Compiler Error CS0553

'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:

// CS0553.cs
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()
      {
      }
   }
}