F＃中的通用限制

Question

Until now, I had a class like this one:

type C<'a when 'a :> A> (...)

But now I created a new type B:

type B (...) =
    inherit A()

But I don't want C to support B, and this doesn't compile:

type C<'a when 'a :> A and not 'a :> B> (...)

How can I do that?

Answer 1

You can't and shouldn't. If B is an A, then C should handle it. If it's reasonable for C not to be able to handle B, then B shouldn't derive from A. Otherwise you're effectively breaking Liskov's Substitution Principle (or at least a variant of the same).

When you declare that B inherits from A, you're saying that it can be used as an A. If that's not the case, you shouldn't be using inheritance.