using System;

using System.Text.Json;

using System.Text.Json.Serialization;

​

namespace StackOverflowExample;

​

public class Program

{   

    static void Main()

    {

        var subclass = new Subclass

        {

            NaN = double.NaN 

        };

​

        var serializeOptions = new JsonSerializerOptions

        {

            NumberHandling = JsonNumberHandling.AllowNamedFloatingPointLiterals,

        };

        serializeOptions.Converters.Add(new SubConvertor()); 

​

        var jsonString = JsonSerializer.Serialize(subclass, serializeOptions);

        Console.WriteLine(jsonString);

        var deserialized = JsonSerializer.Deserialize<Subclass>(jsonString, serializeOptions);

        Console.WriteLine(deserialized.NaN);

    }

​

    public class Subclass

    {

        public double NaN { get; set; } = double.NaN;

    }

    public class SubConvertor : JsonConverter<Subclass>

    {

        public override Subclass Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

        {

            return new Subclass();

        }

​

        public override void Write(Utf8JsonWriter writer, Subclass dateTimeValue, JsonSerializerOptions options)

        {

            writer.WriteStringValue("NaaaN");

            writer.WriteNumberValue(dateTimeValue.NaN);

        }

    }

}