Try to converse it back to delegate or .Net 2.0

Vincent_lambda.cs

Mapper.CreateMap<DTOObject, MainObject>().ForMember(
  d => d.TopObject, 
  mc => mc.MapFrom(s => new SubObject(){ 
    SubPropText = s.TopText, 
    SubPropFont = s.TopFont 
  } 
));

//-->

Mapper.CreateMap<DTOObject, MainObject>().ForMember(
  d => d.TopObject,     // 每一個d 回傳 d.TopObject
  mc => mc.MapFrom(     // 每一個mc 回傳 mc.MapFrom 
      s => new SubObject(){       //用
        SubPropText = s.TopText, 
        SubPropFont = s.TopFont 
  } 
));