ASP Web服务中的动态数据检索

Question

这是我的代码，“列”由来自透视查询的"ID,"01/01/2018","01/02/2018"组成

               var reader = cmd.ExecuteReader();
                var columns = new List<string>();
                for (int i = 0; i < reader.FieldCount; i++)
                {
                    columns.Add(reader.GetName(i));
                    string xxx;
                    xxx = reader.GetName(i);

                    while (reader.Read())
                    {
                        My_cs abc= new My_cs ();
                        abc.FieldName[i] = reader[xxx].ToString();
                        listabc.Add(company);
                    }
                    JavaScriptSerializer jss = new JavaScriptSerializer();
                    Context.Response.Write(jss.Serialize(listabc));
                }

我的错误：

Object reference not set to an instance of an object.

请帮助