How to specify an object that is of one class but not a certain subclass?

Question

class A
{

}

class B : A
{

}

void method(A that is not a B argument) {}
void generic_method(generic_class<A that is not a B> generic_argument) {}
void params_method(params A that is not a B[] params_arguments) {}

Is there any syntactical way to do this? i realize that i could just do

if(argument is B)
    throw new ArgumentException("argument cannot be a B", "argument");

at the beginning of the first method, and do that in a foreach for the second and third, but I'm wondering if there is some keyword or OOP concept that accomplishes this better.

Answer 1

So it sounds like you're trying to express the opposite of a constraint such as:

var Foo<T>() where T : SomeClass

That would constrain T to be SomeClass or a subclass¹... but you're trying to make it explicitly not a T.

No, I'm afraid there's no such constraint in C#.

---

¹ Well, modulo the assumption that SomeClass is a class to start with; it could be an interface, with the meaning you probably expect.

Answer 2

No such constraint exists in C# because it's a bad idea, specifically its a violation of the Liskov substitution principle (one of the SOLID principles)

Substitutability is a principle in object-oriented programming. It states that, in a computer program, if B is a subtype of A, then objects of type A may be replaced with objects of type B (i.e., objects of type B may be substituted for objects of type A) without altering any of the desirable properties of that program

If your program explicitly rejects objects of type B then that's clearly a violation of the LSP and a strong indication that you are doing polymorphism wrong - probably B shouldn't inherit from A in the first place.