MVC5 DropdownListFor上的ArgumentNullException

Question

我刚刚开始学习Asp.net MVC5，我在下一行获得ArgumentNullException。

@Html.DropDownListFor(m => m.Customer.MembershipTypeId, new SelectList(Model.MembershipTypes, "Id", "Name"), "", new { @class = "form-control" })

我正在关注一个教程并且我完全相同，所以我不确定为什么它会抛出这个错误。 我查看了模型和表格数据，并填充了它们。 如果需要任何其他信息来解决这个问题，请告诉我。

New.cshtml

@model Vidly6.VIewModel.NewCustomerViewModel
@{
    ViewBag.Title = "New";
    Layout = "~/Views/Shared/_Layout.cshtml";
}

<h2>New Customer</h2>

@using (Html.BeginForm("Create", "Customers"))

{
    <div class="form-group">
        @Html.LabelFor(m => m.Customer.Name)
       @Html.TextBoxFor(m => m.Customer.Name, new {@class = "form-control"}) 

    </div>   


    <div class="form-group">
        @Html.LabelFor(m => m.Customer.BirthDate)
        @Html.TextBoxFor(m => m.Customer.BirthDate, new {@class = "form-control"}) 

    </div>


    <div class="checkbox">
        <label>
            @Html.CheckBoxFor(m=>m.Customer.IsSubscribedToNewsletter) Subscribed to Newsletter?
        </label>
    </div>

<div class="form-group">
        @Html.LabelFor(m => m.Customer.MembershipTypeId)
        @Html.DropDownListFor(m => m.Customer.MembershipTypeId, new SelectList(Model.MembershipTypes, "Id", "Name"), "", new { @class = "form-control" })
        @Html.ValidationMessageFor(m => m.Customer.MembershipTypeId)
    </div>

}

NewCustomerViewModel

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Vidly6.Models;

namespace Vidly6.VIewModel
{
    public class NewCustomerViewModel
    {
        public IEnumerable<MembershipTypes> MembershipTypes { get; set; }
        public Customers Customer { get; set; }
    }
}

CustomersController

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Vidly6.Models;
using Vidly6.VIewModel;


namespace Vidly6.Controllers
{
    public class CustomersController : Controller
    {


        private ApplicationDbContext _context;
        public CustomersController()
        {

       _context = new ApplicationDbContext();
        // GET: Customers
    }

        protected override void Dispose(bool disposing)
        {
            _context.Dispose();
        }


        public ActionResult New()
        {
            var membershipTypes = _context.MembershipTypes.ToList();
            var viewModel = new NewCustomerViewModel();
            return View(viewModel);
        }
        public ViewResult CustomerGoesHere()
        {

            var customers = _context.Customers.Include(c => c.MembershipTypes).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);

            }





    }
}

MembershipTypes表

Answer 1

public ActionResult New()
{
  var membershipTypes = _context.MembershipTypes.ToList();
  var viewModel = new NewCustomerViewModel();
  return View(viewModel);
}

您实际上并未使用membershipTypes。您需要在viewModel上设置属性，否则它将为null。