Linq - Create new object which contains a collection - entity-framework

Question

I am trying to get data from Entity Framework using linq to populate a custom object. The problem is the object contains a list collection where I am having problems populating.
I have the following
public class Person
{
public string FirstName { get; set; }
public string ListName { get; set; }
public IList<Address> ContactAddresses v{ get; set; }
}
public class Address
{
public string FirstLine { get; set; }
public string SecondLine { get; set; }
public string Town { get; set; }
}
I am trying to populate the Person object but having trouble populating the list of addresses. The situation I have at the moment one address is stored in the Person table and work and other addresses are stored in another. Right now I am only trying to get the address stored in the Person table. Below is my attempt at this but syntax is not working.
Person details = context.Person.Where(p => p.Id == id)
.Select(p => new Person)
{
FirstName = p.FirstName,
LastName = p.LastName,
ContactAddresses = new List<Address>()
.add(new Address
(
FirstLine = m.FirstLineAddress,
SecondLine = m.SecondLine,
Town = m.Town
))
}.FirstOrDefault();
UPDATE:
I worked out how to do this. The following linq is my solution.
Person details = context.Person.Where(p => p.Id == id)
.Select(p => new Person)
{
FirstName = p.FirstName,
LastName = p.LastName,
ContactAddresses = (new List<Address>()
{
new Address()
{
FirstLine = p.FirstLine ,
SecondLine = p.SecondLine,
Town = p.Town
}
}).ToList(),
}.FirstOrDefault();

Answer 1

This is not really a problem of LINQ. It is more about object initialization. First rule (for me) if an object has a collection, it should never be null. It can be empty, but never null. (Another question would be, if a collection property should be settable or not, but that's another story.)
public class Person
{
public string FirstName { get; set; }
public string ListName { get; set; }
public IList<Address> ContactAddresses { get; set; } = new List<Address>();
}
And then you can initialize the object like this:
var person = new Person
{
FirstName = "Foo",
ListName = "Bar",
ContactAddresses = { new Address { FirstLine = "A", SecondLine = "B", Town = "C" } }
};
As you can see, the contact addresses has an initializer that doesn't use new. So the list of objects will be added to the existing list.
If you want to replace the list instead of changing it, you can also provide a new list:
var person = new Person
{
FirstName = "Foo",
ListName = "Bar",
ContactAddresses = new List<Address> { new Address { FirstLine = "A", SecondLine = "B", Town = "C" } }
};
More about object initialization can be found at Microsoft.

Answer 2

You wrote:
The situation I have at the moment one address is stored in the Person table and work and other addresses are stored in another.
So you have a class Person, which has a contact Address, and also a collection of zero or more alternative Addresses. This might be a one-to-many relation, or a many-to-many relation, for a Person class this does not really matter:
class Person
{
public int Id {get; set;}
public Address MainAddress {get; set;}
... // other properties
// every Person has zero or more Alternative Addresses:
public virtual ICollection<AlternativeAddress> AlternativeAddresses {get; set;}
}
class AlternativeAddress
{
public int Id {get; set;}
public Address Address {get; set;}
... // other properties
// if this Address belongs to exactly one Person (one-to-many)
public int PersonId {get; set;}
public virtual Person Person {get; set;}
// or if this address may be the address of several Persons (many-to-many)
public virtual ICollection<Person> Persons {get; set;}
}
In entity framework the columns of a table are represented by the non-virtual properties. The virtual properties represent the relations between the tables (one-to-many, many-to-many, ...)
Address is a class. Because I didn't declare MainAddress virtual, the properties of Address will be columns within Person. Similarly, the properties of the Address in AlternativeAddress will be columns within an AlternativeAddress.
If you didn't use the Class Address to make sure that your MainAddress has exactly the same structure as the AlternativeAddresses you'll have to do a Select.
This is all that entity framework needs to know to detect your tables and the relations between the tables, the primary and foreign keys. Only if you want different identifiers you need attributes or fluent API.
Requirement Given an input parameter personId, Give me (some properties of) the Person with all his Addresses.
var result = dbContext.Persons
.Where(person => person.Id == personId)
.Select(person => new
{
// select only the properties you plan to use:
Id = person.Id,
FirstName = person.FirstName,
...
// For the contact Addresses, concat the MainAddress and the Alternative Addresses
ContactAddresses = new Address[] {person.MainAddress}
.Concat(person.AlternativeAddresses.Select(address => address.Address)
// use a Select if you don't need all Address properties:
.Select(address => new
{
Street = address.Street,
City = address.City,
PostCode = address.PostCode,
...
});
If classes Person and AlternativeAddress don't have an Address Properties, you'll have to select the properties you want:
ContactAddresses =
// Main Address
new []
{
Street = person.Street,
City = person.City,
PostCode = person.PostCode,
...
}
.Concat(person.AlternativeAddresses.Select(address => new
{
Street = address.Street,
City = address.City,
PostCode = address.PostCode,
...
}),