Interfaces inheritance in C#

Question

I'm trying to overrun quite big (for me) problem that I came across while writing my application. Look at this, please (I will try to shorten the code for simplicity):

I have root interface called IRepository<T>. Next, IBookRepository : IRepository<Book> Next, concrete class that implements it: BookRepository : IBookRepository

In the RepositoryManager class I declared private IRepository<IRepoItem> currentRepo;

IRepoItem is an interface that is implemented by Book class.

Now, when I try to do something like this:

currentRepo = new BookRepository();

VisualStudio gives error message:

Cannot implicitly convert type 'BookRepository' to 'IRepository'. An explicit conversion exists (are you missing a cast?)

I tried to cast explicitly, but runtime exception is thrown...

I know (almost for sure) that it is something called covariance and this is (probably) solved in .Net 4.0. Unfortunately I'm writing using framework version 3.5, and I cannot change this. Please give me some advices what to do - how to overrun this problem? I'd like to get currentRepo from RepoFactory that would produce few kinds of repositories depends on user needs. I don't know whether links are allowed here, but I write some kind of blog on http://olgatherer.wordpress.com/ where I describe application creation. My English isn't good, but I hope that it's enough to understand me. Thank you in advance for answer.

Best regards, skrzeczowas

Answer 1

In .NET 3.5 you definitely can't treat an IRepository<Book> as an IRepository<IRepoItem>.

We'd need to know more about what you're using the repository for in RepositoryManager to really know how to solve it... but could you create a non-generic IRepository interface which IRepository<T> extends? Make it include all the members which don't refer to T. Then you can declare currentRepo as just an IRepository.

Answer 2

Try using the intermediate layer (IRep):

    interface IRepository<T>
    {
    }

    interface IRep<T> : IRepository<IRepoItem> where T : IRepoItem
    {
    }

    interface IBookRepository : IRep<Book>
    {
    }

    class BookRepository : IBookRepository
    {
    }

then you can do what you want:

        BookRepository br = new BookRepository();
        IRepository<IRepoItem> currentRepo = br;