Cleanest Way To Map Entity To DTO With Linq Select?

Just use AutoMapper.

Example:

Mapper.CreateMap<Address, AddressDTO>();
Mapper.CreateMap<Person, PersonDTO>();

Your query will execute when the mapping is performed but if there are fields in the entity that you're not interested use Project().To<> which is available both for NHibernate and EntityFramework. It will effectively do a select on the fields specified in the mapping configurations.

If you want to create mappings manually then you can use Select on the collection in the following way:

Some test data:

    var persons = new List<Person>
    {
        new Person() {ID = 1, Name = "name1", Address = new Address() {ID = 1, City = "city1"}},
        new Person() {ID = 2, Name = "name2", Address = new Address() {ID = 2, City = "city2"}},
        new Person() {ID = 3, Name = "name3", Address = new Address() {ID = 1, City = "city1"}}
    };

Mapping methods:

    public static PersonDTO ToPersonDTOMap(Person person)
    {
        return new PersonDTO()
        {
            ID = person.ID,
            Name = person.Name,
            Address = ToAddressDTOMap(person.Address)
        };
    }

    public static AddressDTO ToAddressDTOMap(Address address)
    {
        return new AddressDTO()
        {
            ID = address.ID,
            City = address.City
        };
    }

Actual usage:

var personsDTO = persons.Select(x => ToPersonDTOMap(x)).ToList();

Keep in mind that if this was a real query is would not get executed as long as it was IQueryable, it would be executed once you materialize it (using ToList() for example).

However, I would consider using some framework which could do it (the mappings) for you automatically (if your mapping are as simple as provided example(.