Inheriting an interface with an internal database

Question

I was wondering if there is a way to do the following:

In my project, I defined an interface, say IFruit. This interface has the public method GetName (). I also declare an IApple interface that implements IFruit and provides another method like GetAppleType () or something else. Eat more fruits like Ibanana, Jerry, whatever.

Now outside, I only want to be able to use the actual implementation of the fruit, not IFruit itself. But I can’t declare the IFruit interface private or internal, since the inherited interfaces will say: "Cannot be implemented because the base class is less accessible."

I know this is possible with abstract implementations, but this is not an option in this case: I really need to use interfaces. Is there such an option?

Update I think my example needs clarification :) I use MEF to load interface implementations. Downloaded collections are based on IApple, IBanana, ICherry, etc. But IFruit itself is useless; I cannot use classes based only on this interface. Therefore, I was looking for a way to prevent other developers from implementing exclusively IFruit, believing that their class will be loaded (which will not). So basically it comes down to:

internal interface IFruit { public string GetName(); } 

public interface IApple : IFruit { public decimal GetDiameter(); }

public interface IBanana : IFruit { public decimal GetLenght(); }

But this does not compile due to the less accessible base interface.

Answer 1

It is not possible to do what you are trying to, but you can disable people using the IFruit interface with the [Obsolete] attribute, with a message to say why.

On your IBanana, IApple, ... interfaces, disable the deprecated warning.

 [Obsolete] public interface IFruit { ... } #pragma warning disable 612 public interface IBanana : IFruit { ... } #pragma warning restore 612 

Answer 2

One way to guarantee this unintentionally is to make IFruit internal your assembly, and then use some kind of adapter to properly wrap the type:

 public interface IApple { string GetName(); } public interface IBanana { string GetName(); } internal interface IFruit { string GetName(); } class FruitAdaptor: IFruit { public NameAdaptor(string name) { this.name = name; } private string name; public string GetName() { return name; } } // convenience methods for fruit: static class IFruitExtensions { public static IFruit AsFruit(this IBanana banana) { return new FruitAdaptor(banana.GetName()); } public static IFruit AsFruit(this IApple apple) { return new FruitAdaptor(apple.GetName()); } } 

Then:

 MethodThatNeedsFruit(banana.AsFruit()); 

You can also easily extend this to lazily call GetName on an adapted object if the name can change over time.

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Another option would be to only check for DEBUG, which loads all IFruit implementers and then throws an exception if one of them does not actually implement IBanana / IApple . Since these classes seem to be intended for internal use within your company, this should prevent someone from accidentally implementing the wrong thing.

Answer 3

If you have some kind of code (assuming I understand your state correctly), something like this:

 public class WaterMellon : IFruit, IVegetables... { } 

and you want the user of your infrastructure access to only apply IFruit methods, for me there is no other known method, and then it just executes.

 IFruit fruit = new WaterMelon(); fruit. //CAN ACCESS ONLY TO FRUIT IMPLEMNTATION AVAILABLE IN WATERMELON 

If this is not what you are asking for, please specify.