Asp.net 对象引用未设置为对象的实例（下拉列表）

我刚开始学习，这里有一些问题困扰了我一整天。我无法解决以下错误：

[NullReferenceException: Object reference not set to an instance of an object.]

我有以下代码：

<div class="editor-field">
@Html.DropDownListFor(model => model.DogColors.ColorId, Model.Color, new { @id = "color_id" })
@Html.ValidationMessageFor(model => model.DogColors.ColorId)
</div>

我在数据库中添加了两种颜色，这些颜色显示在下拉列表中。当我单击按钮提交时，此行出现问题：

@Html.DropDownListFor(model => model.DogColors.ColorId, Model.Colors, new { @id = "color_id" })

这个异常经常在那个地方抛出。你知道有什么问题吗

堆栈跟踪：

[NullReferenceException: Object reference not set to an instance of an object.]
ASP._Page_Views_Projects__CreateOrEdit_cshtml.Execute() in c:\Users\Admin\Documents\Visual Studio 2012\Projects\Pro\Pro.Web\Views\Dogs\_CreateOrEdit.cshtml:65
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +197
System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +97
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +88
System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +260
System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +115
System.Web.Mvc.HtmlHelper.RenderPartialInternal(String partialViewName, ViewDataDictionary viewData, Object model, TextWriter writer, ViewEngineCollection viewEngineCollection) +276
System.Web.Mvc.Html.PartialExtensions.Partial(HtmlHelper htmlHelper, String partialViewName, Object model, ViewDataDictionary viewData) +108
System.Web.Mvc.Html.PartialExtensions.Partial(HtmlHelper htmlHelper, String partialViewName, Object model) +32
ASP._Page_Views_Projects_Create_cshtml.Execute() in c:\Users\Admin\Documents\Visual Studio 2012\Projects\Pro\Pro.Web\Views\Dogs\Create.cshtml:14
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +197
System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +97
System.Web.WebPages.StartPage.RunPage() +17
System.Web.WebPages.StartPage.ExecutePageHierarchy() +62
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +76
System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +260
System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +115
System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +295
System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +13
System.Web.Mvc.<>c__DisplayClass1a.<InvokeActionResultWithFilters>b__17() +23
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +242
System.Web.Mvc.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() +21
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +177
System.Web.Mvc.Async.<>c__DisplayClass2a.<BeginInvokeAction>b__20() +89
System.Web.Mvc.Async.<>c__DisplayClass25.<BeginInvokeAction>b__22(IAsyncResult asyncResult) +102
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +57
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +43
System.Web.Mvc.<>c__DisplayClass1d.<BeginExecuteCore>b__18(IAsyncResult asyncResult) +14
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +23
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +57
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +23
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +47
System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +10
System.Web.Mvc.<>c__DisplayClass8.<BeginProcessRequest>b__3(IAsyncResult asyncResult) +25
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +23
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +47
System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9629296
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

[NullReferenceException:对象引用未设置为对象的实例。]
c:\Users\Admin\Documents\Visual Studio 2012\Projects\Pro\Pro.Web\Views\Dogs\中的ASP
System.Web.WebPages.WebPageBase.ExecutePageHierarchy（）+197
System.Web.Mvc.WebViewPage.ExecutePageHierarchy（）+97
System.Web.WebPages.WebPageBase.ExecutePageHierarchy（WebPageContext-pageContext，TextWriter-writer，WebPageRenderingBase-startPage）+88
System.Web.Mvc.RazorView.RenderView（ViewContext、TextWriter、Object实例）+260
System.Web.Mvc.buildmanager compiledview.Render（ViewContext、TextWriter）+115
System.Web.Mvc.HtmlHelper.RenderPartialInternal（字符串partialViewName、ViewDataDictionary viewData、对象模型、TextWriter编写器、ViewEngineCollection ViewEngineCollection）+276
System.Web.Mvc.Html.PartialExtensions.Partial（HtmlHelper HtmlHelper，字符串partialViewName，对象模型，ViewDataDictionary viewData）+108
System.Web.Mvc.Html.PartialExtensions.Partial（HtmlHelper-HtmlHelper，字符串partialViewName，对象模型）+32
ASP.\u页面\u视图\u项目\u创建\u cshtml.Execute（）在c:\Users\Admin\Documents\Visual Studio 2012\Projects\Pro\Pro.Web\Views\Dogs\Create.cshtml:14中
System.Web.WebPages.WebPageBase.ExecutePageHierarchy（）+197
System.Web.Mvc.WebViewPage.ExecutePageHierarchy（）+97
System.Web.WebPages.StartPage.RunPage（）+17
System.Web.WebPages.StartPage.ExecutePageHierarchy（）+62
System.Web.WebPages.WebPageBase.ExecutePageHierarchy（WebPageContext-pageContext、TextWriter-writer、WebPageRenderingBase-startPage）+76
System.Web.Mvc.RazorView.RenderView（ViewContext、TextWriter、Object实例）+260
System.Web.Mvc.buildmanager compiledview.Render（ViewContext、TextWriter）+115
System.Web.Mvc.ViewResultBase.ExecuteResult（ControllerContext上下文）+295
System.Web.Mvc.ControllerActionInvoker.InvokeActionResult（ControllerContext ControllerContext，ActionResult ActionResult）+13
System.Web.Mvc.c__显示Class1A.b_17（）+23
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter（IResultFilter筛选器，ResultExecutingContext预文本，Func`1 continuation）+242
System.Web.Mvc.c__显示Class1C.b__19（）+21
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters（ControllerContext ControllerContext，IList`1过滤器，ActionResult ActionResult）+177
System.Web.Mvc.Async.c__DisplayClass2a.b__20（）+89
System.Web.Mvc.Async.c__显示类25.b__22（IAsyncResult asyncResult）+102
System.Web.Mvc.Async.WrappedAsyncResult`1.End（）+57
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction（IAsyncResult asyncResult）+43
System.Web.Mvc.c__;u displayClassId.b__u18（IAsyncResult asyncResult）+14
System.Web.Mvc.Async.c__显示类4.b__3（IAsyncResult ar）+23
System.Web.Mvc.Async.WrappedAsyncResult`1.End（）+62
System.Web.Mvc.Controller.EndExecuteCore（IAsyncResult asyncResult）+57
System.Web.Mvc.Async.c__显示类4.b__3（IAsyncResult ar）+23
System.Web.Mvc.Async.WrappedAsyncResult`1.End（）+62
System.Web.Mvc.Controller.EndExecute（IAsyncResult asyncResult）+47
System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute（IAsyncResult asyncResult）+10
System.Web.Mvc.c__;u DisplayClass8.b__;u 3（IAsyncResult asyncResult）+25
System.Web.Mvc.Async.c__显示类4.b__3（IAsyncResult ar）+23
System.Web.Mvc.Async.WrappedAsyncResult`1.End（）+62
System.Web.Mvc.MvcHandler.EndProcessRequest（IAsyncResult asyncResult）+47
System.Web.Mvc.MvcHandler.System.Web.IHTTPassynchandler.EndProcessRequest（IAsyncResult结果）+9
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute（）+96296
System.Web.HttpApplication.ExecuteStep（IExecutionStep步骤，布尔值&同步完成）+155

假设您有一个单独的create Post版本，提交视图时，您可能会返回模型，而无需重新填充模型中的“颜色”项：

[HttpPost]
public ActionResult Create(CreateDog model) {
    if (ModelState.IsValid) {
        // do something
    }

    return View(model); // Colors is null at this point, because
                        // It does not get posted.  You have to repopulate it.
}

---

过账仅过账选定值，不过账下拉列表的内容。因此，重新渲染视图后，它将尝试从空集合引用进行渲染。

几乎所有

NullReferenceException

的情况都是相同的。请参阅“”，以获得一些提示。如果您想获得帮助，请发布完整的异常堆栈跟踪。您是否有单独的发布和获取创建版本？@JohnSaunders-我编辑了这个问题。而且，CreateOrEdit.cshtml的第65行在哪里？它可以工作。非常感谢。我错过了我需要在后期版本中再次这样做。

[NullReferenceException: Object reference not set to an instance of an object.]
ASP._Page_Views_Projects__CreateOrEdit_cshtml.Execute() in c:\Users\Admin\Documents\Visual Studio 2012\Projects\Pro\Pro.Web\Views\Dogs\_CreateOrEdit.cshtml:65
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +197
System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +97
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +88
System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +260
System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +115
System.Web.Mvc.HtmlHelper.RenderPartialInternal(String partialViewName, ViewDataDictionary viewData, Object model, TextWriter writer, ViewEngineCollection viewEngineCollection) +276
System.Web.Mvc.Html.PartialExtensions.Partial(HtmlHelper htmlHelper, String partialViewName, Object model, ViewDataDictionary viewData) +108
System.Web.Mvc.Html.PartialExtensions.Partial(HtmlHelper htmlHelper, String partialViewName, Object model) +32
ASP._Page_Views_Projects_Create_cshtml.Execute() in c:\Users\Admin\Documents\Visual Studio 2012\Projects\Pro\Pro.Web\Views\Dogs\Create.cshtml:14
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +197
System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +97
System.Web.WebPages.StartPage.RunPage() +17
System.Web.WebPages.StartPage.ExecutePageHierarchy() +62
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +76
System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +260
System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +115
System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +295
System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +13
System.Web.Mvc.<>c__DisplayClass1a.<InvokeActionResultWithFilters>b__17() +23
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +242
System.Web.Mvc.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() +21
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +177
System.Web.Mvc.Async.<>c__DisplayClass2a.<BeginInvokeAction>b__20() +89
System.Web.Mvc.Async.<>c__DisplayClass25.<BeginInvokeAction>b__22(IAsyncResult asyncResult) +102
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +57
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +43
System.Web.Mvc.<>c__DisplayClass1d.<BeginExecuteCore>b__18(IAsyncResult asyncResult) +14
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +23
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +57
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +23
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +47
System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +10
System.Web.Mvc.<>c__DisplayClass8.<BeginProcessRequest>b__3(IAsyncResult asyncResult) +25
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +23
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +47
System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9629296
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

[HttpPost]
public ActionResult Create(CreateDog model) {
    if (ModelState.IsValid) {
        // do something
    }

    return View(model); // Colors is null at this point, because
                        // It does not get posted.  You have to repopulate it.
}