How can I make this simple C# generics factory work?

Question

I have this design:

public interface IFactory<T> {
  T Create();
  T CreateWithSensibleDefaults();
}

public class AppleFactory : IFactory<Apple> { ... }
public class BananaFactory : IFactory<Banana> { ... }
// ...

The fictitious Apple and Banana here do not necessarily share any common types (other than object, of course).

I don't want clients to have to depend on specific factories, so instead, you can just ask a FactoryManager for a new type. It has a FactoryForType method:

IFactory<T> FactoryForType<T>();

Now you can invoke the appropriate interface methods with something like FactoryForType<Apple>().Create(). So far, so good.

But there's a problem at the implementation level: how do I store this mapping from types to IFactory<T>s? The naive answer is an IDictionary<Type, IFactory<T>>, but that doesn't work since there's no type covariance on the T (I'm using C# 3.5). Am I just stuck with an IDictionary<Type, object> and doing the casting manually?

Answer 1

Unfortunately yes you are stuck with the manual casting. However this casting can be an implementation detail and not seen by the consumer. For example

public class FactoryMap { 
  private Dictionary<Type,object> _map = new Dictionary<Type,object>();
  public void Add<T>(IFactory<T> factory) {
    _map[typeof(T)] = factory;
  }
  public IFactory<T> Get<T>() {
    return (IFactory<T>)_map[typeof(T)];
  }
}