C# Collection classes - yes or no

Question

I'm a relative newbie to C#, although I am a competent programmer, and I confess that I am totally confused as to whether or not it is a good idea to write custom collection classes. So many people seem to say "don't", yet there is a whole set of base classes for it in C#.

Here is my specific case. I have a timetable application. As part of that, I have a service class, and the service class contains collections of things service-y, such as route links. A route link is itself a custom class:

public class Service
{
  public RouteLinks RL;    // A collection of RouteLink types
  ...
}

public class RouteLink
{
    public string FirstStopRef;
    public string LastStopRef;
    public Tracks RouteTrack;    // Another collection, this time of Track types
}

So far I have looked at using Dictionary as the type for RouteLinks, because I need to be able to reference them. This is fine in principle. However, the process of adding a RouteLink to the RouteLinks collection involves checking to see whether it is already there, or whether it extends and existing route link, or... And for that, I need a custom Add function.

So why is is such bad practice to create custom collection classes? Why shouldn't I just inherit CollectionBase or DictionaryBase?

I should perhaps add that I am transferring this code from VBA [please don't shoot me :)] and there I HAD to implement custom collections.

Answer 1

Instead of having RouteLinks be a collection type, an easy solution would be to just define another class, let's say RouteLinksRepository. This class will contain a List<RouteLink> and the AddRoute(RouteLink) functionality as well as any other custom logic for interacting with this collection of RouteLink objects. Your service class will then just contain an instance of this repository class.

public class Service
{
  public RouteLinksRepository RL;    // A collection of RouteLink types
  // ...
}

public class RouteLinksRepository
{
    public List<RouteLink> RouteLinks;
    public bool AddRoute(RouteLink linkToAdd)
    {
        //Custom logic on whether or not to add link
    }
    //Your other logic for the class

}

public class RouteLink
{
    public string FirstStopRef;
    public string LastStopRef;
    public Tracks RouteTrack;    // Another collection, this time of Track types
}