I am trying to create a generic class which new's up an instance of the generic type. As follows:
public class HomepageCarousel<T> : List<T>
where T: IHomepageCarouselItem, new()
{
private List<T> GetInitialCarouselData()
{
List<T> carouselItems = new List<T>();
if (jewellerHomepages != null)
{
foreach (PageData pageData in jewellerHomepages)
{
T item = new T(pageData); // this line wont compile
carouselItems.Add(item);
}
}
return carouselItems;
}
}
But I get the following error:
cannot provide arguments when creating an instance of a variable
type
I found the following related question which is very close to what I need:
Passing arguments to C# generic new() of templated type
However, I can't used Jared's suggested answer as I am
calling the method within the Generic class, not outside of
it, so I can't specify the concrete class.
Is there a way around this?
I have tried the following based on the other question, but
it doesn't work as I don't know the concrete type of T to
specify. As it is called from inside the generic class, not
outside:
public class HomepageCarousel<T> : List<T>
where T: IHomepageCarouselItem, new()
{
private List<T> LoadCarouselItems()
{
if (IsCarouselConfigued)
{
return GetConfiguredCarouselData();
}
// ****** I don't know the concrete class for the following line,
// so how can it be instansiated correctly?
return GetInitialCarouselData(l => new T(l));
}
private List<T> GetInitialCarouselData(Func<PageData, T> del)
{
List<T> carouselItems = new List<T>();
if (jewellerHomepages != null)
{
foreach (PageData pageData in jewellerHomepages)
{
T item = del(pageData);
carouselItems.Add(item);
}
}
return carouselItems;
}
}
********EDIT : ADDED POSSIBLE SOLUTIONS**
So I have tested 2 possible solutions:
First is exactly as explained below by Jon Skeet. This
definitely works but means having an obscure lambda in the
constructor. I am not very comfortable with this as it means
users need to know the correct lambda that is expected.
After all, they could pass a lambda which doesn't new up the
type, but does something entirely unexpected
Secondly, I went down the Factory method route;
I added a Create method to the common interface:
IJewellerHomepageCarouselItem Create(PageData pageData);
Then provided an implementation in each Concrete class:
public IJewellerHomepageCarouselItem Create(PageData pageData)
{
return new JewellerHomepageCarouselItem(pageData, null);
}
And used a two step initialisation syntax:
T carouselItem = new T();
T homepageMgmtCarouselItem = (T) carouselItem.Create(jewellerPage);
Would love to hear some feedback on the merit of each of these approaches.
Best Answer
Have you considered using Activator (this is just another option).