How to access the type of the base class?

Question

C++ / C#

class A {}
class B:A {}
var a = new A();
var b = new B();
      
Console.WriteLine(a.GetType());
Console.WriteLine(b.GetType());
Console.WriteLine((b as A).GetType());

Requires the last output to output "A". The example is very abbreviated, but the question is precisely this: I have an object b and type A, and I need to pull off such a feint. That is, for b to be defined as A. I don't know how else to describe it.
There, in fact, b is transferred to someone else's Dictionary, whose ideology I cannot change. Moreover, I do not work with types, but I pass objects to methods; dictionary - inside someone else's code. That is, I pass b, but I want this foreign dictionary to think that this is an object of the base type A. It occurred to me only to remove inheritance, then I would not have to "replace" the type, but I would like to "replace" it.

Answer 1

b.GetType().BaseType