Cleanest Way To Map Entity To DTO With Linq Select?

Question

I've been trying to come up with a clean and reusable way to map entities to their DTOs. Here is an example of what I've come up with and where I'm stuck.

Entities

public class Person
{
    public int ID { get; set; }
    public string Name { get; set; }
    public Address Address { get; set; }
    // Other properties not included in DTO
}

public class Address
{
    public int ID { get; set; }
    public string City { get; set; }
    // Other properties not included in DTO
}

DTOs

public class PersonDTO
{
    public int ID { get; set; }
    public string Name { get; set; }
    public AddressDTO Address { get; set; }
}

public class AddressDTO
{
    public int ID { get; set; }
    public string City { get; set; }
}

Expressions

This is how I began to handle the mapping. I wanted a solution that wouldn't execute the query before mapping. I've been told that if you pass a Func<in, out> instead of Expression<Func<in, out>> that it will execute the query before mapping.

public static Expressions
{
    public static Expression<Func<Person, PersonDTO>> = (person) => new PersonDTO()
    {
        ID = person.ID,
        Name = person.Name,
        Address = new AddressDTO()
        {
            ID = person.Address.ID,
            City = person.Address.City
        }
    }
}

One issue with this is that I already have an expression that maps an Address to an AddressDTO so I have duplicated code. This will also break if person.Address is null. This gets messy very quick especially if I want to display other entities related to person in this same DTO. It becomes a birds nest of nested mappings.

I've tried the following but Linq doesn't know how to handle it.

public static Expressions
{
    public static Expression<Func<Person, PersonDTO>> = (person) => new PersonDTO()
    {
        ID = person.ID,
        Name = person.Name,
        Address = Convert(person.Address)
    }

    public static AddressDTO Convert(Address source)
    {
        if (source == null) return null;
        return new AddressDTO()
        {
            ID = source.ID,
            City = source.City
        }
    }
}

Are there any elegant solutions that I'm missing?

Answer 1

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.