manage conditional mapping simply for merging

[xbrossard]

I know that expressMapper can manage conditional mapping executed when calling map, but I think that current implementation doesn't manage a true conditional copy in a simple way.
My goal is very simple: i want to manage a merge between two objects of the same type, simply copy the source fields in destination fields only is it's equal to null (or default field value). I try to use different methods, but nothing work as i want:

• the Function operation cannot test source and destination field to compare

• the Before operation cannot conditonnally change comportement of mapping (just make initialisation process)

• the registerCustom need to define one custom mapping class for each type combinaison; I can't define a generic CustomMapper class which doesn't inherit from a statically generic class ICustomMapper<T1,T2>. Furthermore, you have to define one CustomMapper class for all connected types If you have a connected graph of type, and you have to explicit call mapping for each connected object, like in this example:
  for a Address which have a Town as a field object, you have to define 2 Mappers classes like this one

    public class MergeMapperAdress : ExpressMapper.ICustomTypeMapper<Adress, Adress>
    {
        public Adress Map(IMappingContext<Adress, Adress> context)
        {
        if (context.Destination == default(Adress))
            context.Destination = new Adress();
  
       // copy all primitive fields to destination only if it's null
        context.Source.Merge(context.Destination);
  
        if (context.Destination.town== null)
            context.Destination.town= ExpressMapper.Mapper.Map(context.Source.town, typeof(Town), typeof(Town)) as Town;
        else
            ExpressMapper.Mapper.Map(context.Source.town, context.Destination.ville, typeof(Town), typeof(Town));
  
        return context.Destination;
        }
   }
  
    public class MergeMapperTown : ExpressMapper.ICustomTypeMapper<Town, Town>
    {
        public Ville Map(IMappingContext<Town, Town> context)
        {
        if (context.Destination == default(Town))
            context.Destination = new Town();
  
       // copy all primitive fields to destination only if it's null
        context.Source.Merge(context.Destination);
  
        return context.Destination;
        }
    }