A question I got recently was about access to protected methods from a derived class. Clearly that's what "protected" means – that you can access it from a derived class. In that case, why doesn't this work?
class Ungulate {
protected void Eat() { /* whatever */ }
}
class Giraffe : Ungulate {
public static void FeedThem() {
Giraffe g1 = new Giraffe();
Ungulate g2 = new Giraffe();
g1.Eat(); // fine
g2.Eat(); // compile-time error "Cannot access protected member"
}
}
What the heck?
Giraffe is derived from Ungulate, so why can't it always call the protected method?
To understand, you have to think like the compiler. The compiler can only reason from the static type information, not from the fact that we know that at runtime
g2 actually will be a Giraffe. For all the compiler knows from the static type analysis, what we've actually got here is
class Ungulate {
protected virtual void Eat() { /* whatever */ }
}
class Zebra : Ungulate {
protected override void Eat() { /* whatever */ }
}
class Giraffe : Ungulate {
public static void FeedThem() {
Giraffe g1 = new Giraffe();
Ungulate g2 = new Zebra();
g1.Eat(); // fine
g2.Eat(); // compile-time error "Cannot access protected member"
}
}
We can call
Ungulate.Eat legally from Giraffe, but we can't call the protected method Zebra.Eat from anything except Zebra or a subclass of Zebra. Since the compiler cannot determine from the static analysis that we are not in this illegal situation, it must flag it as being illegal.
Incidentally, C++ has the same rule.
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About Eric Lippert
Eric Lippert is a senior developer on the Microsoft C# compiler team. Before that he worked on the framework of Visual Studio Tools For Office. Before that, he worked on the compilers, runtimes and tools for VBScript, JScript, Windows Script Host and other Microsoft Scripting technologies. He lives in Seattle and spends his free time editing books about programming languages, playing the piano, and trying to keep his tiny sailboat upright in Puget Sound.
