Question

关于下拉列表，我在基本问题上遇到了很多问题，我做了很多在stackoverflow中描述的方法，而所有方法都没有用，我将总结这样的代码。我只想简单地创建一个保存到数据库的下拉列表。

这就是我所做的 DefaultModel

public class DefaultModel
  {
    public int Id {get; set;}
    //other things
    public CountryList SelectedCountry {get; set;}
  }
  public DefaultDbContext : DbContext
   {
      public DbSet<DefaultModel> DefaultModels {get; set;}
      public DbSet<CountrtyList> CountryLists {get; set;}
   }

国家列表模型

public class CountryList
  {
    public byte Id {get; set;}
    public string name {get; set;} //country name
    public DefaultModel DefaultModel {get; set;}
  }

ViewModel

 public class CountryListViewModel
 {
   public byte Id {get; set;}
   public List<CountryList> CountryList {get; set;}

DefaultController 不是主控制器

 private DefaultDbContext = db;
 public HomeController
 {
   db = new DefaultDbContext;
 }

 public ActionResult Index()
  {
        var countryLists= _context.CountryLists.ToList();
        var viewModel = new CountryListViewModel
        {
            Default = new DefaultModel(),
            CountryLists = countrylists
        };

        return View(viewModel);
  }

查看

 @model Default.ViewModels.CountryListViewModel
 <div class="form-group">
 @Html.LabelFor(model => 
  model.TransactionModel.SelectedCountry.CountryLists
   , new { @class = "control-label col-md-2" })
    <div class="col-md-10">
   @Html.DropDownListFor(
   model =>    model.TransactionModel.SelectedCountry.CountryLists
   , new     SelectList(Model.CountryList, "Id, Name"), "Select Country")
    </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>

迁移添加国家/地区

 Sql("INSERT INTO COUNTRYLISTS (Id, Name) VALUES(1, UK)");

错误

System.NullReferenceException： “对象引用未设置为对象的实例。”

我只是想从头开始学习如何在MVC5中创建一个下拉列表，我浪费了时间，并且多次尝试都失败了。我看到使用无法使用的方法的教程，我不知道哪里有问题或应该怎么做。

Answer 1

模式

textFieldDidEndEditing

控制器

`

namespace WebApp.Models
{
    public class CountryList
    {
        public byte Id { get; set; }
        public string name { get; set; } //country name
        //public DefaultModel DefaultModel { get; set; }
    }
    public class CountryListViewModel
    {
        public string CountryLists { get; set; }
        public List<CountryList> CountryList { get; set; }
    }
}

查看

        List<CountryList> countrylists = new List<CountryList>();

        //I am using static list items. Just change this to get data from entity framework;

        countrylists.Add(new CountryList  //pass one default list Item here so that your list is never null.
        {
            Id = 0,
            name = "Select Country"
        });

        countrylists.Add(new CountryList
        {
            Id = 1,
            name = "UK"
        });
        var viewModel = new CountryListViewModel
        {
            //Default = new DefaultModel(),
            CountryList=countrylists,
            CountryLists="Country List"
        };

        return View(viewModel);`

DropdownList ASP.net MVC5的问题

1 个答案: