未定义对象

public partial class LayJsonModel
    {
        /// <summary>
        /// 状态
        /// </summary>
        public string code
        {
            get;
            set;
        }
        /// <summary>
        /// 消息
        /// </summary>
        public string msg
        {
            get;
            set;
        }
        public int count
        {
            get;
            set;
        }
        /// <summary>
        /// 数据
        /// </summary>
        public List<dataModel> data
        {
            get;
            set;
        }
 
    }

  public class dataModel
    {
        public string DocEntry
        {get;
        set;}
        public string taxDate
        {
            get;
            set;
        }
    }

public void order_v_list(HttpContext context)
    {
        LayJsonModel msgmodel = new LayJsonModel();

        try
        {

            string message = string.Empty;

            string id = context.Request["id"];

            string sqlstr = "select docentry,taxdate from orders  where cardname='泉州一范'";

            DataTable dt = bdc.GetDataTable(sqlstr);
            StringBuilder JsonSb = new StringBuilder();

            if (dt != null && dt.Rows.Count > 0)
            {
                string jsonString = JsonConvert.SerializeObject(dt);
                msgmodel.code  = "0";
                msgmodel.count = 5000;
                msgmodel.msg = "";

               // JsonSb.Append("[");
                List<dataModel> dmodel = new List<dataModel>();
                for (int i = 0; i < dt.Rows.Count; i++)
                {
                    //dataModel dmodels = new dataModel();
                    //dmodels.DocEntry = dt.Rows[i]["DocEntry"].ToString();
                    //dmodels.taxDate = dt.Rows[i]["taxDate"].ToString();
                    //msgmodel.data.Add(dmodels);//这样写会提示未将对象引用到实例
                    dataModel d = new dataModel();
                    d.DocEntry = dt.Rows[i]["DocEntry"].ToString();
                    d.taxDate = dt.Rows[i]["taxDate"].ToString();
                    dmodel.Add(d);
                   
                }
                msgmodel.data = dmodel;
               
                //string tmp = JsonSb.ToString().Trim(',') + "]";
                //msgmodel.data = tmp;
            }

            string JSONObj = JsonConvert.SerializeObject(msgmodel);


            context.Response.ContentType = "text/plain";
            context.Response.Write(JSONObj); //返回给前台页面  
        }
        catch (Exception ex)
        {
            context.Response.ContentType = "text/plain";
            context.Response.Write(ex.Message); //返回给前台页面  
            context.Response.End();
        }
    }