#1,068 – Generic IEnumerable Interface Is Covariant

A covariant generic interface is one that allows an assignment from a constructed version of the interface on a derived class to a constructed version of the interface for a base class.

For example:

            // If herders is (or implements) IFirstAndLast<BorderCollie>:
            IFirstAndLast<Dog> dogLine = herders;

The IEnumerable<T> interface in System.Collections.Generic is covariant.  This means that you can assign a collection of a given type to an IEnumerable<T> where the T represents a type further up the inheritance chain.

For example:

            List<BorderCollie> someBCs = new List<BorderCollie> {
                new BorderCollie("Shep"),
                new BorderCollie("Kirby")
            };

            // Because BorderCollie derives from Dog,
            // we can do the following
            IEnumerable<Dog> dogList = someBCs;

If MerleBorderCollie inherits from BorderCollie, which in turn inherits from Dog, we can also do:

            List<MerleBorderCollie> merles = new List<MerleBorderCollie>
            {
                new MerleBorderCollie("Lady")
            };
            IEnumerable<Dog> moredogs = merles;