how create dropdown in mvc 4 - asp.net-mvc-4

Question

i have these models:
public class Product
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
[Required]
[Display(Name = "Ürün Adı")]
public string Name { get; set; }
[Required]
[Display(Name = "Açıklama")]
public string Description { get; set; }
[Required]
[Display(Name = "Fiyat")]
public decimal Price { get; set; }
[Required]
[Display(Name = "İndirim")]
public decimal Discount { get; set; }
[Required]
[Display(Name = "İndirim Geçerli Mi?")]
public bool IsDiscountActive { get; set; }
[Required]
[Display(Name="Ürün Var Mı?")]
public bool IsAvailable { get; set; }
[Required]
[Display(Name="Kategori")]
public Category Category { get; set; }
[Display(Name="Resim")]
public Image Image { get; set; }
[Required]
public DateTime CreatedOn { get; set; }
public DateTime ChangedOn { get; set; }
[Required]
public UserProfile CreatedBy { get; set; }
public UserProfile ChangedBy { get; set; }
public IEnumerable<SelectListItem> Categories
{
get
{
return new OnlineShoppingContext().Categories.OrderBy(c=>c.Name).ToList()
.Select(e => new SelectListItem { Text = e.Name, Value = e.Id.ToString() });
}
}
}
[Table("Category")]
public class Category
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
[Required]
[Display(Name = "Kategori Adı")]
public string Name { get; set; }
[Display(Name = "Açıklama")]
public string Description { get; set; }
[Display(Name = "Resim")]
public Image Image { get; set; }
[DatabaseGenerated(DatabaseGeneratedOption.Computed)]
public DateTime CreatedOn { get; set; }
public DateTime ChangedOn { get; set; }
public UserProfile CreatedBy { get; set; }
public UserProfile ChangedBy { get; set; }
public List<Product> Products { get; set; }
}
My category controller
[Authorize(Roles = "Admin")]
public class CategoryController : Controller
{
private OnlineShoppingContext db = new OnlineShoppingContext();
//
// GET: /Category/
public ActionResult Index()
{
return View(db.Categories.ToList());
}
//
// GET: /Category/Details/5
public ActionResult Details(int id = 0)
{
Category category = db.Categories.Find(id);
if (category == null)
{
return HttpNotFound();
}
return View(category);
}
//
// GET: /Category/Create
public ActionResult Create()
{
return View();
}
//
// POST: /Category/Create
[HttpPost]
public ActionResult Create(Category category, HttpPostedFileBase file)
{
if (ModelState.IsValid)
{
if (file != null)
{
string pic = Guid.NewGuid().ToString();
string path = Path.Combine(Server.MapPath("~/Content/images/"), pic + Path.GetExtension(file.FileName));
file.SaveAs(path);
Image i = new Image();
i.Path = path;
i.CreatedBy = db.UserProfiles.Where(u => u.UserId == WebSecurity.CurrentUserId).FirstOrDefault();
category.Image = i;
}
category.CreatedBy = db.UserProfiles.Where(u => u.UserId == WebSecurity.CurrentUserId).FirstOrDefault();
db.Categories.Add(category);
db.SaveChanges();
return RedirectToAction("Index");
}
return View(category);
}
//
// GET: /Category/Edit/5
public ActionResult Edit(int id = 0)
{
Category category = db.Categories.Find(id);
if (category == null)
{
return HttpNotFound();
}
return View(category);
}
//
// POST: /Category/Edit/5
[HttpPost]
public ActionResult Edit(Category category)
{
if (ModelState.IsValid)
{
db.Entry(category).State = EntityState.Modified;
db.SaveChanges();
return RedirectToAction("Index");
}
return View(category);
}
//
// GET: /Category/Delete/5
public ActionResult Delete(int id = 0)
{
Category category = db.Categories.Find(id);
if (category == null)
{
return HttpNotFound();
}
return View(category);
}
//
// POST: /Category/Delete/5
[HttpPost, ActionName("Delete")]
public ActionResult DeleteConfirmed(int id)
{
Category category = db.Categories.Find(id);
db.Categories.Remove(category);
db.SaveChanges();
return RedirectToAction("Index");
}
protected override void Dispose(bool disposing)
{
db.Dispose();
base.Dispose(disposing);
}
}
create.cshtml
#model OnlineShopping.Models.Product
#{
var db = new OnlineShopping.Models.OnlineShoppingContext();
ViewBag.Title = "Create";
}
<h2>Create</h2>
#using (Html.BeginForm("Create", "Product", FormMethod.Post, new { enctype = "multipart/form-data" }))
{
#Html.ValidationSummary(true)
<fieldset>
<legend>Product</legend>
<div class="editor-label">
#Html.LabelFor(model => model.Category)
</div>
<div class="editor-field">
// Error Model is null
#Html.DropDownListFor(model => model.Category, new SelectList(Model.Categories, "Value", "Text"))
#Html.ValidationMessageFor(model => model.Category)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.Name)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.Name)
#Html.ValidationMessageFor(model => model.Name)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.Price)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.Price)
#Html.ValidationMessageFor(model => model.Price)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.Discount)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.Discount)
#Html.ValidationMessageFor(model => model.Discount)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.IsDiscountActive)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.IsDiscountActive)
#Html.ValidationMessageFor(model => model.IsDiscountActive)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.IsAvailable)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.IsAvailable)
#Html.ValidationMessageFor(model => model.IsAvailable)
</div>
<div class="editor-field">
<input type="file" name="file" id="file" />
</div>
<p>
<input type="submit" value="#Translations.Common.Create" />
</p>
</fieldset>
}
<div>
#Html.ActionLink(Translations.Common.BackToList, "Index")
</div>
#section Scripts {
#Scripts.Render("~/bundles/jqueryval")
}
On create.cshtml page i got this error
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
Line 19:
Line 20:
Line 21: #Html.DropDownListFor(model => model.Category, new SelectList(Model.Categories, "Value", "Text"))
Line 22: #Html.ValidationMessageFor(model => model.Category)
Line 23:

Answer 1

In GET your Model is null, so Model.Categories crashes. Try the following:
//
// GET: /Category/Create
public ActionResult Create()
{
var model = new OnlineShopping.Models.Product();
return View(model);
}

Answer 2

Your create view expected you to pass in a model, then in your view, it will look for model's value to generate dropdown list for you. As you haven't pass a model in so you get that object null reference exception.
Also, when you pass a model in, make sure it contains all the data you need to populate the dropdown list.