#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 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;

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.

One Response to #1,068 – Generic IEnumerable Interface Is Covariant

  1. Pingback: Dew Drop – April 4, 2014 (#1758) | Morning Dew

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