'对象引用未设置为对象的实例' - 这是什么意思？

Question

我正在创建一个MVC 3应用程序,其中包含有关"关联"的数据,但我还需要能够通过应用程序删除这些"关联".我正在使用Delete ActionResult,但每次我尝试删除一个条目时,我都会收到以下服务器错误:

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 


Line 14:         <div class="form-grid-1">
Line 15:             <div class="display-label">@Html.LabelFor(model => model.FirstName)</div>
Line 16:             <div class="display-field">@Model.FirstName</div>
Line 17:         </div>
Line 18: 

Source File: c:\Users\Jackie\Documents\Visual Studio 2010\Projects\Associate Tracker versions\Associate Tracker v5\LoginFormExample\Views\Associate\Delete.cshtml    Line: 16 

Stack Trace: 


[NullReferenceException: Object reference not set to an instance of an object.]
   ASP._Page_Views_Associate_Delete_cshtml.Execute() in c:\Users\Jaskharan Shoker\Documents\Visual Studio 2010\Projects\Associate Tracker versions\Associate Tracker v5\LoginFormExample\Views\Associate\Delete.cshtml:16
   System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +207
   System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +81
   System.Web.WebPages.StartPage.RunPage() +19
   System.Web.WebPages.StartPage.ExecutePageHierarchy() +65
   System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +76
   System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +220
   System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +115
   System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +303
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +13
   System.Web.Mvc.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() +23
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +260
   System.Web.Mvc.<>c__DisplayClass1e.<InvokeActionResultWithFilters>b__1b() +19
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +177
   System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +343
   System.Web.Mvc.Controller.ExecuteCore() +116
   System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +97
   System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +10
   System.Web.Mvc.<>c__DisplayClassb.<BeginProcessRequest>b__5() +37
   System.Web.Mvc.Async.<>c__DisplayClass1.<MakeVoidDelegate>b__0() +21
   System.Web.Mvc.Async.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _) +12
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
   System.Web.Mvc.<>c__DisplayClasse.<EndProcessRequest>b__d() +50
   System.Web.Mvc.SecurityUtil.<GetCallInAppTrustThunk>b__0(Action f) +7
   System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action) +22
   System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +60
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8969117
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184

控制器:

//
// GET: /Associate/Delete/AssociateId
public ActionResult Delete(int id)
{
    using (var db = new AssociateDBEntities())
    {
        return View(db.Associates.Find(id));
        }
    }

//
// POST: /Associate/Delete/AssociateId
[HttpPost]
public ActionResult Delete(int id, Associate associate)
{
        try
        {
            using (var db = new AssociateDBEntities())
            {
                db.Entry(associate).State = System.Data.EntityState.Deleted;
                db.SaveChanges();
            }
            return RedirectToAction("ViewAll");
        }
        catch
        {
            return View();
        }

'删除'视图:

@model LoginFormExample.Models.Associate

@{
    ViewBag.Title = "Delete";
}

<h2>Delete</h2>
<h3>Are you sure you want to delete this associate?</h3>
<fieldset>
    <legend>Associate</legend>

        @Html.HiddenFor(model => model.AssociateId)

        <div class="form-grid-1">
            <div class="display-label">@Html.LabelFor(model => model.FirstName)</div>
            <div class="display-field">@Model.FirstName</div>
        </div>

        <div class="form-grid-2">
            <div class="display-label">@Html.LabelFor(model => model.LastName)</div>
            <div class="display-field">@Model.LastName</div>
        </div>

        <div class="form-grid-1">
             <div class="display-label">@Html.LabelFor(model => model.Email)</div>
            <div class="display-field">@Model.Email</div>
        </div>

        <div class="form-grid-2">
            <div class="display-label">@Html.LabelFor(model => model.AddressLine1)     </div>
            <div class="display-field">@Model.AddressLine1</div>
        </div>

        <div class="form-grid-1">
            <div class="display-label">@Html.LabelFor(model => model.AddressLine2)</div>
            <div class="display-field">@Model.AddressLine2</div>
        </div>

        <div class="form-grid-2">
            <div class="display-label">@Html.LabelFor(model => model.Postcode)</div>
            <div class="display-field">@Model.Postcode</div>
        </div>

        <div class="form-grid-1">
            <div class="display-label">@Html.LabelFor(model => model.Phone)</div>
            <div class="display-field">@Model.Phone</div>
        </div>

        <div class="form-grid-2">
            <div class="display-label">@Html.LabelFor(model => model.Mobile)</div>
            <div class="display-field">@Model.Mobile</div>
        </div>
        <br />
</fieldset>

@using (Html.BeginForm()) {
    <p>
        <input type="submit" value="Delete" /> |
        @Html.ActionLink("Back to view all", "ViewAll")
    </p>
}

'助理'课程

namespace LoginFormExample.Models
{
    public partial class Associate
    {
        public int AssociateId { get; set; }

        [Required]
        [Display(Name = "First name:")]
        public string FirstName { get; set; }

        [Required]
        [Display(Name = "Last name:")]
        public string LastName { get; set; }

        [Required(ErrorMessage = "Please enter a valid email address")]
        [DataType(DataType.EmailAddress)]
        [Display(Name = "Email address:")]
        public string Email { get; set; }

        [Display(Name = "Address line 1:")]
        public string AddressLine1 { get; set; }

        [Display(Name = "Address line 2:")]
        public string AddressLine2 { get; set; }
        public int RegionId { get; set; }
        public int CityTownId { get; set; }

        [Display(Name = "Postcode:")]
        public string Postcode { get; set; }

        [Display(Name = "Phone number:")]
        public string Phone { get; set; }

        [Display(Name = "Mobile number:")]
        public string Mobile { get; set; }

有没有人对我为什么一直收到这个错误有任何想法？我已经检查了示例应用程序,我似乎无法发现任何差异.

Answer 1

这意味着在线

return View(db.Associates.Find(id));

要么db或者Associates是null.

这很容易使用Debug.Assert或断点检查.在违规行上设置断点并再次运行程序,或者在违规行之前插入以下代码:

Debug.Assert(db != null);
Debug.Assert(db.Associates != null);