MVC模型返回空值

Question

錯誤錯誤我對 MVC 很陌生，這是我在 stackoverflow 中的第一個問題，所以請耐心等待。 我一直在從我的客戶模型中獲取 ArgumentNullException。 完全相同的代碼似乎在 VS 2013 中有效，如我的一個培訓視頻所示，但在我的 VS 2019 中無效。有人可以建議如何解決此錯誤嗎？

以下是我的客戶控制器代碼

using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
using System.Web.Mvc;
using Vidly.Models;

namespace Vidly.Controllers
{
    public class CustomersController : Controller
    {

        private ApplicationDbContext _context;

        public CustomersController()
        {
            _context = new ApplicationDbContext();
        }

        protected override void Dispose(bool disposing)
        {
            _context.Dispose();
        }
        public ViewResult Index()
        {
            /* Eager Loading */
            var customers = _context.Customers.Include(c => c.MembershipType).ToList();   

            return View(customers);
        }

        public ActionResult Details(int id)
        {
            var customer = _context.Customers.SingleOrDefault(c => c.Id == id);

            if (customer == null)
                return HttpNotFound();

            return View(customer);
        }

        
    }
}

這是我的客戶視圖。

@model IEnumerable<Vidly.Models.Customer>


@{
    ViewBag.Title = "Customers";
    Layout = "~/Views/Shared/_Layout.cshtml";
}

<h2>Customers</h2>
@if (!Model.Any())
{
    <p>We don't have any customers yet.</p>
}
else
{
    <table class="table table-bordered table-hover">
        <thead>
            <tr>
                <th>Customer</th>
                <th>Membership Type</th>
            </tr>
        </thead>
        <tbody>
            @foreach (var customer in Model)
            {
            <tr>
                <td>@Html.ActionLink(customer.Name, "Details", "Customers", new { id = customer.Id }, null)</td>
                <td>@customer.MembershipType.Name</td>

            </tr>
            }
        </tbody>
    </table>
}

Answer 1

我檢查過，沒有發現問題。

在控制器中：

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 Vidly.Data;
using Vidly.Models;

namespace Vidly.Controllers
{
    public class CustomersController : Controller
    {
        private readonly ApplicationDbContext _context;

        public CustomersController(ApplicationDbContext context)
        {
            _context = context;
        }

        // GET: Customers
        //public async Task<IActionResult> Index()
        //{
        //    return View(await _context.Customer.ToListAsync());
        //}

        public ViewResult Index()
        {
            /* Eager Loading */
            var customers = _context.Customers.Include(c => c.MembershipType).ToList();

            return View(customers);
        }

        // GET: Customers/Details/5
        public async Task<IActionResult> Details(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var customer = await _context.Customers
                .FirstOrDefaultAsync(m => m.Id == id);
            if (customer == null)
            {
                return NotFound();
            }

            return View(customer);
        }

        // GET: Customers/Create
        public IActionResult Create()
        {
            return View();
        }

        // POST: Customers/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("Id,Name,MembershipType")] Customer customer)
        {
            if (ModelState.IsValid)
            {
                _context.Add(customer);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            return View(customer);
        }

        // GET: Customers/Edit/5
        public async Task<IActionResult> Edit(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var customer = await _context.Customers.FindAsync(id);
            if (customer == null)
            {
                return NotFound();
            }
            return View(customer);
        }

        // POST: Customers/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("Id,Name,MembershipType")] Customer customer)
        {
            if (id != customer.Id)
            {
                return NotFound();
            }

            if (ModelState.IsValid)
            {
                try
                {
                    _context.Update(customer);
                    await _context.SaveChangesAsync();
                }
                catch (DbUpdateConcurrencyException)
                {
                    if (!CustomerExists(customer.Id))
                    {
                        return NotFound();
                    }
                    else
                    {
                        throw;
                    }
                }
                return RedirectToAction(nameof(Index));
            }
            return View(customer);
        }

        // GET: Customers/Delete/5
        public async Task<IActionResult> Delete(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var customer = await _context.Customers
                .FirstOrDefaultAsync(m => m.Id == id);
            if (customer == null)
            {
                return NotFound();
            }

            return View(customer);
        }

        // POST: Customers/Delete/5
        [HttpPost, ActionName("Delete")]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> DeleteConfirmed(int id)
        {
            var customer = await _context.Customers.FindAsync(id);
            _context.Customers.Remove(customer);
            await _context.SaveChangesAsync();
            return RedirectToAction(nameof(Index));
        }

        private bool CustomerExists(int id)
        {
            return _context.Customers.Any(e => e.Id == id);
        }
    }
}

在視圖中：

@model IEnumerable<Vidly.Models.Customer>


@{
    ViewBag.Title = "Customers";
    Layout = "~/Views/Shared/_Layout.cshtml";
}

<h2>Customers</h2>
@if (!Model.Any())
{
    <p>We don't have any customers yet.</p>
}
else
{
    <table class="table table-bordered table-hover">
        <thead>
            <tr>
                <th>Customer</th>
                <th>Membership Type</th>
            </tr>
        </thead>
        <tbody>
            @foreach (var customer in Model)
            {
                <tr>
                    <td>@Html.ActionLink(customer.Name, "Details", "Customers", new { id = customer.Id }, null)</td>
                    <td>@customer.MembershipType</td>

                </tr>
            }
        </tbody>
    </table>
}

在模型中

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Threading.Tasks;

namespace Vidly.Models
{
    public class Customer
    {
        [Key]
        public int Id { get; set; }
        public string Name { get; set; }
        public MembershipType MembershipType { get; set; }
    }
}