<'T> not typeof(T)

    locality: ResizeArray<Locality>  // Be sure whether you want System.Collections.Generic.List<'T> a.k.a ResizeArray<'T> or FSharp.Collections.List<'T>

}

​

type PostCodeLocality = {

    localities : Localities

}

​

let json0 = """

{

  "localities": {

    "locality":

      {

        "category": "category",

        "id": 101,

        "location": "location",

        "postcode": 22,

        "state": "ZZ",

        "latitude": 81.0,

        "longitude": 202.0

      }

  }

}

"""

​

let p1 = JsonConvert.DeserializeObject<PostCodeLocality>(json0)

​

let p2 : PostCodeLocality = { localities = { locality = new ResizeArray<Locality>([|{category="category"; id=101; location="location"; postcode=22;state="ZZ"; latitude=81.0M; longitude=202M }|]) } }

​

let json1 = JsonConvert.SerializeObject(p1, Formatting.Indented)

​

let json2 = JsonConvert.SerializeObject(p2, Formatting.Indented)

​

if (json1 <> json2) then raise (Exception("json1 != json2"))

​

let p3 = JsonConvert.DeserializeObject<PostCodeLocality>(json2)

​

let json3 = JsonConvert.SerializeObject(p3, Formatting.Indented)

​

if (json1 <> json3) then raise (Exception("json1 != json3"))

​

​