c#的多态问题
本文关键字：问题 多态 | 更新日期: 2023-09-27 18:03:54
我有一个DB表的用户，一些是'代理'和一些是'客户端'。在我的c#项目中，我有一个User超类和一个代理和客户端子类。Agent和Client扩展User.
我有一些基本的问题时，铸造或更改用户对象的代理或客户端对象。我也不知道为什么。这可能是相当基本的，但我不知道哪里出错了。
public class User
{
public int UserId { get; set; }
public string UserType { get; set; }
public DateTime DateCreated { get; set; }
public string Email { get; set; }
public string Password { get; set; }
public string Name { get; set; }
public string Phone { get; set; }
public User()
{
}
}
public class Agent : User
{
public string Company { get; set; }
public string CompanyReg { get; set; }
public string SecurityQuestion { get; set; }
public string SecurityAnswer { get; set; }
public string Description { get; set; }
public int AccountBalance { get; set; }
public bool WantsRequests { get; set; }
public string ImageUrl { get; set; }
public Agent()
{
}
}
public class Client : User
{
public string Country { get; set; }
public string IP { get; set; }
public Client()
{
}
}
为什么我不能这样做呢:
public User GetUser(int userid)
{
User user = new User();
User returnuser = user;
string sql = "SELECT usertype, datecreated, email, name, phone, country, ip, company, companyreg, securityquestion, securityanswer, description, accountbalance, aothcredits, wantsrequests FROM ruser WHERE userid=@userid";
try
{
using (SqlConnection con = new SqlConnection(constr))
using (SqlCommand cmd = new SqlCommand(sql))
{
con.Open();
cmd.Connection = con;
cmd.Parameters.Add("@userid", System.Data.SqlDbType.Int).Value = userid;
using (SqlDataReader rdr = cmd.ExecuteReader())
{
if (rdr.Read())
{
user.UserId = userid;
user.UserType = rdr["usertype"].ToString();
user.DateCreated = (DateTime)rdr["datecreated"];
user.Email = rdr["email"].ToString();
user.Name = rdr["name"].ToString();
user.Phone = rdr["phone"].ToString();
string type = rdr.GetString(0);
if (type == "agent")
{
Agent agent = user as Agent;
agent.Company = rdr["company"].ToString();
agent.CompanyReg = rdr["companyreg"].ToString();
agent.SecurityQuestion = rdr["securityQuestion"].ToString();
agent.SecurityAnswer = rdr["securityanswer"].ToString();
agent.Description = rdr["description"].ToString();
agent.AccountBalance = (int)rdr["accountbalance"];
agent.WantsRequests = Convert.ToBoolean(rdr["wantsrequests"]);
returnuser = agent;
}
else //type == "client"
{
Client client = user as Client;
client.Country = rdr["country"].ToString();
client.IP = rdr["ip"].ToString();
returnuser = client;
}
}
}
}
}
catch (SqlException e)
{
throw e;
}
return returnuser;
}
因为你用
一行创建它User user = new User();
它不能神奇地变成它的一个子类(Agent)。您需要它来创建类型，因为它应该。
你应该做的是在开始…
if (type == "agent")
{
user = new Agent();
基本上我认为你误解了多态性。您可以将实例向上转换为它的父实例之一，即
User user = new Agent();
....Later....
Agent agent = user as Agent;
....or.....
Agent agentTwo = new Agent;
User agentAsUser = agentTwo as User;
但是你不能用另一种方式投射。如果你仔细想想，这是有道理的——当应用程序创建内存来保存数据时，它只知道你用new告诉它什么。
如果您将对象实例化为基类，则不能从基类强制转换为子类。
您正在尝试使用as根据您的数据从User转换为Client或Agent。但是，您在函数的开头明确地创建了一个User对象:
User user = new User();
该对象是User类型，因此as将无法将其转换为Client或Agent，并将返回null。参考这里的文档。
as操作符类似于强制类型转换，只是在转换失败时返回null，而不是引发异常。
您可以这样演示:
User u = new User();
System.Console.WriteLine("u is User: " + (u is User));
System.Console.WriteLine("u is Agent: " + (u is Agent));
System.Console.WriteLine("u is Client: " + (u is Client));
// Should produce:
// u is User: true
// u is Agent: false
// u is Client: false
Agent a = new Agent();
u = a;
System.Console.WriteLine("u is User: " + (u is User));
System.Console.WriteLine("u is Agent: " + (u is Agent));
System.Console.WriteLine("u is Client: " + (u is Agent));
// Should produce:
// u is User: true
// u is Agent: true
// u is Client: false
你需要做的是显式地创建你需要的最具体的类，一个新的Agent或Client，然后在需要的时候将其转换为一个更通用的User。
public User GetUser(int userid)
{
User user;
string sql = "...";
try
{
using (SqlConnection con = new SqlConnection(constr))
using (SqlCommand cmd = new SqlCommand(sql))
{
//.. Snip sql stuff ... //
string type = rdr.GetString(0);
if (type == "agent")
{
Agent agent = new Agent();
agent.Company = rdr["company"].ToString();
agent.CompanyReg = rdr["companyreg"].ToString();
agent.SecurityQuestion = rdr["securityQuestion"].ToString();
agent.SecurityAnswer = rdr["securityanswer"].ToString();
agent.Description = rdr["description"].ToString();
agent.AccountBalance = (int)rdr["accountbalance"];
agent.WantsRequests = Convert.ToBoolean(rdr["wantsrequests"]);
user = agent;
}
else //type == "client"
{
Client client = new Client();
client.Country = rdr["country"].ToString();
client.IP = rdr["ip"].ToString();
user= client;
}
// Now do generic things
user.UserId = userid;
user.UserType = rdr["usertype"].ToString();
user.DateCreated = (DateTime)rdr["datecreated"];
user.Email = rdr["email"].ToString();
user.Name = rdr["name"].ToString();
user.Phone = rdr["phone"].ToString();
return user;
}
}
}
}
catch (SqlException e)
{
throw e;
}
}
你不能将一个实例化为父类的对象强制转换为子类，因为它不是那种类型的，也就是说，User类型的对象永远不能是Agent类型的。
您需要重构代码，以便根据从数据库检索到的类型将对象实例化为正确的具体类。
您已经将用户声明为User，而不是Agent或Client。因此，您不能将该对象强制转换为Agent或Client，因为它不是Agent或Client，而是User。
你必须修改你的代码，使它看起来像这样。(片段):
using (SqlDataReader rdr = cmd.ExecuteReader())
{
if(rdr.Read())
{
User user;
string type = rdr.GetString(0);
if (type == "agent")
{
user = new Agent();
// Fill out Agent specific properties
var agent = user as Agent;
agent.Company = ...
}
else if( type == "client" )
{
user = new Client();
// Fill out Client specific properties
var client = user as Client;
}
else
{
throw new InvalidProgramException ("Unknown user-type");
}
// Fill out common User properties.
}
}
user变量的实例是User类型，在这种情况下，您不能将base类强制转换为派生类
我建议将User改为abstract class，提供新的方法
abstract User BuildFromDataReader(IDataReader)
所以Client和Agent将提供自己的实现从DataReader
多态性意味着您可以将Agent的实例视为User，而不是将User的实例视为Agent。
User returnuser;
string sql = "SELECT usertype, datecreated, email, name, phone, country, ip, company, companyreg, securityquestion, securityanswer, description, accountbalance, aothcredits, wantsrequests FROM ruser WHERE userid=@userid";
try
{
using (SqlConnection con = new SqlConnection(constr))
using (SqlCommand cmd = new SqlCommand(sql))
{
con.Open();
cmd.Connection = con;
cmd.Parameters.Add("@userid", System.Data.SqlDbType.Int).Value = userid;
using (SqlDataReader rdr = cmd.ExecuteReader())
{
if (rdr.Read())
{
string type = rdr.GetString(0);
if (type == "agent")
{
Agent agent = user as Agent;
agent.Company = rdr["company"].ToString();
agent.CompanyReg = rdr["companyreg"].ToString();
agent.SecurityQuestion = rdr["securityQuestion"].ToString();
agent.SecurityAnswer = rdr["securityanswer"].ToString();
agent.Description = rdr["description"].ToString();
agent.AccountBalance = (int)rdr["accountbalance"];
agent.WantsRequests = Convert.ToBoolean(rdr["wantsrequests"]);
returnuser = agent;
}
else //type == "client"
{
Client client = user as Client;
client.Country = rdr["country"].ToString();
client.IP = rdr["ip"].ToString();
returnuser = client;
}
returnuser.UserId = userid;
returnuser.UserType = rdr["usertype"].ToString();
returnuser.DateCreated = (DateTime)rdr["datecreated"];
returnuser.Email = rdr["email"].ToString();
returnuser.Name = rdr["name"].ToString();
returnuser.Phone = rdr["phone"].ToString();
}
}
}
}
catch (SqlException e)
{
throw e;
}
return returnuser;
}
您可以将returnUser定义为User，但必须使用正确的类型创建它，例如:
public User GetUser(int userid)
{
User returnuser;
string sql = "SELECT usertype, datecreated, email, name, phone, country, ip, company, companyreg, securityquestion, securityanswer, description, accountbalance, aothcredits, wantsrequests FROM ruser WHERE userid=@userid";
try
{
using (SqlConnection con = new SqlConnection(constr))
using (SqlCommand cmd = new SqlCommand(sql))
{
con.Open();
cmd.Connection = con;
cmd.Parameters.Add("@userid", System.Data.SqlDbType.Int).Value = userid;
using (SqlDataReader rdr = cmd.ExecuteReader())
{
if (rdr.Read())
{
string type = rdr.GetString(0);
if (type == "agent")
{
Agent agent = new Agent();
agent.Company = rdr["company"].ToString();
agent.CompanyReg = rdr["companyreg"].ToString();
agent.SecurityQuestion = rdr["securityQuestion"].ToString();
agent.SecurityAnswer = rdr["securityanswer"].ToString();
agent.Description = rdr["description"].ToString();
agent.AccountBalance = (int)rdr["accountbalance"];
agent.WantsRequests = Convert.ToBoolean(rdr["wantsrequests"]);
returnuser = agent;
}
else //type == "client"
{
Client client = new Client();
client.Country = rdr["country"].ToString();
client.IP = rdr["ip"].ToString();
returnuser = client;
}
returnuser .UserId = userid;
returnuser .UserType = rdr["usertype"].ToString();
returnuser .DateCreated = (DateTime)rdr["datecreated"];
returnuser .Email = rdr["email"].ToString();
returnuser .Name = rdr["name"].ToString();
returnuser .Phone = rdr["phone"].ToString();
}
}
}
}
catch (SqlException e)
{
throw e;
}
return returnuser;
}
补充其他答案:想象它会像您写的那样工作。考虑这个场景:
var ape = new Ape();
var animal = ape as Animal; // Animal is base class of Ape and Giraffe
var giraffe = animal as Giraffe;
如果最后一行确实会产生一个非空的长颈鹿对象，那么你就神奇地把猿变成了长颈鹿。
所以基本上:你总是可以将子类型强制转换为父类型，但是你只能将父类型强制转换为子类型，如果所讨论的对象实际上是该子类型或其子类型
每辆自行车都是一辆车，但是每辆车都是自行车吗?
一种可能的方法，你可以实现你想要做的是在代理和客户端类上有一个构造函数，它接受一个User参数(本质上使它们成为User类的装饰器)
因此,public class Agent : User
{
public Agent(User user)
{
}
}
在GetUser(int userid)方法中，你可以这样做
if (type == "agent")
{
Agent agent = new Agent(user);
agent.Company = rdr["company"].ToString();
..
..
returnuser = agent;
}
希望对事业有所帮助。