#1,156 – Covariance and Generic Delegate Types

As with generic interfaces, generic delegate types are covariant if you mark output parameters as out.

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

        public delegate T ReturnDelegate<T>();

        public static Terrier GenerateTerrier()
        {
            return new Terrier("Bubba");
        }

        static void Main(string[] args)
        {
            ReturnDelegate<Terrier> del = GenerateTerrier;

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

We can get the delegate type to behave covariantly by marking its type parameter with the out keyword.

        public delegate T ReturnDelegate<out T>();

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