User-1330485181 posted
Hi,
I have a problem with a DropDownList in ASP.Net Core MVC, currently I am using ASP.NET Core 3.1 When I run my program I got an error
"ArgumentNullException: Value cannot be null. (Parameter 'items')" I am creating a Create menu for Products. This menu will contain DropDownList for a field Category. Here I attach the view Create.cshtml
for ProductController
@model WebLoginCore.Models.Product
@{
ViewData["Title"] = "Create";
}
<h1>Create</h1>
<h4>Product</h4>
<hr />
<div class="row">
<div class="col-md-4">
<form asp-action="Create">
<div asp-validation-summary="ModelOnly" class="text-danger"></div>
<div class="form-group">
<label asp-for="asin" class="control-label"></label>
<input asp-for="asin" class="form-control" />
<span asp-validation-for="asin" class="text-danger"></span>
</div>
<div class="form-group">
<label asp-for="asin_image" class="control-label"></label>
<input asp-for="asin_image" class="form-control" />
<span asp-validation-for="asin_image" class="text-danger"></span>
</div>
<div class="form-group">
<label asp-for="asin_descr" class="control-label"></label>
<input asp-for="asin_descr" class="form-control" />
<span asp-validation-for="asin_descr" class="text-danger"></span>
</div>
<div class="form-group">
<label asp-for="textlink" class="control-label"></label>
<input asp-for="textlink" class="form-control" />
<span asp-validation-for="textlink" class="text-danger"></span>
</div>
<div class="form-group">
<label asp-for="CategoryId" class="control-label"></label>
<select asp-for="CategoryId" class="form-control" asp-items="@(new SelectList(ViewBag.ListofCategory, "CategoryId", "CategoryName"))" >
<option value="">-- Select Category --</option>
</select>
</div>
<div class="form-group">
<input type="submit" value="Create" class="btn btn-primary" />
</div>
</form>
</div>
</div>
<div>
<a asp-action="Index">Back to List</a>
</div>
@section Scripts {
@{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}
and here is the code for ProductController.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.EntityFrameworkCore;
using WebLoginCore.Models;
namespace WebLoginCore.Controllers
{
public class ProductsController : Controller
{
private readonly AmazonContext _context;
public ProductsController(AmazonContext context)
{
_context = context;
}
// GET: Products
public async Task<IActionResult> Index()
{
List<Category> CategoryList = new List<Category>();
CategoryList = (from d in _context.Categories
select d).ToList();
CategoryList.Insert(0, new Category { CategoryId = 0, CategoryName = "Select" });
ViewBag.ListofCategory = CategoryList;
return View(await _context.Products.ToListAsync());
}
// GET: Products/Details/5
public async Task<IActionResult> Details(int? id)
{
if (id == null)
{
return NotFound();
}
var product = await _context.Products
.FirstOrDefaultAsync(m => m.ProductId == id);
if (product == null)
{
return NotFound();
}
return View(product);
}
// GET: Products/Create
public IActionResult Create()
{
return View();
}
// POST: Products/Create
// To protect from overposting attacks, enable the specific properties you want to bind to, for
// more details, see http://go.microsoft.com/fwlink/?LinkId=317598.
[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create([Bind("ProductId,asin,asin_image,asin_descr,textlink,CategoryId")] Product product)
{
if (ModelState.IsValid)
{
_context.Add(product);
await _context.SaveChangesAsync();
return RedirectToAction(nameof(Index));
}
return View(product);
}
// GET: Products/Edit/5
public async Task<IActionResult> Edit(int? id)
{
if (id == null)
{
return NotFound();
}
var product = await _context.Products.FindAsync(id);
if (product == null)
{
return NotFound();
}
return View(product);
}
// POST: Products/Edit/5
// To protect from overposting attacks, enable the specific properties you want to bind to, for
// more details, see http://go.microsoft.com/fwlink/?LinkId=317598.
[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Edit(int id, [Bind("ProductId,asin,asin_image,asin_descr,textlink,CategoryId")] Product product)
{
if (id != product.ProductId)
{
return NotFound();
}
if (ModelState.IsValid)
{
try
{
_context.Update(product);
await _context.SaveChangesAsync();
}
catch (DbUpdateConcurrencyException)
{
if (!ProductExists(product.ProductId))
{
return NotFound();
}
else
{
throw;
}
}
return RedirectToAction(nameof(Index));
}
return View(product);
}
// GET: Products/Delete/5
public async Task<IActionResult> Delete(int? id)
{
if (id == null)
{
return NotFound();
}
var product = await _context.Products
.FirstOrDefaultAsync(m => m.ProductId == id);
if (product == null)
{
return NotFound();
}
return View(product);
}
// POST: Products/Delete/5
[HttpPost, ActionName("Delete")]
[ValidateAntiForgeryToken]
public async Task<IActionResult> DeleteConfirmed(int id)
{
var product = await _context.Products.FindAsync(id);
_context.Products.Remove(product);
await _context.SaveChangesAsync();
return RedirectToAction(nameof(Index));
}
private bool ProductExists(int id)
{
return _context.Products.Any(e => e.ProductId == id);
}
}
}
and my Category Model is like this below
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Threading.Tasks;
namespace WebLoginCore.Models
{
public class Category
{
public int CategoryId { get; set; }
[Required]
[MaxLength(50)]
[Column(TypeName = "varchar(50)")]
public String CategoryName { get; set; }
public virtual ICollection<Product> Products { get; set; }
}
}
Is it correct way to bind a DropDownList ?
Regards,
Sentoso
