BsonDocumentWontSerialize.cs

using System;
using MongoDB.Bson;
using MongoDB.Bson.Serialization.Attributes;

namespace BSONSerializationTest
{

    public class ServiceDescription
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public string Id { get; set; } //If you leave this blank one will be assigned by Cosmos
        public string ServiceName { get; set; }
        public DateTime CreatedAt { get; set; }
    }

    public class ServiceDescriptionBad
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public string Id { get; set; } //If you leave this blank one will be assigned by Cosmos

        public string ServiceName { get; set; }
        public DateTime CreatedAt { get; set; }
        public BsonDocument Doc { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var good = new ServiceDescription() { CreatedAt=DateTime.Now, ServiceName="test"  };
            var works = System.Text.Json.JsonSerializer.Serialize(good, typeof(ServiceDescription));

            var bad = new ServiceDescriptionBad() { CreatedAt = DateTime.Now, ServiceName = "test", Doc = good.ToBsonDocument() };
            var broken = System.Text.Json.JsonSerializer.Serialize(bad, typeof(ServiceDescriptionBad));
        }
    }
}