C#•9mo ago

Calling methods from a base class

Hi, I am completing a beginners c# course and i'm stuck on this question - this section of the course is about polymorphism and inhertiance. The code was already written, I have added an 'ALevelComputerStudent' class, and now I must add functionality to pick which type of student is created. I'm not sure how to do this. I've tried using an interface, but then when I use an interface for 'ALevelComputerStudent' + 'GCSEStudent' classes, i can no longer call methods from the base class. I know there are other ways of doing it , but for the purpose of the question I have one base class which the rest add onto I just cant get my head around when to use inheritance and interfaces without changing all of the existing code written around the 'AStudent' object any help/ explanation would be appreciated 🙂

Program.cs

84 Replies

Pobiega•9mo ago

$paste your code here please, instead of as an attachment

MODiX•9mo ago

If your code is too long, you can post to https://paste.mod.gg/ and copy the link into chat for others to see your shared code!

Pobiega•9mo ago

So, all your code here minus the ALevelComputerStudent class were pre-written and given to you? and your goal is to add this new class, and modify the program so that "it asks you what type of student to create: A Level or GCSE"?

JammyG•9mo ago

All the code, except from the ALevelCompterStudent class i added was pre written

Pobiega•9mo ago

JammyG•9mo ago

yeah, stock, the program only works with GCSE students

Pobiega•9mo ago

I can see that.

JammyG•9mo ago

the goal is to add a class ' ALevelStudent' and change the code to work with both

Pobiega•9mo ago

well, you already have the ComputerStudent base class

JammyG•9mo ago

https://paste.mod.gg/njmzysglayyq/0 like this?

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Pobiega•9mo ago

yep okay, so where exactly are you stuck?

JammyG•9mo ago

i need to refactor the Main so that it can use ALevelStudent and GCSEStudent im not sure how to do that without getting a compiler error

Pobiega•9mo ago

Yep.. and I can see you've sort of started on that well, can you name a data type that can hold both GCSEComputerStudent and ALevelComputerStudent objects?

JammyG•9mo ago

can the base class ComputerStudent do that?

Pobiega•9mo ago

it sure can

JammyG•9mo ago

c++
   ComputerStudent AStudent;

   if (choice == 1)
   {
       AStudent = new ALevelComputerStudent();
   }
   else if (choice == 2)
   {
       AStudent = new GCSEComputerStudent();
   }

c++
   ComputerStudent AStudent;

   if (choice == 1)
   {
       AStudent = new ALevelComputerStudent();
   }
   else if (choice == 2)
   {
       AStudent = new GCSEComputerStudent();
   }

Pobiega•9mo ago

there we go

JammyG•9mo ago

this gives me an error

Pobiega•9mo ago

does it really?

JammyG•9mo ago

Pobiega•9mo ago

thats curious because I have your code, written as is, running on the left here and I don't get that error

JammyG•9mo ago

i fixed it, ok, so i have a type computer student that can store either Alevel or GCSE now how do i call methods on it ? it breaks all the code

Pobiega•9mo ago

well, thats the thing both GCSE and ALevel students have methods that the base class does not you'll need to add them to the base class in order to call them

JammyG•9mo ago

Is that the only way of doing that like in the prewritten code, GCSE builds upon the ComputerStudentClass

Pobiega•9mo ago

since this assignment is about learning about polymorphism, I'd say... yes

JammyG•9mo ago

it gives me that diagram too the whole thing before it, was about not needing to touch the base class when making new classes how you can just build upon the base class

Pobiega•9mo ago

And you can. But you are trying to add a new "API surface" to the class you could do pattern matching to check if its ALevel or GCSE before calling the method everywhere, but that would be insanity you could also add an interface to both GCSE and ALevel and use that as your variable but unless the assignment actually mentions that, I find that solution a bit weird the obvious solution here is to use virtual/override keywords, if you ask me

JammyG•9mo ago

i tried to use an interface originally as then i wouldnt have to touch the base class, but in this course, i haven't gone over Interfaces yet so i thought i might be missing something obvious. this is my first language and these are all new concepts too me

Pobiega•9mo ago

idk, I've only been writing C# for 20 years but my gut instict is to add the new control surface to the base class too, and not have to involve either a ton of if statements or a new interface like, otherwise the whole idea of polymorphism is thrown out the window

JammyG•9mo ago

Yeah, overriding the method would make sense, it just confused me that it didnt want me to touch the ComputerStudent class So i need to add methods for ProgressGrade, TargetGrade ect to ComputerStudent, and then override them.

Pobiega•9mo ago

Is there any more information in the assignment? because while you absolutely can solve this without touching ComputerStudent, it will be some of the worst possible code ever written 😄

JammyG•9mo ago

nothing that i havent sent so the best way is to use an override?

Pobiega•9mo ago

absolutely look how both ALevel and GCSE have the same method names and types that should be a surefire indicator that it can be abstracted

JammyG•9mo ago

this is the next page:

JammyG•9mo ago

its stange because the enitre course has been in order

Pobiega•9mo ago

weird

JammyG•9mo ago

not using ideas and concepts it hasnt yet introduced the entire way through so then i got stuck on this question

Pobiega•9mo ago

I wonder if they want you to "do it the hard way" first so that you can later say "omg that would be so much easier if we had override!"

JammyG•9mo ago

yeah, thats probably it lmao

Pobiega•9mo ago

would it be possible to ask your teacher/prof?

JammyG•9mo ago

I will do, theyve been off for the past two weeks and ive been stuck on this. I found this server while looking for help

Pobiega•9mo ago

something like. "Hey, I have a question. The obvious solution is to add virtual methods to the base class, but since that hasn't been introduced yet..."

JammyG•9mo ago

when they get back ill find out which way this question is meant to push you into doing it

Pobiega•9mo ago

yeah

JammyG•9mo ago

as the course only uses things its already gone over, i think you are definitley right its just pushing you to do it the long way, and then afterwards tell you a much easier way

Pobiega•9mo ago

So, the solutions I can come up with are: * Virtual methods on the base class * Introduce new interface on subclasses, use that as base variable type * if (AStudent is GCSEComputerStudent gcse) everywhere

JammyG•9mo ago

Thank you, youve been very helpful if i used an interface for the new methods in the subclasses, how would i access the methods in the base class, do you always have to cast it

Pobiega•9mo ago

nah, just add those methods to the interface the subclasses implement them via the base class

JammyG•9mo ago

so the compiler knows that they are already implemented because of inheriance?

Pobiega•9mo ago

interface was very quick and easy, but feels hacky sorta, yeah more so that subtypes have all the public members of their baseclass they jsut do

JammyG•9mo ago

Why would Overriding be prefered? What are the downsides of the interface

Pobiega•9mo ago

because if you make a third class from ComputerStudent it will not work you have essentially introduced a new baseclass, the interface, that your program needs to work I'd prefer to keep ComputerStudent as the core base type

JammyG•9mo ago

if the third class used the interface though, would everything still work?

Pobiega•9mo ago

yes and as said, at that point you have two base classes and all your new classes must implement both of them which just seems like extra work, when you could have abstract/virtual methods on the base

JammyG•9mo ago

ok, i understand now , i'll use virutal methods then

Pobiega•9mo ago

maybe even abstract, tbh since you dont want them to be optional to implement (I guess)

JammyG•9mo ago

Virutal allows a method to be overridden What does abstract do?

Pobiega•9mo ago

it forces a method to be overriden as in "you must implement this method yourself, you cant use the base version"

JammyG•9mo ago

ohh thats cool. like an interface but not

Pobiega•9mo ago

yeah you could even make the base class abstract itself that would mean you cant do new ComputerStudent you must use one of the child classes I still think you should ask the teacher/prof first, so you dont get shit grades because you "used stuff not covered yet"

JammyG•9mo ago

Nothing i do on assignments like these are graded until i sit my actual exams and in an actual exam you have free reign to solve problems however youd like

Pobiega•9mo ago

okay

JammyG•9mo ago

So right now im just learning ways of doing stuff, and hopefully leaning the 'right' way of doing it so now i know, in situations like these its easier to just keep one base class and not mess around with interfaces or weird casting and if statements like i have been

Pobiega•9mo ago

ok, then imho, go with abstract ComputerStudent and add in

abstract int GetTarget();
abstract void CalcNewTarget();
abstract void UpdateProgress();
abstract int GetProgress();

abstract int GetTarget();
abstract void CalcNewTarget();
abstract void UpdateProgress();
abstract int GetProgress();

and also, these methods should just be properties 😄

JammyG•9mo ago

if i make an entire class abstract , i wouldnt be able to make a 'new ComputerStudent'

Pobiega•9mo ago

correct

JammyG•9mo ago

but i would be able to make a data type of computer student?

Pobiega•9mo ago

but that doesnt seem to be a valid case in the program anyways you must make either an A level, or a GCSE student you can't go "no thanks"

JammyG•9mo ago

how would i store it though? like here

Pobiega•9mo ago

ComputerStudent student;

if(blabla)
{
  student = new ALevelStudent();
}

ComputerStudent student;

if(blabla)
{
  student = new ALevelStudent();
}

JammyG•9mo ago

ohh ok. so even if its abstact , i can still use it as a datatype

Pobiega•9mo ago

absolutely you just cant make instances of it

JammyG•9mo ago

so not make a 'new ComputerStudent'

Pobiega•9mo ago

exactly

JammyG•9mo ago

that makes so much sense and that's perfect for what im doing, thank you

Pobiega•9mo ago

JammyG•9mo ago

one more thing - why/when would i use an interface instead like how would the scenario need to be different to warrant using an interface

Pobiega•9mo ago

when you have no base logic or fields if your "shared surface" is only methods, then an interface is the way to go

JammyG•9mo ago

so if 'class ComputerStudent' didnt exist at all

Pobiega•9mo ago

yeah in this case, all students have names and forms well, and targets... and progress so those two could be moved to the baseclass too 🙂 only UpdateProgress would need to be overriden, I think the rest could be in the base

JammyG•9mo ago

yeah that's the only method that's different Thank you for your help

Pobiega•9mo ago

public abstract class StudentBase
{
    public StudentBase()
    {
        Console.WriteLine("Please enter firstname: ");
        Firstname = Console.ReadLine()!;
        Console.WriteLine("Please enter surname: ");
        Surname = Console.ReadLine()!;
        Console.WriteLine("Please enter form: ");
        Form = Console.ReadLine()!;
    }

    public string Firstname { get; }
    public string Surname { get; }
    public string Form { get; }

    public string Name => $"{Firstname} {Surname}";

    public int Target { get; protected set; }

    public int Progress { get; protected set; }

    public abstract void CalcNewTarget();
    public abstract void UpdateProgress();
}

class ALevelStudent : StudentBase
{
    public ALevelStudent()
    {
        do
        {
            try // replace with TryParse
            {
                Console.WriteLine("What target do you want to set for this student? Input a grade between A and U: ");
                Target = Char.Parse(Console.ReadLine()!.ToUpper());
            }
            catch (Exception)
            {
                Console.WriteLine("You must enter a character between A and U");
            }
        } while (Target <= 'A' && Target >= 'U');
    }

    public override void CalcNewTarget()
    {
        if (Progress > Target)
        {
            Target = Progress + 1;
        }
    }

    public override void UpdateProgress()
    {
        do
        {
            try
            {
                Console.WriteLine(
                    "What progress grade do you want to set for this student? Input a grade between A and U");
                Progress = int.Parse(Console.ReadLine());
            }
            catch (Exception)
            {
                Console.WriteLine("You must enter a character between A and U");
            }
        } while (Progress <= 'A' && Progress >= 'U');
    }
}

public abstract class StudentBase
{
    public StudentBase()
    {
        Console.WriteLine("Please enter firstname: ");
        Firstname = Console.ReadLine()!;
        Console.WriteLine("Please enter surname: ");
        Surname = Console.ReadLine()!;
        Console.WriteLine("Please enter form: ");
        Form = Console.ReadLine()!;
    }

    public string Firstname { get; }
    public string Surname { get; }
    public string Form { get; }

    public string Name => $"{Firstname} {Surname}";

    public int Target { get; protected set; }

    public int Progress { get; protected set; }

    public abstract void CalcNewTarget();
    public abstract void UpdateProgress();
}

class ALevelStudent : StudentBase
{
    public ALevelStudent()
    {
        do
        {
            try // replace with TryParse
            {
                Console.WriteLine("What target do you want to set for this student? Input a grade between A and U: ");
                Target = Char.Parse(Console.ReadLine()!.ToUpper());
            }
            catch (Exception)
            {
                Console.WriteLine("You must enter a character between A and U");
            }
        } while (Target <= 'A' && Target >= 'U');
    }

    public override void CalcNewTarget()
    {
        if (Progress > Target)
        {
            Target = Progress + 1;
        }
    }

    public override void UpdateProgress()
    {
        do
        {
            try
            {
                Console.WriteLine(
                    "What progress grade do you want to set for this student? Input a grade between A and U");
                Progress = int.Parse(Console.ReadLine());
            }
            catch (Exception)
            {
                Console.WriteLine("You must enter a character between A and U");
            }
        } while (Progress <= 'A' && Progress >= 'U');
    }
}

something like this :p doing stream input in a constructor is very weird in general btw normally you'd just take in parameters, and optionally provide a Create static method that does the input

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Calling methods from a base class