Converting IQueryable<T> object to another object?

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

Provided that SimpleUser is assigneable to User (User is an interface of baseclass of SimpleUser), you can

 var users = simpleUsers.Cast<User>();

optionally with

 var users = simpleUsers.Cast<User>().AsQueryable();

And if you're not sure whether all items are actually Users, then you can use OfType<User> instead of Cast<User>

Answer 2

This question is 9 years old so idk if it existed then but for anyone searching now, using .FirstOrDefault() works if you pick a single user.