using System;

using System.Text.Json;

using System.Text.Json.Serialization;

public class Program

{

    public static void Main()

    {

        Outer _Outer = new Outer{ A = "a", B = new Inner { C = JsonSerializer.Serialize(new SuperInner{ D = "d" }) } };

        Console.WriteLine(JsonSerializer.Serialize(_Outer));

        Outer2 _Outer2 = new Outer2{ A = "a", B = new Inner2 { C = JsonSerializer.Serialize(new SuperInner{ D = "d" }) } };

        Console.WriteLine(JsonSerializer.Serialize(_Outer2));

    }

}

​

public class Outer

{

    public string A {get;set;}

    public Inner B {get;set;}

}

​

public class Inner

{

    public string C {get;set;}

}

​

public class Outer2

{

    public string A {get;set;}

    public Inner2 B {get;set;}

}

​

public class Inner2

{

    [JsonConverter(typeof(RawJsonConverter))]

    public string C {get;set;}

}

​

public class SuperInner

{

    public string D {get;set;}

}

​

public sealed class RawJsonConverter : JsonConverter<string>

    {

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

        {

            if (JsonDocument.TryParseValue(ref reader, out JsonDocument _Document))

            {

                using (_Document)

                {

                    return _Document.RootElement.GetRawText();

                }

            }

​

            return reader.GetString();

        }

​

        public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)

        {

            try

            {

                writer.WriteRawValue(value);

            }

            catch (JsonException)

            {

                writer.WriteStringValue(value);

            }

        }

    }