MVC DropDownListFor null reference

Question

This model is used to define a view:

namespace OnlineStore.ViewModels
{
    public class SubCategoryVM
    {
        [Key]
        public int ID { get; set; }
        [Required]
        public virtual string Name { get; set; }
        [Required(ErrorMessage = "Parent Category Name is required")]
        public virtual string ParentName { get; set; }
        public IEnumerable<SelectListItem> categoryNames { get; set; }
    }
}

Inside controller:

public ActionResult createSubCategory()
{
    SubCategoryVM model = new SubCategoryVM();
    var cNames = db.Categories.ToList();
    model.categoryNames = cNames.Select(x
        => new SelectListItem
        {
            Value = x.Name,
            Text = x.Name
        });
    return View(model);
}
[HttpPost]
public ActionResult createSubCategory(int? id, SubCategoryVM model)
{
    SubCategory sc = new SubCategory();
    if (ModelState.IsValid)
    {
        sc.ParentName = model.ParentName;
        sc.Name = model.Name;
    }
    return View();
}

and View:

@model OnlineStore.ViewModels.SubCategoryVM        

<div class="form-group">
            @Html.LabelFor(model => model.ParentName, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownListFor(model => model.ParentName, Model.categoryNames, "--Please select an option--", new { @class = "form-control" })
                @Html.ValidationMessageFor(model => model.ParentName, "", new { @class = "text-danger" })
        </div>

This code is throwing a null-reference exception on line @Html.DropDownListFor(model => model.ParentName, Model.categoryNames, "--Please select an option--", new { @class = "form-control" }) saying:

Model.categoryName (Object reference not set to an instance of an object).

Please help me debug it.

Thanks in advance.

Suggestin: Your model property "categoryNames" seems to be null, make a null check before returning from the controller and return an empty list if null — Andreas Gustafsson, Feb 04 '19 at 07:34
Why is it irrelevant? There is one cause of "Object reference not set to an instance of an object": something is null. — ProgrammingLlama, Feb 04 '19 at 07:36
@AndreasGustafsson - if "categoryNames" is null, how come I'm seeing a list in the dropdownlist when the get method stretches the view? — R K, Feb 04 '19 at 07:36
@John - and that is what I'm asking. If its null, how come I'm seeing a list inside the dropdownlist to select from. — R K, Feb 04 '19 at 07:37
Does your exception appear after you submit your form, or when you load the form at the beginning? (I suspect the former) — Rafalon, Feb 04 '19 at 07:40
@AndreasGustafsson just look at his code, answer is **no**. R.K: check your Post method, your list is null there — Rafalon, Feb 04 '19 at 07:43
@RK repopulate the dropdown again in the submit method before returning. — Andreas Gustafsson, Feb 04 '19 at 07:45

TanvirArjel · Accepted Answer · 2019-02-04T08:03:16.997

Problem is when you are posting the form and returning the View with the form data in case of invalid form, categoryNames in the model is becoming null and you have to repopulate the categoryNames before returning the view with model again.

So update your createSubCategory post method as follows:

[HttpPost]
public ActionResult createSubCategory(int? id, SubCategoryVM model)
{
    SubCategory sc = new SubCategory();
    if (ModelState.IsValid)
    {
        sc.ParentName = model.ParentName;
        sc.Name = model.Name;
    }

    var cNames = db.Categories.ToList();
    model.categoryNames = cNames.Select(x
        => new SelectListItem
        {
            Value = x.Name,
            Text = x.Name
        });

    return View(model);
}

Have to admit that I've done this for trillions of times, but always forget all about it. Thanks for the help. — R K, Feb 04 '19 at 08:00

MVC DropDownListFor null reference

1 Answers1