多态问题C＃

Question

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;
    }

Answer 1

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。

Answer 2

如果将对象实例化为基类，则无法从基类强制转换为子类。

您尝试使用as从User转换为Client或Agent具体取决于您的数据。 但是，您在函数的开头部分创建了一个User对象：

User user = new User();

这个对象类型的User ，以便as将无法将其转换为一个Client或Agent ，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

您需要做的是显式创建您需要的最具体的类，无论是新Agent还是Client ，然后根据需要将其强制转换为更通用的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;
        }
    }

Answer 3

您不能将已实例化为超类的对象转换为子类，因为它不属于该类型，即User类型的对象永远不能是一种代理。

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。

Answer 4

您已将用户声明为User ，而不是Agent或Client 。 因此，您无法将该对象强制转换为Agent或Client ，因为它不是Agent或Client ，而是User 。

您必须更改代码，使其看起来像这样。 （SNIPPIT）：

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.
    }
}

Answer 5

user变量的实例属于User类型，在这种情况下，您无法将base类强制转换为Derived

我建议将User作为abstract class ，提供新方法

abstract User BuildFromDataReader(IDataReader) 

因此， Client和Agent都将提供自己的DataReader构建方法

Answer 6

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;
}

Answer 7

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;
}

Answer 8

一种可能的方法，你可以实现你想要做的是在Agent和Client类上有一个构造函数，它接受一个User参数（实际上是使它们成为User类的Decorator）

因此，

public class Agent : User
{
   public Agent(User user)
   {
   }
}

因此，在您的GetUser（int userid）方法中，您现在可以执行类似的操作

if (type == "agent")                        
{                            
    Agent agent = new Agent(user);
    agent.Company = rdr["company"].ToString();                           
    ..
    ..
    returnuser = agent;                        
 }

希望有助于事业。

Answer 9

增加其他答案：想象一下，它会像你写的那样起作用。 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;

如果最后一行确实会导致非空的Giraffe对象，那么你就会将Ape神奇地变成长颈鹿。

所以基本上：你总是可以将一个孩子投射到一个父母身上，但是如果有问题的对象实际上是那个孩子类型或者它的后代，你只能从父母施放到孩子。

Answer 10

每辆自行车都是车辆，但每辆车都是自行车吗？