c# - Why have class-level access modifiers instead of object-level?

Question

While using C#, I recently realised that I can call a Foo object's private functions from Foo's static functions, and even from other Foo objects. After everything I have learned about access modifiers, this sounds very strange to me.

As far as I know, you make a function private when it does something that's part of some kind of internal process. Only the object itself knows when to use those functions, because other objects shouldn't/can't control the object's flow. Is there any reason why other objects of the same class should be excepted from this pretty straightforward rule?

As per request, an example:

public class AClass {
    private void doSomething() { /* Do something here */ }
    public void aFunction() {
        AClass f = new AClass();
        f.doSomething(); // I would have expected this line to cause an access error.
    }
}

Answer 1

The private modifier enforces Encapsulation principle.

The idea is that 'outer world' should not make changes to AClass internal processes because AClass implementation may change over time (and you would have to change the whole outer world to fix the differences in implementation - which is nearly to impossible).

When instance of AClass accesses internals of other AClass instance - you can be sure that both instances always know the details of implementation of AClass. If the logic of internal to AClass processes is changed - all you have to do is change the code of AClass.

In some other languages, however, private works on instance level, but this is not true in C#.