'conversion routine' : user defined conversion to/from interface
You cannot create a user-defined conversion to or from an interface. If you need the conversion routine, resolve this error by making the interface a class or derive a class from the interface.
The following sample generates CS0552:
namespace x { interface ii { } public class a { // delete the routine to resolve CS0552 public static implicit operator ii(a aa) // CS0552 { return new ii(); } public static void Main() { } } }