I see your point but I am not yet convinced. At least if the interface method is not explicitly implemented, I can call the same method not through the interface. But I don't know the specs well enough to judge if this would be a viable optimization avoiding boxing or if this could cause different behavior, for example if you start handing out the this pointer. Thinking a bit more about it, it probably does. Maybe I will have a closer look at this out of curiosity.
Yes, if the method is not explicitly implemented then you can call it without going through the interface.
But at that point it's just a regular method that might also happen to implement part of an interface that you the programmer are not using (at least at that point).
Likewise in the case of foreach, the compiler is looking for a particular method and does not need to care whether or not that method happens to also implement part of an interface, so there is not point in making the programmer go through the trouble of marking the type as such.
