Compiler Error CS0552

'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:

// CS0552.cs
public 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()
   {
   }
}