C# 多态问题#
我有一个包含用户的DB表,有些是“代理”,有些是“客户”。在我的C#项目中,我有一个用户超类和一个代理和客户端子类。代理和客户端扩展用户 在将用户对象强制转换或更改为代理或客户端对象时,我遇到了一些基本问题。我真的不知道为什么。这可能是很基本的,但我不知道怎么了C# 多态问题#,c#,polymorphism,C#,Polymorphism,我有一个包含用户的DB表,有些是“代理”,有些是“客户”。在我的C#项目中,我有一个用户超类和一个代理和客户端子类。代理和客户端扩展用户 在将用户对象强制转换或更改为代理或客户端对象时,我遇到了一些基本问题。我真的不知道为什么。这可能是很基本的,但我不知道怎么了 public class User { public int UserId { get; set; } public string UserType { get; set; } public DateTime D
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()
{
}
}
现在为什么我不能这样做:
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;
}
如果将对象实例化为基类,则无法从基类强制转换为子类 您正试图使用
as
从用户
转换为客户
或代理
,具体取决于您的数据。但是,您在函数的开头明确地创建了一个User
对象:
User user = new User();
此对象属于User
类型,因此as
将无法将其转换为客户端
或代理
,并将返回null。请参阅文档
as运算符类似于强制转换,只是在转换失败时生成null,而不是引发异常
您可以通过以下方式演示这一点:
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
您需要做的是显式地创建您需要的最具体的类,或者是一个新的代理
或者客户端
,然后在需要时将其转换为更通用的用户
例如:
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;
}
}
因为是你用这条线创造的
User user = new User();
它以后不能神奇地变形为它的子类(代理)。您需要它来创建应该的类型
你应该做的是在开始的时候
if (type == "agent")
{
user = new Agent();
基本上我认为你误解了多态性。您可以将实例向上投射到其父实例之一,即
User user = new Agent();
....Later....
Agent agent = user as Agent;
....or.....
Agent agentTwo = new Agent;
User agentAsUser = agentTwo as User;
但你不能投另一种方式。如果你仔细想想,这是很有道理的——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。你不能将你实例化为超类的对象强制转换为子类,因为它不是那种类型,即User类型的对象永远不能是Agent类型
您需要重新构造代码,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。您已将用户声明为
用户,而不是代理或客户端。因此,您不能将该对象强制转换为代理
或客户端
,因为它不是代理
或客户端
,而是用户
您必须更改代码,使其看起来像这样。(窃笑):
user
变量的实例属于user
类型,在这种情况下,不能将base
类强制转换为派生类
我建议将用户
作为抽象类
,提供新的方法
abstract User BuildFromDataReader(IDataReader)
因此,Client
和Agent
都将提供如何从DataReader构建的自己的实现多态性意味着您可以将代理实例视为用户,而不是将用户实例视为代理
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;
}
一种可能的方法是,在代理类和客户端类上有一个构造函数,它接受一个用户参数(基本上使它们成为用户类的装饰器),从而实现您想要做的事情
所以,
public class Agent : User
{
public Agent(User user)
{
}
}
因此,在GetUser(intuserid)方法中,您现在可以执行以下操作
if (type == "agent")
{
Agent agent = new Agent(user);
agent.Company = rdr["company"].ToString();
..
..
returnuser = agent;
}
希望这有助于解决问题。要补充其他答案,请想象它在编写时会起作用。考虑这种情况:
var ape = new Ape();
var animal = ape as Animal; // Animal is base class of Ape and Giraffe
var giraffe = animal as Giraffe;
如果最后一行的结果确实是一个非空的长颈鹿对象,那么你将神奇地把一只猿变成长颈鹿
因此,基本上:你可以将一个孩子强制转换为一个家长,但你只能将所讨论的对象从家长强制转换为一个孩子,如果该对象实际上是该儿童类型或其后代。每辆自行车都是一辆车,但每辆车都是一辆自行车吗?什么问题?恐怕你没提那件事EntityFramework(如果您想使用ORM工具)或LinqToSql(用于较低的数据库访问)等框架很好地完成了您在这里所做的工作。有什么理由不使用这个吗?也许Jesper应该重新格式化他的问题。问题就在他的代码示例上说明了,但有点看不见;)顺便说一句:如果它是唯一的构造函数,则不需要提供默认构造函数(不带参数的构造函数)。@J.Steen,除非对象实际上是子类的类型,否则他不能提供默认构造函数。在所有情况下,我建议Jesper将用户类标记为抽象:公共抽象类user
。这将有助于正确设计应用程序。这一个(由xan提供)运行良好。return语句需要在if(rdr.read())-部分中,否则用户永远不会被实例化。我想感谢你们提供的信息和答案…@Steve B,是的,我删除了我的评论。不知什么原因,我是以另一种方式阅读的同意抽象类
,但是我不建议将此方法添加到客户端
和代理
,因为它将违反关注点分离。如果这样做:public User GetUser(int userid){User User User User;…If(rdr.Read()){string type=rdr.GetString(0);if(type==“agent”){user=new agent();user.Company=rdr[“Company”].ToString();它不起作用。公司下面有一条红线,上面写着:用户不包含公司的定义。为了访问特定属性,您当然需要强制转换。我想您需要一些OO方面的阅读/课程。(检查我编辑的代码)
public class Agent : User
{
public Agent(User user)
{
}
}
if (type == "agent")
{
Agent agent = new Agent(user);
agent.Company = rdr["company"].ToString();
..
..
returnuser = agent;
}
var ape = new Ape();
var animal = ape as Animal; // Animal is base class of Ape and Giraffe
var giraffe = animal as Giraffe;