Casting between two types derived from the (same) interface

Question

I have an interface and two types that derive from it.

However, I cannot do the following:

B objectB = (B) objectA

Where B derives

Answer 1

The fact that both types implement the same interface (or have the same base-type, for that matter) does not make them interchangeable; an A is always an A, and a B is always a B. In an inheritance chain, an object can be cast as itself or any parent type. You have:

A : ISomeInterface
B : ISomeInterface

which lets you cast an A as A or ISomeInterface, and a B as B or ISomeInterface

or (depending on your meaning of "derived from")

SomeBaseType
 > A
 > B

which lets you cast an A as A or SomeBaseType, and a B as B or SomeBaseType

(plus object, in each case)