'conversion routine' : user defined conversion to/from base class
User-defined conversions to values of a base class are not allowed; you do not need such an operator.
The following sample generates CS0553:
namespace x { public class ii { } public class a : ii { // delete the conversion routine to resolve CS0553 public static implicit operator ii(a aa) // CS0553 { return new ii(); } public static void Main() { } } }