NullReferenceException when Assigning String to ViewModel

Question

I'm experiencing the error NullReferenceException was unhandled by user code followed by Object reference not set to an instance of an object. I've read a number of questions including the very detailed answer here but can't understand why.

The line is occurring on the line vm.HomeView.ComputerName = Environment.MachineName;

When the error occurs, Environemnt.MachineName is correctly showing the machines name as expected so there is definitely a string variable trying to be assigned.

Could anyone help explain why.

Controller:

[HttpGet]
public ActionResult Index()
{
    InventoryLogViewModel vm = new InventoryLogViewModel();

    vm.HomeView.ComputerName = Environment.MachineName;
    vm.HomeView.RACFID = Environment.UserName;

    vm.AssetTypes = (from at in db.AssetTypes
                     orderby at.Name
                     select new AssetType
                     {
                         AssetTypeId = at.AssetTypeId,
                         Name = at.Name
                     }).ToList();

    return View(vm);

ViewModel:

namespace ControlsTeam.Models
{
    public class InventoryLogViewModel : _LayoutViewModel
    {
        public IEnumerable<Department> Departments { get; set; }
        public IEnumerable<AssetType> AssetTypes { get; set; }
        public Inventory Inventory { get; set; }
        public HomeView HomeView { get; set; }
    }

    public class HomeView
    {
        public string RACFID { get; set; }
        public string ComputerName { get; set; }
    }
}

You need to learn to debug your code. If `vm.HomeView` is null (you have not initialized it anywhere) then that exception will be thrown. Suggest you include a parameterless constructor in `InventoryLogViewModel` that initializes properties `HomeVew`, Inventory` `AssetTypes and `Departments` — , May 10 '15 at 00:49

score 1 · Accepted Answer · answered May 10 '15 at 00:43

How, given this code:

InventoryLogViewModel vm = new InventoryLogViewModel();

could vm.HomeView (as used on the next line) be anything else then null ?

The InventoryLogViewModel constructor does not have an implementation, so its HomeView member after construction can only be null.

You may want to do the following:

InventoryLogViewModel vm = new InventoryLogViewModel() {
   HomeView = new HomeView() {
       RACFID = Environment.UserName,
       ComputerName = Environment.MachineName
   }
};

Oh jeez, well I feel like a bit of a idiot. It's late here in England. Thanks! — Gareth, May 10 '15 at 00:49

NullReferenceException when Assigning String to ViewModel

1 Answers1