Question

I’d like to define an generic interface that allows nullable value types (int?, double?, etc.) and class types (which can also be null). I do not want to allow a simple value type. Is there a way to do this?

Answer 1

EDIT: Given the question title, I assume you want to constrain the type parameter to not be a non-nullable value type. It would probably be a good idea to specify that in the question body too.

No – there’s no such constraint. In fact, both class and struct constraints prohibit arguments which are nullable value types.

You could potentially create an interface without a constraint, but only create two implementations:

interface IFoo<T> { }

class FooClass<T> : IFoo<T> where T : class {}

class FooNullableValue<T> : IFoo<Nullable<T>> where T : struct {}

That wouldn’t stop anyone else from implementing IFoo<int> of course. If you can give us more background, we may be able to help more.