Should I pass a list of objects or an object to a domain object method? I am developing a registration portal for a gym.  When a user registers; a list of sports are recommended based on their age; sex; fitness level etc.  Here is the tables in the database:

CREATE TABLE Person (ID int, name varchar(100), dateofbirth datetime, Gender char(1), primary key (ID))
CREATE TABLE Sport (ID int, description varchar(30), primary key (ID))
CREATE TABLE PersonCanPlaySport (Person ID int references Person(ID), SportID int references Sport(ID), PRIMARY KEY (SportID, PersonID))


The tables show what sports a person can participate in. Now see the class below:

public class Person
{
   public List&lt;Sport&gt; Sports;

   public Person()
   {
     Sports = new List&lt;Sport&gt;();
   }

   public void AddSports(List&lt;Sports&gt; allSports)
   {
      Sports.Clear();
      ((List&lt;Sport&gt;)Sports).AddRange(allSports
      .Where(sport =&gt; sport.CanPlay(this)));
   }

}


This works very well.  However, in this question: https://softwareengineering.stackexchange.com/questions/361040/can-two-objects-have-multiple-types-of-relationships (under question comments); a user critics the design but does not really explain why.  What is "wrong" with the design? Should I be passing single Sport objects to Person.AddSports similar to how James does it in his answer here: https://stackoverflow.com/questions/1103693/how-to-model-a-many-to-many-relationship-in-code/1103862?noredirect=1#comment81696466_1103862?
I will simply go from top to bottom without ordering by priority.

Person is not supposed to be a base class (at least now) then it should be marked as sealed.

Sports is a public field. There is not any level of protection. Caller might do anything with it: set it to null, clear the content. Add/remove items in an uncontrolled way (is, for example, null a valid item?) The very first thing to do is to make it a read-only property:

public List&lt;Sport&gt; Sports { get; }


You can assign an instance in your ctor (as you're doing now) or directly where it's declared:

public List&lt;Sport&gt; Sports { get; } = new List&lt;Sport&gt;();


The second problem is the type. You're exposing List&lt;T&gt; which is, at best, an implementation detail. Which operations the caller is allowed to do? Add/remove items? Clear the collection? You may change its type to IEnumerable&lt;Sport&gt; like this:

private readonly List&lt;Sport&gt; _sports = new List&lt;Sport&gt;();
public IEnumerable&lt;Sport&gt; Sports =&gt; _sports;


However we're just postponing the problem to the future. I'd introduce a SportCollection class which exposes exactly what you need (and to begin with it might be as little as IEnumerable&lt;Sport&gt;).

public SportCollection Sports { get; } = new SportCollection();


We fixed the declaration, we fixed the type but we still need to fix the name. It does not communicate almost anything. It's the list of sports that this person may play but its name is saying nothing: you have to read the code to understand how it's populated.

public SportCollection LikeableSports { get; } = new SportCollection();


I'm sure someone else can suggest a much better name but I hope you understood the point.

AddSports() method again does not say what it's actually doing. It's not adding generic sports but only sports this person may play/like. Change the name to reflect its behavior.

allSports parameters is List&lt;Sport&gt; but you if you change it to IEnumerable&lt;Sport&gt; you code compiles. Do it, always pick the less restrictive requirement for your parameters (what if the list comes from a DB? Do you want to materialize the whole thing in memory because AddSports() wants a list?)

You're clearing the list before adding the new items. It's not what I expect from a method named Add*(). Change the logic or change the name to reflect it.

You do not need to cast Sports to List&lt;Sport&gt;, just drop it.

Again a word about naming: I read sport.CanPlay(this) as "Can Sport play Person?" and it sounds weird. If this responsibility is in the Sport class then it should be at least named CanBePlayedBy().

Sport has the responsibility to check if it can be played/liked by Person. It's debatable if this responsibility should be in Sport, in Person or much better (IMO) in a separate class. I'd go with a separate class because:


I can switch algorithm in any moment, even from configuration and at run-time without changing anything else.
It makes testing much easier.
It keeps responsibilities separate and clear.
If you add more rules to determine a match (proximity of the structure to play that sport and public transportation to/from that place, for example) you will have a single clear place where these knowledge has to be managed (unless you want to pollute your Sport class with references to StructureCollection and PublicTransportationTripPlanner, of course...)


Do not ignore this suggestion, it's the last one but I think this is more important than most of the other considerations in my post: you can live with a non sealed class but putting responsibilities in the wrong place will make you code a nightmare to maintain. Micro design is easy to change (with better knowledge of the domain and of the language) but wrong architectural decisions are much much MUCH more expensive to deal with (on the long term).