Tell AutoMapper that a field is a top-level member - automapper

Question

I am trying to take some of the pain out of creating mapping expressions in AutoMapper, using AutoMapper.QueryableExtensions
I have the following, which gives a critical performance gain:
private MapperConfiguration CreateConfiguration() {
return new MapperConfiguration(cfg => cfg.CreateMap<Widget, WidgetNameDto>()
.ForMember(dto => dto.Name,
conf => conf.MapFrom(w => w.Name)));
}
To understand the performance gain, see here: https://github.com/AutoMapper/AutoMapper/blob/master/docs/Queryable-Extensions.md The key is that the query is limited by field at the database level.
It's terrific that this works. But I anticipate needing to do a lot of this kind of projecting. I am trying to take some of the pain out of the syntax in the ForMember clause above.
For example, I've tried this:
public static IMappingExpression<TFrom, TTo> AddProjection<TFrom, TTo, TField>(this IMappingExpression<TFrom, TTo> expression,
Func<TFrom, TField> from,
Func<TTo, TField> to
)
=> expression.ForMember(t => to(t), conf => conf.MapFrom(f => from(f)));
The problem is that everything I do runs into an error:
AutoMapper.AutoMapperConfigurationException : Custom configuration for members is only supported for top-level individual members on a type.
Even if the passed in Funcs are top-level individual members, that fact is lost in the passing, so I hit the error. I've also tried changing Func<Whatever> to Expression<Func<Whatever>>. It doesn't help.
Is there any way I can simplify the syntax of the ForMember clause? Ideally, I would just pass in the two relevant fields.

Answer 1

First, there is no need to add mapping for the fields/properties that match by name - AutoMapper maps them automatically by convention (that's why it is called convention-based object-object mapper). And for including just some of the properties in the projection you could use the Explicit expansion feature.
Second, what you call a pain in the ForMember syntax is in fact a flexibility. For instance, the explicit expansion and other behaviors can be controlled by conf argument, so it's not only for specifying the source.
With that being said, what you ask is possible. You have to change the from/ to type to Expression:
Expression<Func<TFrom, TField>> from,
Expression<Func<TTo, TField>> to
and the implementation simply as follows:
=> expression.ForMember(to, conf => conf.MapFrom(from));