ASP.NET MVC DropDownFor驗證（值不能爲空參數名稱：源）

Question

我仍在努力學習ASP.NET MVC。我所有的表單條目都是必需的，所以我想對它們進行驗證。爲簡潔起見，我已將模型配對到Description（文本框）和Paradigm（下拉框）。我包括Entry.cs，Paradigm.cs和EntryViewModel.cs模型類和Display.cshtml視圖。

[Bind(Exclude = "EntryId")] 
public class Entry 
{ 
    [ScaffoldColumn(false)] 
    public int EntryId { get; set; } 

    [Required(ErrorMessage = "You must include a description.")] 
    public string Description { get; set; } 

    [Display(Name = "Type")] 
    [Required(ErrorMessage = "You must select a type.")] 
    public int ParadigmId { get; set; } 

    public virtual Paradigm Paradigm { get; set; } 
} 

public class Paradigm 
{ 
    [ScaffoldColumn(false)] 
    public int ParadigmId { get; set; } 

    [Required] 
    public string Name { get; set; } 

    public List<Entry> Entries { get; set; } 
} 

public class EntryViewModel 
{ 
    public Entry Entry { get; set; } 
    public IEnumerable<Entry> Entries { get; set; } 
} 

@model Pylon.Models.EntryViewModel 

@{ 
    ViewBag.Title = "Display"; 
} 

<hr /> 

@using (Html.BeginForm()) 
{ 
    @Html.ValidationSummary(true) 
    <fieldset> 
     <legend>Entry</legend> 

     <div class="editor-label"> 
      @Html.LabelFor(model => model.Entry.Description) 
     </div> 
     <div class="editor-field"> 
      @Html.TextAreaFor(model => model.Entry.Description) 
      @Html.ValidationMessageFor(model => model.Entry.Description) 
     </div> 

     <div class="editor-label"> 
      @Html.LabelFor(model => model.Entry.ParadigmId) 
     </div> 
     <div class="editor-field"> 
      @Html.DropDownListFor(model => model.Entry.ParadigmId, ((IEnumerable<Pylon.Models.Paradigm>)ViewBag.PossibleParadigms).Select(option => new SelectListItem { 
       Text = (option == null ? "None" : option.Name), 
       Value = option.ParadigmId.ToString(), 
       Selected = (Model != null) && (option.ParadigmId == Model.Entry.ParadigmId) 
      })) 
      <a href="@Url.Action("Paradigm")"><img src="../../Content/Images/add_icon.gif" /></a> 
      @Html.ValidationMessageFor(model => model.Entry.ParadigmId) 
     </div> 

     <p> 
      <input type="submit" value="Create" /> 
     </p> 
    </fieldset> 
} 

如果我在未輸入描述的情況下提交表單，我希望進行驗證，然後說「您必須包含說明」。但是，我在DropDownFor行上收到一個ArgumentNullException。 http://www.wvha.org/temp/ArgumentNullException.png

我該怎麼做？作爲一個涵蓋ASP.NET MVC 3/Razor的合適書籍。我可以沿着基本的方向走，但是當我需要偏離更多先進的特徵時，我會誤入歧途。

public class EntriesController : Controller 
    { 
     private readonly PylonContext _context = new PylonContext(); 

     public ActionResult Display() 
     { 
      // DropDown 
      ViewBag.PossibleParadigms = _context.Paradigms; 

      var viewModel = new EntryViewModel {Entries = _context.Entries.ToList()}; 
      return View(viewModel); 
     } 

     [HttpPost] 
     public ActionResult Display(EntryViewModel viewModel) 
     { 
      if (ModelState.IsValid) 
      { 
       _context.Entries.Add(viewModel.Entry); 
       _context.SaveChanges(); 
       return RedirectToAction("Display"); 
      } 

      return View(viewModel); 
     } 
} 

Answer 1

這是相當困難的，沒有看到你的控制器代碼的說法，但看起來像你的ViewBag.PossibleParadigms可能爲空。

您的插入/更新控制器操作看起來像這樣嗎？

if (ModelState.IsValid) { 
    ///... 
} else { 
    return View(model); 
} 

如果是這樣，你需要把PossibleParadigms回ViewBag（這麼說）你返回到視圖之前。

如果您可以發佈相關的控制器操作代碼，那麼肯定會更容易知道。