#1,069 – Contravariance and Generic Interfaces

Generic interfaces in C# are contravariant, provided that their type parameters are constrained with the in keyword.  A contravariant generic interface allows an assignment from a constructed version of the interface on a base class to a constructed version of the interface for a derived class.

For example, if BorderCollie derives from Dog and if IAddRemove<T> is contravariant, we can do the following:

            IAddRemove<Dog> dogAddRemove;
            // Assign dogAddRemove to some class that implements IAddRemove<Dog>
            IAddRemove<BorderCollie> bcAddRemove = dogAddRemove;

Below is a full example of defining a contravariant interface. Note that the LineOf<T> class implements both the covariant interface IFirstAndLast<T> and the contravariant interface IAddRemove<T>.

        public interface IFirstAndLast<out T>
        {
            T First();
            T Last();
        }

        public interface IAddRemove<in T>
        {
            void AddToEnd(T item);
            void RemoveFromFront(T item);
        }   

        public class LineOf<T> : IFirstAndLast<T>, IAddRemove<T>
        {
            private Queue<T> theQueue = new Queue<T>();

            public void AddToEnd(T d)
            {
                theQueue.Enqueue(d);
            }

            public void RemoveFromFront(T d)
            {
                theQueue.Dequeue();
            }

            public T First()
            {
                return theQueue.Peek();
            }

            public T Last()
            {
                return theQueue.Last();
            }
        }

        static void Main(string[] args)
        {
            LineOf<Dog> dogs = new LineOf<Dog>();

            IAddRemove<BorderCollie> bcAddRemove = dogs;
            bcAddRemove.AddToEnd(new BorderCollie("Kirby"));
            bcAddRemove.AddToEnd(new BorderCollie("Shep"));
        }
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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.

2 Responses to #1,069 – Contravariance and Generic Interfaces

  1. Jason says:

    Been looking for a simple explanation of this. Now I think “In=Assign Base to Derived”. Thanks for clearing that up.

  2. Steve says:

    Wow! That’s a good example to demonstrate how to use in or out in generic interface. Good to know it. Thanks.

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