Question

I have a DB table with users, some are ‘agents’ and some are ‘clients’. In my C# project, I have a User superclass and an Agent and Client subclass. Agent and Client extends User.

I am having some basic problems when casting or changing a User object to an Agent or Client object. I don’t really know why. It’s probably rather basic, but I don’t know what’s wrong.

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()
    {
    }
}

Now why can’t I do this:

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

You can’t cast from a base class to a child class if you instantiate the object as the base class.

You are attempting to use as to cast from a User to a Client or an Agent depending on your data. However, you are explicately creating a User object at the start of your function:

User user = new User();

This object is of type User so as will not be able to convert it to a Client or an Agent and will return null. See the documentation here.

The as operator is like a cast except that it yields null on conversion failure instead of raising an exception.

You can demonstrate this as follows:

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

What you need to do is to explicitly create the most specific class you need, either a new Agent or Client, then cast this to a more generic User when and as you need to.

For example:

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;
        }
    }