Why can private member variable be changed by class instance?

Question

class TestClass
{
    private string _privateString = "hello";
    void ChangeData()
    {
        TestClass otherTestClass = new TestClass();
        otherTestClass._privateString = "world";
    }
}

This code compiles in C# and the equivalent works in PHP, but can someone explain the reason why otherTestClass._privateString can be changed here ?

I would have thought an instance of a class should not be able to change a private member variable under any circumstances, and that trying to access otherTestClass._privateString would give an 'inaccessible due to protection level' error.

This is not the case though, so why does instantiating an object inside its own class let you access the private members ? And should it, doesn't this break encapsulation to an extent ? Or am I missing something obvious ?

• (I'm not asking if the above class design is good practice, just wondering about the theory behind it.)

Edit - Thanks for the answers and comments. To clarify, I'm also interested in knowing if being able to do this is regarded as a positive feature, or if it's a necessary tradeoff for better compile-time checking/code clarity/because most other languages do it that way or whatever. It seems to me ideally the compiler would prevent or warn you about this, but then I'm far from a language designer. Any examples of how it being this way lets you do something useful (without violating encapsulation) that would otherwise be difficult or impossible would be great.

Answer 1

Private members are accessible to any code within the program text of that class (including within nested types). It has nothing to do with which instance of the class you're dealing with.

I don't believe this violates encapsulation - the API is still separated from the implementation, but the implementation "knows" about itself regardless of which instance it's looking at.

I believe that in some other languages this isn't how accessibility works, but it definitely is for C# and Java. (Java has slightly different rules about what can access private members, but the translated code for what you've written would still work.)

Answer 2

This is because C# enforces class-level privacy and not object-level privacy.

Most mainstream languages enforce the same policy, i.e. C#, C++ and Java. I think the reason are:

1) because developers are accustomed to that kind of policy;

2) because object-level privacy would become much too tedious in return of very few advantages.