Can you attach an interface to a defined class

Question

Here is the situation. In some cases I find myself wanting a class, let's call it class C that has the same functionalities as class A, but with the addition that it has interface B implemented. For now I do it like this:

class C : A,B
{
   //code that implements interface B, and nothing else
}

The problem will come if class A happens to be sealed. Is there a way I can make class A implement interface B without having to define class C (with extension methods or something)

Answer 1

Basically: no. That is part of what "mixins" could bring to the table, but the C# languauge doesn't currently support that (it has been discussed a few times, IIRC).

You will have to use your current approach, or (more commonly) just a pass-through decorator that encapsulates A rather than inheriting A.

class C : IB
{
    private readonly A a;
    public C(A a) {
        if(a == null) throw new ArgumentNullException("a");
        this.a = a;
    }

    // methods of IB:
    public int Foo() { return a.SomeMethod(); }
    void IB.Bar() { a.SomeOtherMethod(); }
}

Answer 2

The only way I see, is to change inheritance to aggregation, like this:

class C : B
{
    public C(A instanceToWrap)
    {
        this.innerA = instanceToWrap;
    }

    //coda that implements B

    private A innerA;
}

There seems to be a possibility to inject interface in run-time, as it is done with Array class and IEnumerable<T> interface, but it seems a bit of an overkill.