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.

please read: http://stackoverflow.com/questions/245607/how-is-generic-covariance-contra-variance-implemented-in-c-sharp-4-0 — Dave Bish, Apr 18 '12 at 14:09

Sergey Berezovskiy · Accepted Answer · 2012-04-18T14:36:26.400

7

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

edited Apr 18 '12 at 14:36

answered Apr 18 '12 at 14:11

Sergey Berezovskiy

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... and that's why it isn't allowed to do what the OP wants to do. +1 – Daniel Hilgarth Apr 18 '12 at 14:13
But `ISpecificEntity` implements `IIdentifier`. And `TIdentifier` in `IManager` accepts only `IIdentifier`. So, there should not be an exception in that line. Am I wrong? – Palama Apr 18 '12 at 14:32
Hmmm. You are right. I think I have to change my summarised code. It does not fit to the problem in the original code. – Palama Apr 18 '12 at 15:04

score 2 · Answer 2 · answered Apr 18 '12 at 14:32

2

Why not:

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

?

answered Apr 18 '12 at 14:32

Omar

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Because `SpecificEntity` is not an `IManager` but an `IIdentifier`. – Palama Apr 18 '12 at 14:41

How to implement generic polymorphism in c#?

2 Answers2

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