Question

.NET 4 introduces covariance. I guess it is useful. After all, MS went through all the trouble of adding it to the C# language. But, why is Covariance more useful than good old polymorphism?

I wrote this example to understand why I should implement Covariance, but I still don’t get it. Please enlighten me.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Sample
{
    class Demo
    {
        public delegate void ContraAction<in T>(T a);

        public interface IContainer<out T>
        {
            T GetItem();
            void Do(ContraAction<T> action);
        }

        public class Container<T> : IContainer<T>
        {
            private T item;

            public Container(T item)
            {
                this.item = item;
            }

            public T GetItem()
            {
                return item;
            }

            public void Do(ContraAction<T> action)
            {
                action(item);
            }
        }

        public class Shape
        {
            public void Draw()
            {
                Console.WriteLine("Shape Drawn");
            }
        }

        public class Circle:Shape
        {
            public void DrawCircle()
            {
                Console.WriteLine("Circle Drawn");
            }
        }

        public static void Main()
        {
            Circle circle = new Circle();
            IContainer<Shape> container = new Container<Circle>(circle);
            container.Do(s => s.Draw());//calls shape

            //Old school polymorphism...how is this not the same thing?
            Shape shape = new Circle();
            shape.Draw();
        }
    }
}

Answer 1

Consider an API which asks for an IContainer<Shape>:

public void DrawShape(IContainer<Shape> container>) { /* ... */ }

You have a Container<Circle>. How can you pass your container to the DrawShape API? Without covariance, the type Container<Circle> is not convertible to IContainer<Shape>, requiring you to rewrap the type or come up with some other workaround.

This is not an uncommon problem in APIs that use a lot of generic parameters.