Why are private fields private to the type, not the instance?

Question

In C# (and many other languages) it's perfectly legitimate to access private fields of other instances of the same type. For example:

public class Foo
{
    private bool aBool;

    public void DoBar(Foo anotherFoo)
    {
        if (anotherFoo.aBool) ...
    }
}

As the C# specification (sections 3.5.1, 3.5.2) states access to private fields is on a type, not an instance. I've been discussing this with a colleague and we're trying to come up with a reason why it works like this (rather than restricting access to the same instance).

The best argument we could come up with is for equality checks where the class may want to access private fields to determine equality with another instance. Are there any other reasons? Or some golden reason that absolutely means it must work like this or something would be completely impossible?

Answer 1

I think one reason it works this way is because access modifiers work at compile time. As such, determining whether or not a given object is also the current object isn't easy to do. For example, consider this code:

public class Foo
{
    private int bar;

    public void Baz(Foo other)
    {
        other.bar = 2;
    }

    public void Boo()
    {
        Baz(this);
    }
}

Can the compiler necessarily figure out that other is actually this? Not in all cases. One could argue that this just shouldn't compile then, but that means we have a code path where a private instance member of the correct instance isn't accessible, which I think is even worse.

Only requiring type-level rather than object-level visibility ensures that the problem is tractable, as well as making a situation that seems like it should work actually work.

EDIT: Danilel Hilgarth's point that this reasoning is backwards does have merit. Language designers can create the language they want, and compiler writers must conform to it. That being said, language designers do have some incentive to make it easier for compiler writers to do their job. (Though in this case, it's easy enough to argue that private members could then only be accessed via this (either implicitly or explicitly)).

However, I believe that makes the issue more confusing than it needs to be. Most users (myself included) would find it unneccessarily limiting if the above code didn't work: after all, that's my data I'm trying to access! Why should I have to go through this?

In short, I think I may have overstated the case for it being "difficult" for the compiler. What I really meant to get across is that above situation seems like one that the designers would like to have work.