ASP.NET MVC DropDownList NullReferenceException

Question

我的dropDownList給我一個NullReferenceException的問題，我不知道爲什麼。

我想發佈一篇文章，您可以選擇DropDownList，它屬於哪個類別。

下面是我的一些代碼：

PostModel

public class Post 
    { 
     [Key] 
     public int PostID { get; set; } 

     [Display(Name = "Idea Title")] 
     public String Title { get; set; } 

     [Display(Name = "Description")] 
     public String DescText { get; set; } 

     [Display(Name = "Votes")] 
     public int Votes { get; set; } 

     [Display(Name = "Comments")] 
     public int CommentCount { get; set; } 

     [Display(Name = "Posted:")] 
     [DataType(DataType.DateTime)] 
     [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:dd/MM/yyyy}")] 
     public DateTime CreatedDate { get; set; } 

     [Display(Name = "Last Edited:")] 
     [DataType(DataType.DateTime)] 
     [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:dd/MM/yyyy}")] 
     public DateTime? EditedDate { get; set; } 

     public String UserName { get; set; } 

     public virtual Status Status { get; set; } 
     public virtual Category Category { get; set; } 
     public virtual Department Department { get; set; } 
     public virtual ICollection<Comment> Comments { get; set; } 
    } 

PostViewModel

public class PostCreateViewModel 
    { 
     [Required] 
     [Display(Name = "Title")] 
     [MinLength(10, ErrorMessage = "The title must be at least 10 characters long.")] 
     [StringLength(100, ErrorMessage = "The title cannot be longer than 100 characters.")] 
     public String Title { get; set; } 

     [Required] 
     [DataType(DataType.MultilineText)] 
     [MinLength(50, ErrorMessage = "The description of your idea should be at least 50 characters long.")] 
     public String DescText { get; set; } 

     [Required] 
     [Display(Name = "Category")] 
     public int SelectedCategoryId { get; set; } 
     public IEnumerable<SelectListItem> Categories { get; set; } 
    } 

CategoryModel

public class Category 
    { 
     [Key] 
     public int CategoryID { get; set; } 

     [Display(Name = "Category Name")] 
     public String CategoryName { get; set; } 
    } 

    public class DbCategories 
    { 
     private IdeaContext context = new IdeaContext(); 

     public List<Category> GetCategories() 
     { 
      return context.Categories.ToList(); 
     } 
    } 

PostController中修訂

 private IEnumerable<SelectListItem> GetCategories() 
     { 
      var dbCategories = new DbCategories(); 
      var categories = dbCategories 
          .GetCategories() 
          .Select(x => 
            new SelectListItem 
            { 
             Value = x.CategoryID.ToString(), 
             Text = x.CategoryName 
            }); 
      return new SelectList(categories, "Value", "Text"); 
     } 

     [HttpGet] 
     public ActionResult Create() 
     { 
      var model = new PostCreateViewModel { Categories = GetCategories() }; 
      return View(model); 
     } 

     [HttpPost] 
     public ActionResult Create(PostCreateViewModel model) 
     { 
      model.Categories = GetCategories(); 
      if (ModelState.IsValid) 
      { 
       var post = new Post(); 

       // From ViewModel 
       post.Title = model.Title; 
       post.DescText = model.DescText; 

       var dbCategories = new DbCategories(); 
       var categories = dbCategories.GetCategories(); 

       post.Category = categories.FirstOrDefault(x => x.CategoryID == model.SelectedCategoryId); 

       // Controller created 
       post.CreatedDate = DateTime.Now; 
       post.Votes = 0; 
       post.CommentCount = 0; 
       post.Status.StatusName = "Active"; 
       post.Department = null; 
       post.UserName = User.Identity.Name; 

       context.Posts.Add(post); 
       context.SaveChanges(); 

       return RedirectToAction("Index"); 
      } 
      else 
      { 
       return View(); 
      } 
     } 

而我查看

@model WebApplication3.Models.ViewModels.PostCreateViewModel 

@{ 
    ViewBag.Title = "Create"; 
} 

<h2>Create</h2> 

@using (Html.BeginForm()) 
{ 
    @Html.AntiForgeryToken() 

    <div class="form-group top-space-large"> 
     @Html.LabelFor(model => model.Title, htmlAttributes: new { @class = "control-label col-md-2" }) 
     <div class="col-md-10"> 
      @Html.EditorFor(model => model.Title, new { htmlAttributes = new { @class = "form-control" } }) 
      @Html.ValidationMessageFor(model => model.Title, "", new { @class = "text-danger" }) 
     </div> 
    </div> 

    <div class="form-group"> 
     @Html.LabelFor(m => m.SelectedCategoryId, htmlAttributes: new { @class = "control-label col-md-2" }) 
     <div class="col-md-10"> 
      @Html.DropDownListFor(m => m.SelectedCategoryId, Model.Categories, new { htmlAttributes = new { @class = "form-control" } }) 
      @Html.ValidationMessageFor(model => model.SelectedCategoryId, "", new { @class = "text-danger" }) 
     </div> 
    </div> 

    <div class="form-group top-space-large"> 
     @Html.LabelFor(model => model.DescText, htmlAttributes: new { @class = "control-label col-md-2" }) 
     <div class="col-md-10"> 
      @Html.EditorFor(model => model.DescText, new { htmlAttributes = new { @class = "form-control" } }) 
      @Html.ValidationMessageFor(model => model.DescText, "", new { @class = "text-danger" }) 
     </div> 
    </div> 

    <div class="form-group top-space-large"> 
     <button type="submit" class="btn btn-default">Submit</button> 
    </div> 
} 

堆棧異常

System.NullReferenceException was unhandled by user code 
    HResult=-2147467261 
    Message=Object reference not set to an instance of an object. 
    Source=App_Web_jqgoppva 
    StackTrace: 
     at ASP._Page_Views_Post_Create_cshtml.Execute() in \\etj\My Documents\Visual Studio 2013\Projects\WebApplication3\WebApplication3\Views\Post\Create.cshtml:line 24 
     at System.Web.WebPages.WebPageBase.ExecutePageHierarchy() 
     at System.Web.Mvc.WebViewPage.ExecutePageHierarchy() 
     at System.Web.WebPages.StartPage.RunPage() 
     at System.Web.WebPages.StartPage.ExecutePageHierarchy() 
     at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) 
     at System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) 
     at System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) 
     at System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) 
     at System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) 
     at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) 
     at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) 
    InnerException: 

這是怎麼發生的？當我嘗試發佈表單時，我在DropDownList的行的視圖中得到「對象引用未設置爲對象的實例」。 context.Categories不是空的。它包含ID爲28的「Web」和ID爲31的「Environment」。

Answer 1

我看到這種情況發生的唯一原因是因爲在POST動作中你想使用Model.Categories這個不是持久的。您必須再次調用GetCatgories()方法的行動Create。

你得到NullReference異常，因爲post.Category是null，你不能訪問null對象，post.Category.CategoryID = model.SelectedCategoryId;的屬性。您必須根據SelectedCategoryId從數據庫中查詢Category。

[HttpPost] 
public ActionResult Create(PostCreateViewModel model) 
{ 
    model.Categories = GetCategories(); // call again, Categories don't persist in a POST. 
    if (ModelState.IsValid) 
    { 
     var post = new Post(); 

     // ... 
     post.Category = context.Categories.FirstOrDefault(x => x.CategoryId == model.SelectedCategoryId); 
     // ... 

     return RedirectToAction("Index"); 
    } 

    return View(model); 
}