EF6如何写update方法

[cnuusw]

public void UpdateWithLog(T entity, Guid creatorGuid)
        {
            dbContext.Entry(entity).State = EntityState.Modified;
            //dbContext.Set<T>().Attach(entity);
            //dbContext.Entry(entity).State = EntityState.Modified;
            dbContext.Set<ModelLog>().Add(GetModelLog(entity, creatorGuid, "Update"));
        }

如上方法报错：




具体操作过程：
Find----序列化-----反序列化-----修改model值----update

求助，该如何写EF的Update方法。



补充：

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                if (!string.IsNullOrEmpty(Request.QueryString["userid"]) && CheckValue.IsInteger(Request.QueryString["userid"]))
                {
                    LoadInfo();
                }
                else
                {
                    this.btnSave.Enabled = false;
                }
            }
        }

        protected void btnSave_Click(object sender, EventArgs e)
        {
            BLL.UserService bll = new BLL.UserService();
            Model.User model = (Model.User)ViewState["model"];
            //Model.User model = bll.GetById(int.Parse(Request.QueryString["userid"]));
            model.OrganizationID = int.Parse(this.OrganizationList.SelectedValue);
            bll.UpdateWithLog(model);
            ShowMsgHelper.ShowScript("art.dialog({title: \"恭喜，操作成功\", content: \"恭喜，操作成功，用户信息已成功更新！\", ok:function(){ art.dialog.close(); art.dialog.opener.ListGrid(); }});");
        }


        private void LoadInfo()
        {
            BLL.UserService bll = new BLL.UserService();
            BLL.UserTypeService usertype = new BLL.UserTypeService();
            BLL.OrganizationService org = new BLL.OrganizationService();
            Model.User model = bll.GetById(int.Parse(Request.QueryString["userid"]));

            CSS = model.Sex ? "man" : "woman";
            OfficeID = model.OfficeID;
            this.ltlUserName.Text = model.UserName;
            this.ltlRealName.Text = model.RealName;
            this.ltlUserType.Text = usertype.GetById(model.UserTypeID).UserTypeName;
            this.ltlOrganizationName.Text = org.GetOrganizationFullName(model.OrganizationID);
            ViewState["model"] = model;

            this.OrganizationList.SelectedValue = model.OrganizationID.ToString();
        }

使用viewstate后就出该问题，如果不使用viewstate 而是在此getbyid就无问题。 求解。