Question

I have no idea what the keywords are so here is an example of what I want:

return from i in userRepo.GetUsers()
  select new SimpleUser{
        i.UserId,
        i.Name
  };

userRepo.GetUsers() returns type IQueryable<User>. I’d like to convert this to IQueryable<SimpleUser> to I can restrict access to certain properties of the User domain.

How do I do this without hard coding the translation like this? What about tools like automapper or ValueInjecter, how can they do this?

Also, what is this technique called?

Answer 1

You must hardcode the translation or you must first convert it to IEnumerable. IQueryable represents expression tree translated to some execution in used provider – in your case I believe it will be Entity framework. You can’t use any automatic mapping in such query because it will be translated to SQL which will not understand your .net methods or AutoMapper. Projections to custom types are part of the query and must be hardcoded. You can create custom extension method to IQueryable and reuse it where you need:

public static IQueryable<SimpleUser> ProjectToSimpleUser(this IQueryable<User> query)
{
    return query.Select(u => new SimpleUser
        {
            // Here map your fields
        });
}

Now you can use:

return repo.GetUsers().ProjectToSimpleUser();

In case of Entity framework SimpleUser mustn’t be an mapped entity.