为什么我不能从私有类/接口继承？

Question

Why is this illegal in C#?

class Foo: Foo.BaseFoo //Circular base class dependency compile time error
{
   private class BaseFoo {...}

   ...
}

I'm not arguing when this could be useful or not, but I'd like to know what are the reasons that would disallow such code to compile. A similar restriction happens with private interfaces.

UPDATE

Seeing that its a duplicate I'll center the question more in why this isn't valid with interfaces which seems more useful?

And, what's more, why does it seem to be legal with the Roslyn preview as shown here

Answer 1

This is not a problem of the access modifier -that the class is private. This is a circular base class dependency. You try to define a class called Foo that inherits a nested class called BaseFoo .