Can I force descendants to have a parameterless constructor?

Question

I am trying to create a generic factory-pattern-like mechanism.

The factory will be like:

public class APlugin<ActionType> where ActionType : IAction
{
    // create a new action. Note: ActionType should contain
    // an empty constructor
    public ActionType CreateAction()
    {
        return Activator.CreateInstance<ActionType>();
    }
}

Descendants of IAction might hide the parameterless constructor and this will cause the factory to fail.

Answer 1

Pretty late but i think this is worth knowing..

You can force constructors with 0 parameters on non-abstract classes. This is a type constraint to use whenever you want to enforce those constraints on a class structure. It requires you to specify the type that implements the interface which might be found disturbing. That's the price if you want some neat type safety.

public interface IEmptyConstructor<TThis> where TThis : IEmptyConstructor<TThis>, new() {}

for example a singleton structure

public abstract class Singleton<TThis> : ISingleton where TThis : Singleton<TThis>, new() { /* some singleton madness */ }

Answer 2

You can't force an empty constructor or any type of constructor on derived types.

Constructors do not get inherited, but do get called.

If not chaining to the base constructor (using the : base() syntax), the default base constructor will be called implicitly.

From Using Constructors on MSDN:

In a derived class, if a base-class constructor is not called explicitly using the base keyword, then the default constructor, if there is one, is called implicitly.