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.

Answer 1

You could either use AutoMapper or write extension methods like these:

public static class PersonMapper
{
    public static PersonDTO ConvertToDTO(this Person person)
    {
        return new PersonDTO { ID = person.ID, Name = person.Name, Address = person.Address.ConvertToDTO() };
    }

    public static IEnumerable<PersonDTO> ConvertToDTO(this IEnumerable<Person> people)
    {
        return people.Select(person => person.ConvertToDTO());
    }
}

public static class AddressMapper
{
    public static AddressDTO ConvertToDTO(this Address address)
    {
        return new AddressDTO { ID = address.ID, City = address.City };
    }

    public static IEnumerable<AddressDTO> ConvertToDTO(this IEnumerable<Address> addresses)
    {
        return addresses.Select(address => address.ConvertToDTO());
    }
}

You could then map a Person object to a PersonDTO object like this:

public class Program
{
    static void Main(string[] args)
    {
        Person person = new Person { ID = 1, Name = "John", Address = new Address { ID = 1, City = "New Jersey" } };
        PersonDTO personDTO = person.ConvertToDTO();
        Console.WriteLine(personDTO.Name);
    }
}

Answer 2

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.

Answer 3

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(.

Answer 4

Automapper is the best way .

For me, I use this for simple objects only, but I don't recommend it

  public static class ObjectMapper
{
    public static T Map<T>(object objfrom, T objto)
    {
        var ToProperties = objto.GetType().GetProperties();
        var FromProperties = objfrom.GetType().GetProperties();

        ToProperties.ToList().ForEach(o =>
            {
                var fromp = FromProperties.FirstOrDefault(x => x.Name == o.Name && x.PropertyType == o.PropertyType);
                if (fromp != null)
                {
                    o.SetValue(objto, fromp.GetValue(objfrom));
                }
            });

        return objto;
    }
}

And I call it like that wherever I want

   var myDTO= ObjectMapper.Map(MyObject, new MyObjectDTO());