.Net SDK problem, which way to go?

Question

We're writing an SDK for a CAD program and have run into a slight disagreement regarding a specific type of function (not just disagreement between different people, also disagreement between my two brain-halves).

Imagine there's a lot of classes for specific curve types (ellipse, circle, arc, line, bezier etc.). All of these classes can be represented by a Nurbs curve. So should we put the Circle->Nurbs function on Circle:

public NurbsCurve Circle.ToNurbsCurve()
{
  // Return a circular NurbsCurve or null if the Circle is invalid.
}

or should it be a static on NurbsCurve:

public static NurbsCurve NurbsCurve.CreateFromCircle(Circle)
{
  // Return a circular NurbsCurve or null if the Circle is invalid.
}

Answer 1

I would put it in the Circle class, because it's the one that knows how it should be converted to a NurbsCurve. The NurbsCurve class isn't supposed to know about all specific types of curve. That way, if you create a new type of curve, you won't have to modify the NurbsCurve class.

BTW, I suggest you declare the ToNurbsCurve method in an interface implemented by all curves (or declare it virtual in an abstract base class)

Answer 2

I think I'd go for the first (e.g. on the shape classes, maybe even with a common base class or interface like IConvertibleToNurbsCurve), because this makes it easier if you add other shapes later which are also convertible to a NurbsCurve.

The NurbsCurve seems to be less specialized and therefore should not "know" about the more specialized types IMHO.