Confused about a cast error was working before..

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Hello all,
I’m not sure why I’m getting a cast error. I can login as an Admin with no problem. I am using inheritance with customer class inheriting properties from user. A admin is just a normal user. Customer only has a few extra properties but before a lil earlier I was able to run my program no errors now all a sudden im getting this error. I’m so confused it came out of no where I thought maybe it’s a compiler error so I cleaned/rebuilt and still a error…..

C#

//LOGIN SCREEN AND WHERE THE ERROR OCCURS IN MY CATCH CLAUSE *** 			private void BtnLoginUser_Click(object sender, RoutedEventArgs e)    {
                try     {
                    loginUser = SQLuserAccess.UserLogin(txtUsername.Text, txtPassword.Password.ToString());
                    if (txtUsername.Text == loginUser.Username && txtPassword.Password == loginUser.Password)   {
                        if (loginUser.IsAdmin == true){
                            Window showAdminSrc = new AdminWindow(loginUser);
                            showAdminSrc.Show();
                            Close();
                        }else if (loginUser.IsAdmin == false)   {
                            Customer CastUser = new Customer();
                                CastUser = (Customer)loginUser;
          Window nonAdmin = new CustomerScreen(CastUser);//((Customer)loginUser);
                            nonAdmin.Show();
                            Close();
                        }  else
                            lblInvalidText.Content = "Admin is not True or False!";
                    }
                    else  {
                        lblInvalidText.Visibility = Visibility.Visible;
                    }
                } catch(Exception ex)     {
                    ex.Message.ToString();
                    throw ex;
                }
            }
        }	//CUSTOMER SCREEN LOADING CONSTRUCTOR NOTHING SPECIAL..       		 public CustomerScreen(User user) {
            customerUser = (Customer)user;
            InitializeComponent();
            //Display Users 'username' on Src / *future - Data Binding.
            TxtBlockName.Text = "Hello:" + Environment.NewLine + customerUser.Username;
        }

What I have tried:

I have tried casting a few different ways.
I tried having it as I do in the Admin section because it works there.
This was working earlier but now with very few changes, I get this error. I DID NOT CHANGE ANYTHING IN THIS SECTION for getting this error.

Posted 7-Nov-18 19:36pm

TheBigBearNow

Updated 7-Nov-18 19:47pm

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OriginalGriff · Accepted Answer · 2018-11-07T19:47:00

Solution 1

WE can't see the definitions of loginUser and user which are the only explicit casts in that code; we also can't see what type SQLuserAccess.UserLogin returns, or what is in user before you cast that.

So run your code in the debugger, and look at exactly what the variables contain.

At a guess, you say that Customer is derived from User but I'd suspect that UserLogin returns a member of the base User class, not a Customer - and you can't "up-cast" anything.

C#

public class User {}
public class Customer : User {}
...
Customer c;
User u = new Customer ();  // Fine, Customer derived from User.
c = (Customer) u;          // Also fine, u contains a Customer.
u = new User();
c = (Customer) u;          // Error because you can't upcast the User.

Posted 7-Nov-18 19:47pm

OriginalGriff

Comments

TheBigBearNow 8-Nov-18 2:23am

Here is UserLogin
User class is just a class with Username password id property and customer just adds on a few properties like firstname etc..

I think i understand what you are getting at because Yes Login does return a User not a Customer i will try to figure it out otherwise.
In the debugger it gets the correct user and i put that user in the next screen and cast it into a customer i take a user object in CustomerScreen and in the constructor i cast that specific user object over to a Customer.
[code]
public static User UserLogin(string username, string password)
{
//SQL Login Query.
string SQLloginQuery = "SELECT * FROM Users WHERE Username=@username AND Password=@password";
SqlCommand cmdLogin = new SqlCommand(SQLloginQuery, connection);
cmdLogin.Parameters.AddWithValue("@username", username);
cmdLogin.Parameters.AddWithValue("@password", password);
try
{
connection.Open();
//int result = (int)cmdLogin.ExecuteScalar();
SqlDataReader reader = cmdLogin.ExecuteReader(CommandBehavior.SingleRow);
if (reader.Read())
{
User user = new User
{
UserID = Convert.ToInt32(reader["UserId"]),
Username = reader["Username"].ToString(),
Password = reader["Password"].ToString(),
IsAdmin = Convert.ToBoolean(reader["IsAdmin"]),
UserCreatedDate = Convert.ToDateTime(reader["UserCreatedDate"])
};
return user;
}
else
return null;
}
[/code]

Richard Deeming 9-Nov-18 9:15am

You're storing passwords in plain text. Don't do that!

Secure Password Authentication Explained Simply[^]
Salted Password Hashing - Doing it Right[^]

TheBigBearNow 8-Nov-18 2:26am

What do i do to upcast with
[code]c = (Customer) u;[/code]

because YES this is exactly what the issue is.
You have it dead on.

OriginalGriff 8-Nov-18 2:34am

You can't upcast at all! Think about it - if you could do this:

public class A {}
public class B : A {public string s;}
...
A a = new A();
B b = (B) a;
Console.WriteLine(b.s);

Where does the string "s" come from? What does it contain? The system can;t magically create it, and the instance of A in the variable doesn;t have any space allocated for the string - so when you tried to print it's value you would be accessing memory that isn't allocated to the instance - and that could make your code crash in very interesting ways!

TheBigBearNow 8-Nov-18 2:42am

Ok what would i do to fix this error?
In userlogin create another loginmethod one for Customer and keep mine for User or just return customer in loginmethod instead of user because my customer will inherit all the user properties and also have its own customer properties.
I tried code like this but then i get a nullreference error just like you said because no allocated memory here.
[code]
Customer CastUser = loginUser as Customer;
Window nonAdmin = new CustomerScreen(CastUser);
[/code]

OriginalGriff 8-Nov-18 2:55am

The simplest solution is probably to create a Customer constructor that accepts a User and builds a new instance based on it.
But that's clumsy (and may cause problems elsewhere is you do reference comparisons) - I think you would be better off taking a hard look at your class hierarchy and how you are using it: I'd say off hand that your whole design is flawed and you are papering over fundamental mistakes!

OriginalGriff 8-Nov-18 2:57am

And "as" won't solve anything: it doesn't do magic!

Customer CastUser = loginUser as Customer;

is a short form for this:

Customer CastUser = null;
if (loginUser is Customer) CastUser = (Customer) loginUser;

TheBigBearNow 8-Nov-18 3:07am

thank you
I have it working somewhat now. just took away customer with login switched back to user and going to look into it so i can use my customer properties in my later screens. I am still learning. Could you take a quick look at these classes is this how i should have it?
thanks again. P.S. how do i [code][/code] in these comments?

[code]
//My USER class
public class User
{
//IMPLEMENTING/UPDATING C# CODE TO VERSION 7.0.
//create UserID property
private int _userId;
public int UserID
{
get => _userId; //get { return _userId; }
set => _userId = value; //set { _userId = value; }
}
//create username property
private string _username;
public string Username
{
get => _username; //get { return username; }
set => _username = value; //set { _username = value; }
}
//create password property
private string _password;
public string Password
{
get => _password; //get { return _password; }
set => _password = value; //set { _password = value; }
}
//create isAdmin property
private bool _isAdmin;
public bool IsAdmin
{
get => _isAdmin; //get { return _isAdmin; }
set => _isAdmin = value; //set { _isAdmin = value; }
}
//Creating a new property thought of this while creating the Database.
//create UserCreatedDate property
private DateTime _userCreatedDate;
public DateTime UserCreatedDate
{
get => _userCreatedDate; //get { return _userCreatedDate; }
set => _userCreatedDate = value; //set { _userCreatedDate = value; }
}

//Default Constructor for User class
public User() { }

//Constructor without UserId (USED FOR CREATING NEW USER)
public User(string uname, string pass, bool isadmin, DateTime userDate)
{
Username = uname;
Password = pass;
IsAdmin = isadmin;
UserCreatedDate = userDate;
}
//Constructor with UserId (USED FOR UPDATING CURRENTLY SELECTED USER)
public User(int id, string uname, string pass, bool isadmin, DateTime userDate)
{
UserID = id;
Username = uname;
Password = pass;
IsAdmin = isadmin;
UserCreatedDate = userDate;
}
//Constructor for UPDATE and to keep orginal DateCreated ~~(UPDATE: DON'T THINK I NEED)~~11/2/18~
public User(string uname, string pass, bool isadmin)
{
Username = uname;
Password = pass;
IsAdmin = isadmin;
}
//MY CUSTOMER CLASS
public class Customer : User
{
//Properties for the newly created Customer class. V 7.0
//CustomerId Property Foreign key of UserID.
//private int _customerId;
//public int CustomerId
//{
// get => _customerId;
// set => _customerId = value;
//}
////First name property.
//private string _firstname;
//public string FirstName
//{
// get => _firstname;
// set => _firstname = value;
//}

// ~ SWITCHING OVER TO USE AUTO-IMPLEMENTED PROPERTIES ~
//CustomerId Property inherits/foreign key of UserId.
public int CustomerId { get; set; }

//FirstName Property.
public string FirstName { get; set; }

//LastName Property.
public string LastName { get; set; }

//Address Property.
public string Address { get; set; }

//City Property.
public string City { get; set; }

//State Property.
public string State { get; set; }

//Zip Code Property.
publ