C# does not have a non-generic List class: the Generic version is List<T> and as such, your code will not compile without your writing your own List class - and if you wrote your own, then only you know how it works, what you can do with it, and so on.
The generic version would require different definitions:
static List<Sweep> ribsPoints()
{
List<Sweep> _rPoint = new List<Sweep>();
_rPoint.Add(new XYZ(firstRibSpacing, OuterStiffinerThick, 0));
_rPoint.Add(new XYZ(firstRibSpacing, PanelWidth - (2 * OuterStiffinerThick), 0));
return _rPoint;
}
And the
XYZ
class would have to be derived from the
Sweep
class.
If you have written your own List class which returns a holds a collection of Sweep objects, then it's at best poorly named from maintenance point of view!
So look at your code, and see what it is doing - we can't do that for you.