Get Concrete Implementation of Interface with Generic Parameter

Question

I'm feeling pretty dumb right now. Not sure why I can't solve this. I have a repository interface:

public interface ICollateralItemBaseImplementation<T> where T : CollateralItemBase
{
    int Add(T collateralItem);
    T Get(int collateralID);
}

and many implementations, each implementing one of the 10 models that inherit from CollateralItemBase. For example:

internal sealed class CollateralItemCertifiedDepositRepository : ServiceBaseRepository, ICollateralItemBaseImplementation<CollateralItemCertifiedDeposit>
{
   int Add(CollateralItemCertifiedDeposit collateralItem) { /*...*/ }
   CollateralItemCertifiedDeposit Get(int collateralID)  { /*...*/ }
}  

Now i just need to switch on the incoming CollateralItemBase type to get the repository I need:

private ICollateralItemBaseImplementation<???> GetRepository(CollateralItemBase item) 
{
    switch (item.GetType().Name)
    {
            case "CollateralItemCertifiedDeposit": return new CollateralItemCertifiedDepositRepository();
            //...
    }

I just need to figure out what to return from this method, for other methods to act on whichever repository I return. How do I refactor this to get it working? I'm pretty sure I have a covariance/contravariance problem. Again, I'm feeling pretty dumb, just drawing a blank.

Thanks.

Answer 1

You could do it in two stages. Add a non-generic base interface to ICollateralItemBaseImplementation then cast to the generic version.

public interface ICollateralItemBaseImplementation
{
}

public interface ICollateralItemBaseImplementation<T> : ICollateralItemBaseImplementation
    where T : CollateralItemBase
{
    int Add(T collateralItem);
    T Get(int collateralID);
}

public static class RepositoryFactory
{
    public static ICollateralItemBaseImplementation<T> GetRepository<T>(T item)
        where T : CollateralItemBase
    {
        return (ICollateralItemBaseImplementation<T>)GetRepositoryImpl(item);
    }

    private static ICollateralItemBaseImplementation GetRepositoryImpl<T>(T item)
            where T : CollateralItemBase
    {
        switch (item.GetType().Name)
        {
            case "CollateralItemCertifiedDeposit":
                return new CollateralItemCertifiedDepositRepository();
        }
        return null;
    }
}

internal static class Program
{

    static void Main()
    {
        var repo = RepositoryFactory.GetRepository(new CollateralItemCertifiedDeposit());
        Debug.Assert(repo is CollateralItemCertifiedDepositRepository);

    }
}