Sealing an interface after implementing it

Question

I am working on a small project and I came across that problem.

The project output is a library containing an interface. I would like to implement that interface and

Answer 1

Why not use an abstract class like below.

Haven't tested it but this should work?

public abstract class Test
{
    public virtual void SomeMethod() {}
    //OR
    public abstract void SomeMethod();//MSDN says:
    //an abstract method is implicitly virtual
}

class A : Test
{
    public sealed override SomeMethod()
    {

    }
}