FirstOrDefault() 我得到一个 ArgumentNullException

Question

我得到一个ArgumentNullException，我不知道如何解决这个问题。我试过DefaultIfEmpty，但它仍然不起作用。

代码如下:

if (controller != "Home")
{
var defaultActionForController = ((List<DefaultActionForControllers>)ViewBag.DefaultActionsForControllers).Where(s => s.Controller == controller).Select(s => s.Action).FirstOrDefault();
if (controller == "ManagementObjects")
{
var controllerName = firstLayerNameSet;
@:> @Html.ActionLink(firstLayerNameSet, defaultActionForController, controller)
}
else if (controller == "MangementCategories")
{
var controllerName = secondLayerNameSet;
@:> @Html.ActionLink(secondLayerNameSet, defaultActionForController, controller)
}
else if (controller == "View")
{
@:> @Html.ActionLink(Resources.ResourceManager.GetString(controller), "", "")
}
else
{
@:> @Html.ActionLink(Resources.ResourceManager.GetString(controller), defaultActionForController, controller)
}
}

错误如下:

3005 An unhandled exception has occurred.
ArgumentNullException
Value cannot be null.
Parameter name: source
at System.Linq.Enumerable.Where[TSource](IEnumerable`1 source, Func`2 predicate)
at ASP._Page_Views_Shared__LoginPartial_cshtml.Execute() in d:inetpubwwwrootTestMapViewsShared_LoginPartial.cshtml:line 30
at System.Web.WebPages.WebPageBase.ExecutePageHierarchy()
at System.Web.Mvc.WebViewPage.ExecutePageHierarchy()
at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage)
at System.Web.Mvc.Html.PartialExtensions.Partial(HtmlHelper htmlHelper, String partialViewName, Object model, ViewDataDictionary viewData)
at ASP._Page_Views_Shared__TopNavbar_cshtml.Execute() in d:inetpubwwwrootTestMapViewsShared_TopNavbar.cshtml:line 13
at System.Web.WebPages.WebPageBase.ExecutePageHierarchy()
at System.Web.Mvc.WebViewPage.ExecutePageHierarchy()
at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage)
at System.Web.Mvc.Html.PartialExtensions.Partial(HtmlHelper htmlHelper, String partialViewName, Object model, ViewDataDictionary viewData)
at ASP._Page_Views_Shared__Layout_cshtml.Execute() in d:inetpubwwwrootTestMapViewsShared_Layout.cshtml:line 73
at System.Web.WebPages.WebPageBase.ExecutePageHierarchy()
at System.Web.Mvc.WebViewPage.ExecutePageHierarchy()
at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage)
at System.Web.WebPages.WebPageBase.<>c__DisplayClass40_0.<RenderPageCore>b__0(TextWriter writer)
at System.Web.WebPages.WebPageBase.Write(HelperResult result)
at System.Web.WebPages.WebPageBase.RenderSurrounding(String partialViewName, Action`1 body)
at System.Web.WebPages.WebPageBase.PopContext()
at System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass3_1.<BeginInvokeAction>b__5(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult)
at System.Web.Mvc.Controller.<>c.<BeginExecuteCore>b__152_1(IAsyncResult asyncResult, ExecuteCoreState innerState)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult)
at System.Web.Mvc.MvcHandler.<>c.<BeginProcessRequest>b__20_1(IAsyncResult asyncResult, ProcessRequestState innerState)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult)
at System.Web.HttpApplication.CallHandlerExecutionStep.InvokeEndHandler(IAsyncResult ar)
at System.Web.HttpApplication.CallHandlerExecutionStep.OnAsyncHandlerCompletion(IAsyncResult ar)

下面是我的defaulftactionforcontrollers的代码:

using System.Collections.Generic;
namespace Heijligers.Mas.Web.Models
{
public class DefaultActionForControllers
{
public string Controller { get; set; }
public string Action { get; set; }
}
}

我把代码改成这样，它不工作了:

var defaultActionForController = ((List<DefaultActionForControllers>)ViewContext.Controller.ViewBag.DefaultActionForControllers).FirstOrDefault(s => s.Controller == controller).Action;

它仍然给了我一个ArgumentNullException。当我去调试时，它说var defaultActionForController = null在ViewBag中。DefaultActionsForControllers count 12

我不知道这些信息是否有帮助，但我想只是发布它，你永远不知道

Answer 1

错误堆栈显示问题所在:

ArgumentNullException
Value cannot be null.
Parameter name: source
at System.Linq.Enumerable.Where[TSource](IEnumerable`1 source, Func`2 predicate)
at ASP._Page_Views_Shared__LoginPartial_cshtml.Execute() in 
d:inetpubwwwrootTestMapViewsShared_LoginPartial.cshtml:line 30
at System.Web.WebPages.WebPageBase.ExecutePageHierarchy()

Where是一个扩展方法。因此，它在一个源上运行，错误告诉您source参数是null。

检查填充ViewBag.DefaultActionsForControllers的代码。您需要在呈现视图之前设置控制器动作的值。

最后，您的方法调用可以简化为FirstOrDefault也允许谓词:

var defaultActionForController = ((List<DefaultActionForControllers>)ViewBag.DefaultActionsForControllers).FirstOrDefault(s => s.Controller == controller).Action;