#1,157 – Contravariance and Generic Delegate Types

As with generic interfaces, generic delegate types are contravariant if you mark input parameters as in.

In the example below, we’re unable to assign an instance of ReturnDelegate<Dog> to ReturnDelegate<Terrier>.  The delegate type is not contravariant.

        public delegate void ReportOnDelegate<T>(T param);

        public static void ReportOnDog(Dog d)
        {
            Console.WriteLine(d.Name);
        }

        static void Main(string[] args)
        {

            ReportOnDelegate<Dog> del = ReportOnDog;

            // Error: Cannot implicitly convert ReportOnDelegate<Dog>
            // to ReportOnDelegate<Terrier>
            ReportOnDelegate<Terrier> del2 = del;
        }

We can get the delegate type to behave contravariantly by marking its type parameter with the in keyword.

        public delegate void ReportOnDelegate<in T>(T param);

We can now assign an instance of ReportOnDelegate<Dog> to a variable of type ReportOnDelegate<Terrier>.