How to implement generic polymorphism in c#?

Question

to avoid confusion I summarised some code:

namespace ConsoleApplication1
{
    class Program
    {
        static void Main()
        {
            IManager<ISpecificEntity> specificManager = new SpecificEntityManager();
            IManager<IIdentifier> manager = (IManager<IIdentifier>) specificManager;
            manager.DoStuffWith(new SpecificEntity());
        }
    }

    internal interface IIdentifier
    {
    }

    internal interface ISpecificEntity : IIdentifier
    {
    }

    internal class SpecificEntity : ISpecificEntity
    {
    }

    internal interface IManager<TIdentifier> where TIdentifier : IIdentifier
    {
        void DoStuffWith(TIdentifier entity);
    }

    internal class SpecificEntityManager : IManager<ISpecificEntity>
    {
        public void DoStuffWith(ISpecificEntity specificEntity)
        {
        }
    }
}

When I debug the code I get an InvalidCastException in Main().

I know that ISpecificEntity implements IIdentifier. But obviously a direct cast from an IManager<ISpecificEntity> into an IManager<IIdentifier> does not work.

I thought working with covariance could do the trick but changing IManager<TIdentifier> into IManager<in TIdentifier> does not help either.

So, is there a way do cast specificManager into an IManager<IIdentifier>?

Thanks and all the best.

Answer 1

With IManager<IIdentifier> you can do such thing:

IIdentifier entity = new NotSpecificEntity();
manager.DoStuffWith(entity);

That will lead to exception, in your SpecificEntityManager, because it accepts only parameters of type ISpecificEntity

UPDATE: You can read more about covariance and contravariance in C# at Eric Lippert's blog

Answer 2

Why not:

ISpecificEntity bankAccountManager = new SpecificEntity();
IManager<IIdentifier> manager = (IManager<IIdentifier>)bankAccountManager;
manager.DoStuffWith(new SpecificEntity());

?