server/src/Core/Models/Api/Response/ResponseModel.cs

using System;
using Newtonsoft.Json;

namespace Bit.Core.Models.Api
{
    public abstract class ResponseModel
    {
        public ResponseModel(string obj)
        {
            if(string.IsNullOrWhiteSpace(obj))
            {
                throw new ArgumentNullException(nameof(obj));
            }

            Object = obj;
        }

        [JsonProperty(Order = -200)] // Always the first property
        public string Object { get; private set; }
    }
}