#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).

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About Sean
Software developer in the Twin Cities area, passionate about .NET technologies. Equally passionate about my own personal projects related to family history and preservation of family stories and photos.

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