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?
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.
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(.
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