Selected Items in MVC select list are null - asp.net-mvc

Question

I have the following model -
public class RoleModel
{
public int Id { get; set; }
public string RoleName { get; set; }
public virtual IEnumerable<UserModel> Users { get; set; }
public virtual IEnumerable<UserModel> SelectedUsers { get; set; }
public RoleModel()
{
}
}
The users IEnumerable is a list of users who are currently assigned to the selected role. When the View loads, this is populated correctly.
When I press Save in the view, the SelectedUsers IEnumerable and the Users IEnumerable is null, meaning I cannot unassign all users and reassign the selected users to the role.
Any ideas why the IEnumerables are null when pressing Save?
View
#Html.ListBoxFor(model => model.SelectedUsers, new SelectList(Model.Users, "Id", "SurnameFirstName"))
Controller
[Authorize]
[HttpGet]
public ActionResult Details(int id = 0)
{
RoleModel role = _roleService.GetById(id);
if (role == null)
{
return HttpNotFound();
}
return View(role);
}
[HttpPost]
public ActionResult Details(RoleModel model)
{
if (ModelState.IsValid)
{
_roleService.ReassignRights(model);
_roleService.ReassignUsers(model);
_roleService.Update(model);
return RedirectToAction("Index");
}
return View(model);
}
Reassign Users method in RoleService.cs
public void ReassignUsers(RoleModel role)
{
var roleDal = _roleRepository.FindById(role.Id);
//_roleRepository.ClearUsersForRole(role.Id);
foreach (var user in role.SelectedUsers)
{
}
}

Answer 1

Try replacing the type of UserModel into type of of UserModel's PK. If UserModel::Id is int then change the SelectedUsers as below.
public virtual IEnumerable<int> SelectedUsers { get; set; }
While the Users property will still be null because there is no input that is related to Users, it's just being used to render the html options in the ListBoxFor. You need to set the value again when it's submitted.
DEMO