How to implement an interface which already has some other interfaces in it?

Question

the example looks like this:

interface IA
{
    ICollection<IB> Bs {get;set;}
}

interface IB
{
}


public class BBase : IB
{

}

public class ABase : IA
{
    public ICollection<BBase> Bs { get; set; }
}

The question is that, when I wanted to implement the interface IA with BBase , just as I did in ABase , an error occured. Is that to say I can only use IB instead of BBase to implement the IA in ABase ?

Answer 1

What you need is to make IA generic:

interface IA<T> where T : IB
{
    ICollection<T> Bs { get; set; }
}

interface IB
{
}


public class BBase : IB
{

}

public class ABase : IA<BBase>
{
    public ICollection<BBase> Bs { get; set; }
}

The implementation of an interface should exactly match its definition, so in a non-generic case you are expected to have ICollection<IB> Bs {get;set;} in ABase exactly, that is it may accept any of IB implemetations.

While when the interface is generic ( interface IA<T> where T : IB ), it's implementation should provide any T satisfying the given constraint (ie here some exact implementation of IB ). Consequently ABase class becomes generic as well.

For more info read:

1. Generic Interfaces (C# Programming Guide)
2. where (generic type constraint) (C# Reference)

Answer 2

You can't implement property by specifying different type for it - see Interfaces (C# Programming Guide) :

To implement an interface member, the corresponding member of the implementing class must be public, non-static, and have the same name and signature as the interface member

In your particular case you either need to use ICollection<IB> as a type for property in ABase or follow Konstantin Vasilcov suggestion to use generic IA<T> .

If you can't go generic route consider making property in the interface 'get' - only. This way you'll be able to not provide setter in the class and validate all "add to to collection" operations by having custom methods to add item(s) to the collection.