.NET MVC -- Using Composition in a View Model - asp.net-mvc

Question

I am trying to wrap my head around the idea of Composition. Never used it before. I have a class that looks like this (thinned down):
public class AccountProfile
{
public string AccountNumber { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public void GetAccountProfile()
{
AccountNumber = "123456"; // eventual these will become values from the database
FirstName = "John";
LastName = "Smith";
}
}
Then, in my view model, I'd like to have access to AccountNumber, FirstName, and LastName. I don't want to use inheritance, as this view model will need access to multiple external, unrelated classes. So far the model is simple:
public class AccountProfileViewModel
{
public AccountProfileViewModel() { }
}
Here's what I've tried so far, none which are correct:
public class AccountProfileViewModel
{
AP= new AccountProfile();
public AccountProfileViewModel() { }
}
That one (above) throws multiple errors and won't compile. I've also tried this:
public class AccountProfileViewModel
{
public AccountProfile AP { get; set; }
public AccountProfileViewModel() { }
}
This one (the one above) compiles just fine, but it throws a run-time error in the controller when I try and use it:
model.AP.GetAccountProfile();
The error: {"Object reference not set to an instance of an object."}
I'm out of ideas. Thanks!

Answer 1

you have to initialize the object at least.
public class AccountProfileViewModel
{
public AccountProfile AP { get; set; }
public AccountProfileViewModel() {
AP = new AccountProfile();
}
}

Answer 2

I think what you are trying to achieve is something like this:
public class AccountProfileViewModel
{
public AccountProfile AP { get; set; }
public AccountProfileViewModel() { }
}
or if AccountProfileViewModel really needs AccountProfile you can do
public class AccountProfileViewModel
{
public AccountProfile AP { get; set; }
public AccountProfileViewModel(AccountProfile profile) {
this.AP = profile;
}
}
and in your controller you can do something like this
public class controller {
public ActionResult Index(){
var vm = new AccountProfileViewModel();
var ap = //Get accountProfile
vm.AP = ap;
return View(vm);
}
}
or in the case of example where you need the AccountProfile
public class controller {
public ActionResult Index(){
var ap = //Get accountProfile
var vm = new AccountProfileViewModel(ap);
return View(vm);
}
}
you want AccountProfileViewModel to have an instance of the AccountProfile but you want to set it in the controller.
then in your view you can do Model.AP.AccountNumber for instance

Answer 3

If you need an object reference in this class then it's my personal preference to only create the object if it's required like so:
public class AccountProfileViewModel
{
private AccountProfile _ap;
public AccountProfile AP
{
get { return _ap ?? (_ap = new AccountProfile()); }
set { _ap = value; }
}
}
If you actually use yourObject.AP then it will have a reference created / return the existing one but if it's not used then no reference has been created.