C#

public IMyInterface CreateMyInterfaceClass(Type type) {
   return (IMyInterface)type.GetConstructor(new Type[0]).Invoke(new object[0]);
}

public enum MyEnum {
   ImplA,
   ImplB
}

public IMyInterface CreateMyInterfaceClass(MyEnum type) {
   switch (type) {
      case MyEnum.ImplA:
         return new MyInterfaceImplA();
      case MyEnum.ImplB:
         return new MyInterfaceImplB();
      default:
         throw new ArgumentException("type");
   }
}

public interface IMyInterface {
   void Notify();
}

public class MyInterfaceImplA : IMyInterface {
   public void Notify() { //do something }

   //add as much other methods as you like
}

public class MyInterfaceImplB : IMyInterface {
   public void Notify() { //do something different }

   //add as much other methods as you like
}

interface INotifier {
   ...
}

public static INotifier CreateInstance(...) {
   ...
}