C# generics problem - newing up the generic type with parameters in the constructor

Question

I am trying to create a generic class which new\'s up an instance of the generic type. As follows:

public class HomepageCarousel : List
            


        

Answer 1

Just to add to other answers:

What you are doing here is basically called projection. You have a List of one type and want to project each item (using a delegate) to a different item type.

So, a general sequence of operations is actually (using LINQ):

// get the initial list
List pageDataList = GetJewellerHomepages();

// project each item using a delegate
List carouselList =
       pageDataList.Select(t => new ConcreteCarousel(t));

Or, if you are using .Net 2.0, you might write a helper class like:

public class Project
{
    public static IEnumerable From
        (IEnumerable source, Func projection)
    {
        foreach (Tsource item in source)
            yield return projection(item);
    }
}

and then use it like:

// get the initial list
List pageDataList = GetJewellerHomepages();

// project each item using a delegate
List carouselList =
       Project.From(pageDataList, 
           delegate (PageData t) { return new ConcreteCarousel(t); });

I'm not sure how the rest of the code looks like, but I believe that GetInitialCarouselData is not the right place to handle the initialization, especially since it's basically duplicating the projection functionality (which is pretty generic and can be extracted in a separate class, like Project).

[Edit] Think about the following:

I believe right now your class has a constructor like this:

public class HomepageCarousel : List
    where T: IHomepageCarouselItem, new()
{
    private readonly List jewellerHomepages;
    public class HomepageCarousel(List jewellerHomepages)
    {
        this.jewellerHomepages = jewellerHomepages;
        this.AddRange(GetInitialCarouselData());
    }

    // ...
}

I presume this is the case, because you are accessing a jewellerHomepages field in your method (so I guess you are storing it in ctor).

There are several problems with this approach.

• You have a reference to jewellerHomepages which is unneccesary. Your list is a list of IHomepageCarouselItems, so users can simply call the Clear() method and fill it with anything they like. Then you end up with a reference to something you are not using.

• You could fix that by simply removing the field:

  public class HomepageCarousel(List jewellerHomepages)
  {
      // do not save the reference to jewellerHomepages
      this.AddRange(GetInitialCarouselData(jewellerHomepages));
  }
  

  But what happens if you realize that you might want to initialize it using some other class, different from PageData? Right now, you are creating the list like this:

  HomepageCarousel list =
       new HomepageCarousel(listOfPageData);
  

  Are you leaving yourself any option to instantiate it with anything else one day? Even if you add a new constuctor, your GetInitialCarouselData method is still too specific to use only PageData as a source.

Conclusion is: Do not use a specific type in your constructor if there is no need for it. Create actual list items (concrete instances) somewhere else.