using System;

using System.IO;

using System.Text;

using System.Text.Json;

using AutoFixture;

public class Program

{

    public static void Main()

    {

        var fixture = new Fixture();

        fixture.Customize<Car>(

            c => c.With(

                x => x.GeneralCarInfo,

                (CarInfo ci) => JsonDocument.Parse(JsonSerializer.Serialize(ci))));

        var actual = fixture.Create<Car>();

        Console.WriteLine(actual.GeneralCarInfo.ToJsonString());

    }

}

​

public class Car 

{

   public string Name;

   public int Age;

   public JsonDocument GeneralCarInfo;

}

​

public class CarInfo

{

    public string Model {get;set;}

    public int Year {get;set;}

}

​

public static class Extensions

{

    public static string ToJsonString(this JsonDocument jdoc)

    {

        using (var stream = new MemoryStream())

        {

            Utf8JsonWriter writer = new Utf8JsonWriter(stream, new JsonWriterOptions { Indented = true });

            jdoc.WriteTo(writer);

            writer.Flush();

            return Encoding.UTF8.GetString(stream.ToArray());

        }

    }

}