Are Interfaces Compatible With Polymorphism

Question

I am having trouble with the concept of interfaces interacting with polymorphic types (or even polymorphic interfaces). I\'m developing in C# and would appreciate answers st

Answer 1

Interfaces are not only compatible with polymorphism--they're essential to it. Part of the idea you seem to be missing is that if one has an interface like IPaintable, the expectation is that every object that implements it will provide some default method of being painted, typically encapsulating other methods which would configure the object in some useful fashion.

For example, an IPaintable interface might define a paint method:

void Paint(ICanvas canvas);

Note that the Paint method says nothing about what should be painted, nor what color, nor anything else. Various paintable objects would expose properties to control such things. A Polygon method, for example, might dispose OutlineColor and FillColor properties, along with a SetVertices() method. A routine that wants to paint a whole lot of objects could accept an IEnumerable(Of IPaintable) and simply call Paint on all of them, without having to know anything about how they'll be painted. The fact that an object is painted by calling Paint with no parameters other than the canvas upon which it should be painted in no way prevents the Paint method from doing all sorts of fancy gradient fills, texture mapping, ray tracing, or other graphical effects. The code which is commanding the painting would know nothing of such things, but the IPaintable objects themselves would hold all the information they needed and thus wouldn't need the calling code to supply it.