using System;

using System.Collections.Generic;

using MongoDB.Bson.Serialization.Attributes;

using System.Text.Json;

using System.Text.Json.Serialization;

public class Program

{

    public static void Main()

    {

        var demo = new List<QuestionAnswerClass>()

        {

            new QuestionAnswerClass()

            {

                Name = "John",

                AdditionalData = new Dictionary<string, object>() 

                {

                    { "Question1", "Answer1" },

                    { "Question2", "Answer2" }

                }

            },

            new QuestionAnswerClass()

            {

                Name = "Mary",

                AdditionalData = new Dictionary<string, object>() 

                {

                    { "Question3", "Answer3" },

                    { "Question4", "Answer4" }

                }

            }

        };

        Console.WriteLine(JsonSerializer.Serialize(demo, new JsonSerializerOptions() { WriteIndented = true }));

    }

}

​

public class QuestionAnswerClass

{

    public string Name { get;set;}

    [BsonExtraElements]

    [JsonExtensionData]

    public Dictionary<string,object> AdditionalData {get; set;}

}