dropdownlistfor yields error ,{"Object reference not set to an instance of an object."} , when I click submit button

Question

I am trying to create sub category for categories. User first select the categories from dropdownlist and then type the subcategory name and clicks submit. Even though dropdownlist elements are properly fill the dropdown list. When I click submit button It creates error. How can I solve this? My View:

@model CETAPPSUGG.Models.CategorySubCategoryModel

@{
    ViewBag.Title = "Create";
}

<h2>Create</h2>

@using (Html.BeginForm())
{
    @Html.HiddenFor(model => model.selectedId, new { id = "3" });


   // @Html.AntiForgeryToken()

    <div class="form-horizontal">
        <h4>SubCatagories</h4>

        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        <div class="form-group">
            @Html.LabelFor(model => model.SubCategory.SubCategoryName, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.SubCategory.SubCategoryName, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.SubCategory.SubCategoryName, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
Upper cat:            <div class="col-md-10">
                          @Html.DropDownListFor(Model => Model.Categories, Model.categoryList)


            </div>
        </div>

        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Create" class="btn btn-default" />
            </div>
        </div>
    </div>
}

<div>
    @Html.ActionLink("Back to List", "Index")
</div>

My Controller:

public ActionResult Create()
    {
        var categories = db.Categories.ToList();
        CategorySubCategoryModel deneme = new CategorySubCategoryModel();
        var list = new List<SelectListItem>();
        deneme.Categories = categories;
        foreach (Categories c in categories)
        {
            list.Add(new SelectListItem() { Text = c.CategoryName, Value = c.Id.ToString() });
        }

        deneme.categoryList = list;

        return View(deneme);
    }


   // POST: SubCatagories/Create
    // To protect from overposting attacks, please enable the specific properties you want to bind to, for 
    // more details see http://go.microsoft.com/fwlink/?LinkId=317598.
    [HttpPost]
   // [ValidateAntiForgeryToken]
public ActionResult Create( CategorySubCategoryModel model)
{
    string strDDLValue = model.selectedId;
    SubCatagories newSubCategory = new SubCatagories();
    Categories cat = new Categories();
    cat = db.Categories.Find(Convert.ToInt32(strDDLValue));
    //    cat =  db.Categories.Find(Convert.ToInt32(strDDLValue));

    newSubCategory.SubCategoryName = model.SubCategory.SubCategoryName;
    newSubCategory.UpperCategory = Convert.ToInt32(strDDLValue);
    newSubCategory.Categories = cat;

    db.SubCatagories.Add(newSubCategory);
    db.SaveChanges();

    return View();
}

My Model

namespace CETAPPSUGG.Models
{
    public class CategorySubCategoryModel
    {
        SubCatagories SubCatagories { get; set; }

        public IEnumerable<Categories> Categories { get; set; }
        public IEnumerable<SubCatagories> SubCategories { get; set; }

        public IEnumerable<SelectListItem> categoryList { get; set; }

        public SubCatagories SubCategory { get; set; }

        public string selectedId;

    }
}

It creates error in view

Answer 1

You have a bunch of problems here.

Your primary problem is that you are not passing a model back to the View on post, thus the model is null. So, when you attempt to dereference items from the model in the View, a null reference is generated.

First, you are using selectedId but do not set this anywhere. It doesn't get set by magic. What you probably want is @Html.DropDownListFor(model => model.selectedId, Model.categoryList) (note the lowercase m in model in the first parameter, and uppercase M in the second)

Second, don't use a Model in your lambda in the DropDownListFor, use the lowercase model, because uppercase Model is reserved for the actual Model instance. If you want to reference the Model instance, then do something like DropDownListFor(_ => Model.Foo, Model.Foos). Note that I replaced the Model before the lambda with an underscore or some other value that is not Model. Frankly i'm surprised this even works, but there's probably a scoping rule here that overrides the outer Model. Avoid this because it can cause you confusion down the road.

Third, you are passing an IEnumerable to the DropDownListFor as the selected item variable, this won't work on a number of levels. This needs to be a single string value in most cases (sometimes a numerical one, but always a single more basic type that can have ToString() called on it and get a sensible string since DropDownListFor can't display complex objects).

Fourth, You also need to re-populate your DropDownListFor in the Post action, because the contents of a dropdownlist are not posted back, and thus will be null in the model. This, along with the SubCategory derefences in your view are ultimately what is generating the Null Reference exception.

You also need to pass the model back to your view in the Post, but as stated above, it needs to be re-initialized with the Categories as well as SubCategories.

There are probably more problems here, but fix these and you should be on your way.