How to extend a Class in C#? [closed]

Question

I have my own Object, and I'd like to extend it, saving data from a person and adding new info.

So the code would be:

public class Student : Person {     public string code { get; set; } } 

but when I try to init it and add the new value:

Person person = new Person("Paul", "Catch"); Student student = (Person)person; student.code = "1234"; 

I got System.InvalidCastException: Unable to cast object of type 'MyLayer.Person' to type 'Student'.

Am I missing some point?

EDIT: maybe I wrong putting that Person class. You must suppose it become from a DB as object, such as Person person = new MyPersons().First();

So I won't to populate the new one with properties one by one, just extend one property thanks to the new object that extend the old one.

Answer 1

You cannot convert a Person to Student directly.

Inheritance in OOP comes with hierarchy level, i.e. you can cast the derived class to base class, but the opposite is not possible. You cannot cast base class to derived class

One possible solution is :

Create Student from Person using constructor overload.

public class Person {     public string FName { get; set; }     public string LName { get; set; }      public Person(string fname, string lname)     {         FName = fname;         LName = lname;     } }  public class Student : Person {     public Student(Person person, string code)         : base(person.FName, person.LName)     {         this.code = code;     }     public Student(Person person)         : base(person.FName, person.LName)     {      }      public string code { get; set; } }    static class Program {     static void Main(string[] args)     {         Person person = new Person("Paul", "Catch");          // create new student from person using          Student student = new Student(person, "1234");          //or         Student student1 = new Student(person);         student1.code = "5678";          Console.WriteLine(student.code); // = 1234         Console.WriteLine(student1.code); // = 5678     } }