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Likely follow-ups
Can one instance of a class read another instance's private fields?Yes. Private is scoped to the type, not the object, so a method can read private fields of any instance of the same class.
Inside a derived class, can you call a protected instance member through a base-class variable?No. The access must go through the derived type or one of its subclasses, so a reference typed as the base class fails to compile.
How would unit tests reach internal members without making them public?Name InternalsVisibleTo on the assembly under test, and its cost. The tests now depend on internal details that were meant to change freely.
Why expose a public property instead of a public field?A property lets you add validation or change storage later, and the setter can take a narrower modifier, like private set.
Do access modifiers stop other code from reading your private data?No. Reflection can read private members. Modifiers are a compile-time design boundary, not a security control.
What you can say
Access modifiers decide which code is allowed to use a type or a member, so they're how a class keeps its internals hidden.
The basic four are public for anyone, private for the containing class only, protected for the class and its subclasses, and internal for anything in the same assembly.
Protected internal is the looser combination, same assembly or any derived class. Private protected is the tighter one, derived classes inside the same assembly only.
If you don't write a modifier, class members are private and top-level types are internal.
I start with the most restrictive modifier that works, because opening access later is easy, while narrowing it breaks code that already depends on it.
Weak answers to avoid
Mixes up protected internal and private protectedThey're opposites. Protected internal is same assembly or any derived class. Private protected is derived classes in the same assembly only. Say which one is wider.
Says internal means same namespaceInternal is scoped to the assembly, the compiled DLL. Namespaces only organize names and never restrict access.
Gets the defaults wrongClass members default to private and top-level types to internal. Guessing public suggests you've never checked what you get when you leave the modifier off.
Recites the list with no design reasonReciting six definitions only shows recall. Say you start with the narrowest access, since every public member becomes a contract other code depends on.
Interview lens
Likely follow-ups, what you can say, and the weak answers to avoid.
