How is covariance cooler than polymorphism...and not redundant?
- by P.Brian.Mackey
.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();
}
}
}
