Cleanest Way To Map Entity To DTO With Linq Select?

c# entity-framework linq linq-to-entities linq-to-sql

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?

1
21
4/13/2015 7:45:54 PM

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Accepted Answer

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.

10
4/14/2015 12:23:40 PM

Popular Answer

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(.

Andrew B

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