c# - 如何在不获取 NULLREFERENCEEXCEPTION 的情况下添加到列表？

Question

我一直在尝试添加到列表列表中，添加来自 sql 数据库的信息。将新信息添加到列表时出现的错误是:

“在 StatusScope.dll 中发生类型为‘System.NullReferenceException’的异常，但未在用户代码中处理

附加信息:未将对象引用设置到对象的实例。"

我该如何解决这个错误？

map 逻辑.cs

public static List<MapModel.ClientInfo> GetClientsData()
    {
        SqlConnection Connection = site.Models.Shared.DBConnection.GetConnection();
        SqlDataReader Reader = null;
        SqlCommand Command = new SqlCommand("SELECT DocInfo.DocID, DocInfo.DocName, DocInfo.DocPic, DocInfo.PatientAcceptance, ClientInfo.ClientName, ClientInfo.AddressLocal, ClientInfo.AddressBroad, ClientInfo.Phone, ClientInfo.Lat, ClientInfo.Long, ClientInfo.ClientID FROM DocInfo INNER JOIN ClientInfo ON DocInfo.ClientID = ClientInfo.ClientID;", Connection);
        Reader = Command.ExecuteReader();

        var ClientsData = new List<MapModel.ClientInfo> { };
        int IDCounter = 0;
        bool FirstRun = false;

        while (Reader.Read())
        {
            if (!FirstRun)
            {
                ClientsData.Add(new MapModel.ClientInfo { Id = IDCounter, ClientID = Reader["ClientID"].ToString(), ClientName = Reader["ClientName"].ToString(), DocPic = Reader["DocPic"].ToString(), PatientAcceptance = Reader["PatientAcceptance"].ToString(), AddressLocal = Reader["AddressLocal"].ToString(), AddressBroad = Reader["AddressLocal"].ToString(), Phone = Reader["Phone"].ToString(), latitude = Reader["Lat"].ToString(), longitude = Reader["Long"].ToString(), DocNames = { } });
                FirstRun = true;
            }
            else
            {
                for (var x = 0; x < ClientsData.Count; x++)
                {
                    if (ClientsData[x].ClientID == Reader["ClientID"].ToString())
                    {
                        ClientsData[x].DocNames.Add("123"); //error occurs here
                    }
                    else
                    {
                        ClientsData.Add(new MapModel.ClientInfo { Id = IDCounter, ClientID = Reader["ClientID"].ToString(), ClientName = Reader["ClientName"].ToString(), DocPic = Reader["DocPic"].ToString(), PatientAcceptance = Reader["PatientAcceptance"].ToString(), AddressLocal = Reader["AddressLocal"].ToString(), AddressBroad = Reader["AddressLocal"].ToString(), Phone = Reader["Phone"].ToString(), latitude = Reader["Lat"].ToString(), longitude = Reader["Long"].ToString() });
                    }
                }
            }




            IDCounter++;
        }

        Connection.Close();
        return ClientsData;
    }

map 模型.cs

public class ClientInfo
        {
            public int Id { get; set; }
            public string ClientID { get; set; }
            public string ClientName  { get; set; }

            public List<string> DocNames { get; set; }

            public string DocPic { get; set; }

            public string PatientAcceptance { get; set; }
            public string AddressLocal { get; set; }
            public string AddressBroad { get; set; }
            public string Phone { get; set; }
            public string latitude { get; set; }
            public string longitude { get; set; }


        }

Answer 1

您是否曾经将 DocNames 设置为任何内容？如果不是，它将为空。

在 ClientInfo 的构造函数中你应该有

DocNames = new List<string>();

初始化一个空列表。