Compiler Error CS0554

'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.

See chapter 6 in the C# language specification for more information on user-defined conversions.

The following sample generates CS0554:

// CS0554.cs
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()
      {
      }
   }
}