Here,
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cp
conqualifyingnettypesforinteroperation.asp,
at MSDN documentation you can read:

" Types must have a public default constructor to be activated from COM.
Managed, public types are visible to COM. However, without a public default
constructor (a constructor with no arguments), COM clients cannot create the
type. COM clients can still use the type if it is activated by some other
means. "

You always can have a public class with a public constructor and a method
that returns an new instance of your class. You must put one internal
constructor in your current class which only would be callable from inside
your assembly.