#1,066 – Constraining a Type Parameter on a Generic Interface

You can constrain a type parameter on a generic interface so that it can be used only as the output type of methods in the interface and not as a type of any method parameters.  You do this using the out keyword on the type parameter.

        public interface FirstAndLast<out T>
        {
            T First();
            T Last();
            void AddThing(T item);   // won't compile
        }

This seems like an odd thing to do, but ensuring that a particular type parameter is used only as output allows us to later use this generic interface covariantly.  (I’ll explain this in an upcoming post).