保存或编辑下拉列表中的值multiselect C＃WebAPI

Question

我正在用Post方法执行保存或编辑操作，我必须包括或编辑dropdow的值，但是我不知道该怎么做。

包括我已经在做的，但是当我编辑具有多选功能的下拉列表的值并删除该组件的数据时，将保留我已经拥有的那些，而不是删除并仅插入选定的内容以进行更新记录。

在api控制器中关注我的帖子代码

  [JWTAuthFilter]
    [System.Web.Http.HttpPost]
    [System.Web.Http.Route("api/Register")]
    public HttpResponseMessage Post([FromBody] UserDataModel userModel)
    {

        string pl = userModel.Name;
        if (Regex.IsMatch(pl, @"^[a-zA-Z áéíóã]*$"))
        {
            var msg = "Nome válido";

        }
        else
        {
            var msg = "Nome inválido";
        }

        User user = new User();

        if (userModel.Name == null || userModel.Name.Trim().Equals(""))
        {
            throw new Exception("ERRO");
        }



        if (userModel.IdUser != 0)
        {

            //editar -- edit
            user = ctx.User.Find(userModel.IdUser);
        }
        else
        {
            //criar -- create
            ctx.User.Add(user);
        }
        try
        {
            user.Area = ctx.Area.FirstOrDefault(it => it.IdArea == userModel.areasModel.Id);

            user.ApplicantSubstituteUser = ctx.User.FirstOrDefault(it => it.IdUser == userModel.Applicant.apId);

            /* comentário de teste */
            //verificar
            if (userModel.typesModel != null)
            {
                foreach (var typesModelScreen in userModel.typesModel)
                {
                    user.RequestType.Add(ctx.RequestType.Single(it => it.IdRequestType == typesModelScreen.Id));
                }

            }

            user.Name = userModel.Name;
            user.IdTr = userModel.IdTr;
            user.Password = userModel.Password;
            user.Enabled = userModel.Enabled;
            user.IsAdmin = userModel.IsAdmin;
            user.IsRequester = userModel.IsRequester;
            user.IsAnalyst = userModel.IsAnalyst;
            user.Email = userModel.Email;
            user.Manager = userModel.Manager;
            user.ActingType = userModel.ActingType;
            user.Telephone = userModel.Telephone;
            user.DateRegister = DateTime.UtcNow;

            ctx.SaveChanges();

            var message = Request.CreateResponse(HttpStatusCode.Created, userModel + "Usuário registrado com sucesso!");

            return message;
        }
        catch (Exception ex)
        {
            return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ex);
        }
    }