我在我的dropDownList遇到问题，给了我NullReferenceException，我不知道为什么。
我想发布一个帖子，您可以在其中选择DropDownList所属的类别。
这是我的一些代码：
后模型
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; }
}
分类模型
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更新
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();
}
}
我的看法
@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>
}
堆栈异常
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:
为什么会这样呢？当我尝试发布表单时，在DropDownList行的“视图”中得到“对象引用未设置为对象的实例”。 context.Categories不为空。它包含ID为28的“ Web”和ID为31的“ Environment”。
最佳答案
我看到这种情况发生的唯一原因是因为在POST操作上您想使用Model.Categories而不持久化。您必须在操作GetCatgories()中再次调用Create方法。
因为NullReference是post.Category，并且无法访问null对象null的属性，所以会出现post.Category.CategoryID = model.SelectedCategoryId;异常。您必须基于Category从数据库中查询SelectedCategoryId。
[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);
}
关于c# - ASP.NET MVC DropDownList NullReferenceException，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/30892389/