我有一个用户的数据库表,有些是'代理',有些是'客户'.在我的C#项目中,我有一个User超类和一个Agent和Client子类.代理和客户扩展用户.
在将User对象转换为或更改为Agent或Client对象时,我遇到了一些基本问题.我真的不知道为什么.这可能是相当基本的,但我不知道什么是错的.
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()
{
}
}
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;
}
因为你用线创建了它
User user = new User();
它以后不能神奇地变成它的一个子类(代理).你需要它来创建它应该的类型.
你应该做的就是开始......
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其父级之一,即
User user = new Agent();
....Later....
Agent agent = user as Agent;
....or.....
Agent agentTwo = new Agent;
User agentAsUser = agentTwo as User;
但你不能抛弃其他方式.如果你考虑它,这是有道理的 - 当应用程序创建内存来保存数据时,它只知道你用新的东西告诉它.
如果将对象实例化为基类,则无法从基类强制转换为子类.
您试图使用as从a转换User为a Client或Agent取决于您的数据.但是,您User在函数的开头创建了一个对象:
User user = new User();
此对象是类型,User因此as将无法将其转换为a Client或an Agent并将返回null.请参阅此处的文档.
as运算符就像一个转换器,除了它在转换失败时产生null而不是引发异常.
你可以证明如下:
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
你需要做的是明确地创建你需要的最具体的类,或者新的,Agent或者在你需要的时候Client将它转换为更通用的类User.
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;
}
}
您不能将已实例化为超类的对象强制转换为子类,因为它不属于该类型,即User类型的对象永远不会是一种代理.
您需要重新构建代码,以便根据从数据库中检索的类型将对象实例化为正确的具体类.