'conversion routine' : user defined conversion to/from derived class
User-defined conversions to values of a derived class are not allowed; you do not need such an operator.
The following sample generates CS0554:
namespace x { public class ii { // delete the conversion routine to resolve CS0554 public static implicit operator ii(a aa) // CS0554 { return new ii(); } } public class a : ii { public static void Main() { } } }