Call Constructor from abstract class with readonly values

Question

Im trying to call this constructor from Student into the collegestudent.

Here is the constructor:

abstract class Student
{
    public readonly string FirstName;
    public readonly string LastName;
    public readonly string StudentID;

    public Student(string Value)
    {
        FirstName = Value;
        LastName = Value;
        StudentID = Value;
    }

    public string Value
    {
        get { return FirstName + LastName + StudentID; }
    }
}

Here is CollegeStudent:

class CollegeStudent : Student, IMathClass
{
    public Student(string Tim, string russell, string studentid)
    {
        FirstName = Tim;
        LastName = russell;
        studentID = studentid;
    }
}

Im getting following errors

"Method must have a return type"
"A readonly field cannot be assigned to"

Does this help? https://stackoverflow.com/questions/5601777/constructor-of-an-abstract-class-in-c-sharp — Jacob Huckins, Jul 30 '20 at 20:17
Why just don't call `base` constructor? Btw, `CollegeStudent` declaration is invalid, constructor can't be named as `Student` — Pavel Anikhouski, Jul 31 '20 at 08:04

Alex · Accepted Answer · 2020-07-30T21:07:40.820

The problem might be the initialization order. A readonly variable can only be assigned once (within the constructor when it's a class variable). When you create the object, the variable will already be initialized by the Student constructor because this will be called first (before the constructor of CollegeStudent). When you try to initialize it (again) in the CollegeStudent constructor, it's already initialized and therefore an error raise.

Furthermore, I'd suggest some changes:

First: Make the properties not readonly but define only a public getter and private/protected setter. This might already solve your problem.

public string Firstname {get; protected set;}

Second: The next thing is the constructor and the parameter names of your CollegeStundet. As these are parameters (variables containing information) they look really strange. Better would be:

public CollegeStudent(string firstName, string lastName, string studentId)

You will use it like this to create a new CollegeStudent object:

// this uses polymorphism as all CollegeStudnet are Student
Student studentX = new CollegeStundet("Tim", "Russel", "123AAA");

Third: Setting all the properties to the same value in the default constructor is really odd. In general, this will never be a good idea. Provide at least on general constructor to initialize all three to different values. You can still let them be "empty" (null - when using a default constructor) when you already followed my first suggestion.

 Student(string firstName, string lastName, string studentId);

 // the base keyword will pass the values to the constructor of Student
 CollegeStudent(string firstName, string lastName, string studentId) : base(firstName, lastName, studentId)

But in general, I'd suggest to initialize them in the class where they are defined (Student constructor in this case). When you need to change the values from outside Student and you really want the values to be not writeable, there went something wrong with your concept.

I see what you are saying and this helped out alot, Im just getting into c# so knowing this stuff really helped out, thanks Alex! — William Daly, Jul 30 '20 at 21:27

Call Constructor from abstract class with readonly values

1 Answers1