Impossible to implement reference behavior without making attribute public

[DaniD3v]

in 01 Types/04_Collections.md Übung 1 you provide a reference output:

s sitzt in der Klasse 3AHIF mit dem KV KV1.
In der 3AHIF sind folgende Städte: ["CTY1","CTY2"].
3AHIF vor ChangeKlasse:
[{"City":"CTY1","Id":1001,"Lastname":"LN1","Firstname":"FN1"},{"City":"CTY1","Id":1002,"Lastname":"LN2","Firstname":"FN2"},{"City":"CTY2","Id":1003,"Lastname":"LN3","Firstname":"FN3"}]
3AHIF nach ChangeKlasse:
[{"City":"CTY1","Id":1002,"Lastname":"LN2","Firstname":"FN2"},{"City":"CTY2","Id":1003,"Lastname":"LN3","Firstname":"FN3"}]
3BHIF nach ChangeKlasse:
[{"City":"CTY1","Id":1011,"Lastname":"LN4","Firstname":"FN4"},{"City":"CTY1","Id":1012,"Lastname":"LN5","Firstname":"FN5"},{"City":"CTY1","Id":1013,"Lastname":"LN6","Firstname":"FN6"},{"City":"CTY1","Id":1001,"Lastname":"LN1","Firstname":"FN1"}]
s sitzt in der Klasse 3BHIF mit dem KV KV2.

with the following main method:

static void Main(string[] args)
        {
            Dictionary<string, SchoolClass> classes = new();
            classes.Add("3AHIF", new SchoolClass(name: "3AHIF", classTeacher: "KV1"));
            classes.Add("3BHIF", new SchoolClass(name: "3BHIF", classTeacher: "KV2"));
            classes.Add("3CHIF", new SchoolClass(name: "3CHIF", classTeacher: "KV3"));

            classes["3AHIF"].AddStudent(new Student(id: 1001, firstname: "FN1", lastname: "LN1", city: "CTY1"));
            classes["3AHIF"].AddStudent(new Student(id: 1002, firstname: "FN2", lastname: "LN2", city: "CTY1"));
            classes["3AHIF"].AddStudent(new Student(id: 1003, firstname: "FN3", lastname: "LN3", city: "CTY2"));
            classes["3BHIF"].AddStudent(new Student(id: 1011, firstname: "FN4", lastname: "LN4", city: "CTY1"));
            classes["3BHIF"].AddStudent(new Student(id: 1012, firstname: "FN5", lastname: "LN5", city: "CTY1"));
            classes["3BHIF"].AddStudent(new Student(id: 1013, firstname: "FN6", lastname: "LN6", city: "CTY1"));

            Student s = classes["3AHIF"].Students[0];
            Console.WriteLine($"s sitzt in der Klasse {s.SchoolClass?.Name} mit dem KV {s.SchoolClass?.ClassTeacher}.");
            Console.WriteLine($"In der 3AHIF sind folgende Städte: {JsonSerializer.Serialize(classes["3AHIF"].Cities)}.");

            Console.WriteLine("3AHIF vor ChangeKlasse:");
            Console.WriteLine(JsonSerializer.Serialize(classes["3AHIF"].Students));
            s.ChangeClass(classes["3BHIF"]);
            Console.WriteLine("3AHIF nach ChangeKlasse:");
            Console.WriteLine(JsonSerializer.Serialize(classes["3AHIF"].Students));
            Console.WriteLine("3BHIF nach ChangeKlasse:");
            Console.WriteLine(JsonSerializer.Serialize(classes["3BHIF"].Students));
            Console.WriteLine($"s sitzt in der Klasse {s.SchoolClass?.Name} mit dem KV {s.SchoolClass?.ClassTeacher}.");
        }

this means that SchoolClass.Add needs to call Student.ChangeClass. Student.ChangeClass expects the student to be added to the class as well. This means we need to call SchooClass.Add from this method.

This results in infinite recursion. Without a RawAdd method or a public setter this is impossible to prevent. This behavior is also nonsensical because Student.ChangeClass does not except classes of value null so SchoolClass.Remove can't reset the students class.

I suggest to either

• remove the requirement of SchoolClass.Add to change the class on the student or
• change the signature of Student.ChangeClass to accept null values and make it not necessary to add the student to the class.

This behavior should also be documented!

This excercise is pretty bad anyways because in multi threading having circular references like this can result in funky behavior and wouldn't even be possible in e.g. rust.

[DaniD3v]

It's also a good idea to write tests in the reference instead of providing an example output.

running

wl-paste > /tmp/reference
dotnet run > /tmp/own
diff /tmp/own /tmp/reference

every single compile-run-debug cycle can get bothersome