Which error code to return when a passed object doesn't implement a necessary interface?

E_INVALIDARG is a fine choice, but the most important thing is to ensure that the precise conditions for each return code that you use are well documented.

Yes. At least, that is what I've learned to expect a microsoft API call to return in such a case.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.