Difficulty repopulating viewmodels on post - asp.net-mvc

Question

I have various viewmodels that have properties that must be populated. For example, imagine a database table full of countries. I have a create user page, and one of the properties on the viewmodel is List<string> Countries.
Originally I populated this in a parameterless constructor, ie.
public CreateUserViewModel()
{
this.Countries = new CountryManager().GetCountries();
}
But I read this is a bad practice, and I should pass them in.
public CreateUserViewModel(IEnumerable<string> countries)
{
this.Countries = countries;
}
But in my post, this data is lost, and if validation fails it redirects to the view, but then the countries property is null.
I am wondering how I should be repopulating this value. Manually putting some code into the controller post method seems bad, eg.
[HttpPost]
public ActionResult CreateUser(CreateUserViewModel vm)
{
if (Model.IsValid)
{
new UserManager().CreateUser(vm);
}
else
{
vm.Countries = new CountryManager().GetCountries();
return View(vm);
}
}
I am struggling to google what is probably a very common question. Any ideas?

Answer 1

The rationale behind avoiding parameterless constructors is for inversion of control. The logic in this case would be that Countries is a dependency and by externalizing that dependency (so that it is inject into the class, instead), you make your class less brittle and more open to extension.
However, I would argue that doesn't apply actually in your scenario, because Countries is not really a dependency of your class, but rather of your view. The view model is there to serve the view and is somewhat closed and unextensible anyways as a result. In other words, do follow inversion of control for things like services, repositories, utility classes, etc., but for view models, it's not really necessary or important.
Anyways, here's how I handle this type of thing:
public class FooViewModel
{
...
// Countries is not initialize by a constructor
public IEnumerable<SelectListItem> Countries { get; set; }
}
Then in your controller:
public class FooController : Controller
{
internal void PopulateCountryChoices(FooViewModel model)
{
// fetch countries
model.Countries = countries.Select(m => new SelectListItem
{
Text = m.Name,
Value = m.Id.ToString()
});
}
public ActionResult Bar()
{
var model = new FooViewModel();
PopulateCountryChoices(model);
return View(model);
}
[HttpPost]
public ActionResult Bar(FooViewModel model)
{
if (ModelState.IsValid)
{
// save and redirect
}
PopulateCountryChoices(model);
return View(model);
}
}

Answer 2

Instead of populating this static data in model, We can populate Dropdownlist values in the view itself -
#Html.DropDownListFor(model => model.State,
new SelectList(Utils.GetCountries()),
"value",
"text",
2)
Where Utils is a Helper class which returns all the countries.
This way of populating all the static data (I mean all options of Dropdownlist or Listbox etc) will be taken over by the View, which makes model to be free from holding this data.