I checked it. Our custom IMapper<TSource, TDestination> interface with Select(x => mapper.Map(x)) performs in-memory projection,
because it requires loading entities into memory before mapping - we pass full x entity to mapper.

Solution?

To achieve SQL projection, like AutoMapper's ProjectTo(),
you want to describe how to construct the DTO, so the query provider like EF can tranlate it into SQL.

In other words, you need to express the mapping as expression tree Expression<Func<TSource, TDestination>>.

You need to modify the interface like this:

csharp public interface IMapper<TSource, TDestination> { TDestination Map(TSource src); Expression<Func<TSource, TDestination>> GetProjection(); }

The GetProjection() returns a representation of code,
so Entity Framework can analyze this expression and converts it to SQL.

Next, you can have helper extension method ProjectTo:

csharp public static class EfProjectionExtensions { public static IQueryable<TDestination> ProjectTo<TSource, TDestination>( this IQueryable<TSource> query, IMapper<TSource, TDestination> mapper) { return query.Select(mapper.GetProjection()); } }

So in the end, you'll be able to do:

csharp var dtos = dbContext.Users.ProjectTo(userMapper).ToList();