为什么我的元素变量在这个foreach循环中总是为空？

Question

下面是代码：

public IEnumerable<UserSummary> getUserSummaryList() 
{ 
    var db = new entityContext(); 
    List<UserSummary> model = new List<UserSummary>(); 

    List<aspnet_Users> users = (from user in db.aspnet_Users 
       select user).ToList<aspnet_Users>(); 
    foreach (aspnet_Users u in users) //u is always null while users is a list that contains 4 objects 
    { 
     model.Add(new UserSummary() 
     { 
      UserName = u.UserName, 
      Email = u.aspnet_Membership.Email, 
      Role = Roles.GetRolesForUser(u.UserName).First(), 
      AdCompany = u.AD_COMPANIES.ad_company_name != null ? u.AD_COMPANIES.ad_company_name : "Not an Advertiser", 
      EmployeeName = u.EMPLOYEE.emp_name != null ? u.EMPLOYEE.emp_name : "Not an Employee" 
     }); 
    } 

    return model; 
} 

出于某种原因，在foreach循环的u变量总是空。我已经完成了代码并且users集合总是被填充。 db.aspnet_Users的表格实体是随asp.net成员资格服务一起提供的users表格。我只添加了几个关联。

编辑：调试

下面的图像是堆栈跟踪：

[NullReferenceException: Object reference not set to an instance of an object.] 
    OutAd.Models.AccountMembershipService.getUserSummaryList() in C:\Users\stephen\Desktop\outad\OutAd\OutAd\Models\AccountModels.cs:301 
    OutAd.Controllers.AdminController.UserList() in C:\Users\stephen\Desktop\outad\OutAd\OutAd\Controllers\AdminController.cs:78 
    lambda_method(Closure , ControllerBase , Object[]) +96 
    System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +17 
    System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +208 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +27 
    System.Web.Mvc.<>c__DisplayClass15.<InvokeActionMethodWithFilters>b__12() +55 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +263 
    System.Web.Mvc.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() +19 
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodWithFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +191 
    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.Mvc.<>c__DisplayClassa.<EndProcessRequest>b__9() +23 
    System.Web.Mvc.<>c__DisplayClass4.<Wrap>b__3() +12 
    System.Web.Mvc.ServerExecuteHttpHandlerWrapper.Wrap(Func`1 func) +38 
    System.Web.Mvc.ServerExecuteHttpHandlerWrapper.Wrap(Action action) +65 
    System.Web.Mvc.ServerExecuteHttpHandlerAsyncWrapper.EndProcessRequest(IAsyncResult result) +71 
    System.Web.HttpServerUtility.ExecuteInternal(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage, VirtualPath path, VirtualPath filePath, String physPath, Exception error, String queryStringOverride) +1072 

[HttpException (0x80004005): Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'.] 
    System.Web.HttpServerUtility.ExecuteInternal(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage, VirtualPath path, VirtualPath filePath, String physPath, Exception error, String queryStringOverride) +3049403 
    System.Web.HttpServerUtility.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage) +77 
    System.Web.HttpServerUtility.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm) +28 
    System.Web.HttpServerUtilityWrapper.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm) +22 
    System.Web.Mvc.Html.ChildActionExtensions.ActionHelper(HtmlHelper htmlHelper, String actionName, String controllerName, RouteValueDictionary routeValues, TextWriter textWriter) +497 
    System.Web.Mvc.Html.ChildActionExtensions.Action(HtmlHelper htmlHelper, String actionName, String controllerName, RouteValueDictionary routeValues) +88 
    System.Web.Mvc.Html.ChildActionExtensions.Action(HtmlHelper htmlHelper, String actionName, String controllerName) +10 
    ASP._Page_Views_Admin_Users_cshtml.Execute() in c:\Users\stephen\Desktop\outad\OutAd\OutAd\Views\Admin\Users.cshtml:7 
    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() +8969201 
    System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184 

Answer 1

我要去猜测u本身不是空的，但它的一个特性是null或Roles.GetRolesForUser(u.UserName)调用返回null。如果是这种情况，那么当你尝试访问这些空值的成员时，你会得到NullReferenceException。

例子：

public class Person 
{ 
    public Address HomeAddress { get; set; } 
} 

public class Address 
{ 
    public String StreetAddress { get; set; } 
} 

public void SomeFunc() 
{ 
    var person = new Person(); 

    //NullReferenceException because HomeAddress is null. 
    //NOT because person is null... 
    var address = person.HomeAddress.StreetAddress; 
} 

---

您可以通过鼠标悬停在每个项目在调试，以确保它不为空验证这一点，但我敢打赌，你的目标之一就是。为确保GetRolesForUser方法不返回null，只需在即时窗口中调用该方法，并查看返回的内容。如果你回到空，那就是你的问题。

或者，只需将每个右侧的任务分配到您的本地观察窗口中，其中一个将呈现为问题。

---

哦，还有一些其他的东西......

1. 如果您正计划返回IEnumerable<T>不创建一个列表，然后返回一个列表，而不是只yield return你的对象。

2. 必要时使用null合并运算符。

3. 为您的IDisposable对象使用using声明。

修正版本：

public IEnumerable<UserSummary> getUserSummaryList() 
{ 
    using (var db = new EntityContext()) 
    { 
     foreach (aspnet_Users user in users) 
     { 
      // Retrieve the username (with logic if it is null or empty) 
      var username = user.UserName; 

      // Retrieve the email (with logic if it is null or empty) 
      var email = (user.aspnet_Membership != null) 
       ? user.aspnet_Membership.Email ?? String.Empty 
       : String.Empty; 

      // Retrieve the role (with logic if it is null) 
      var roles = Roles.GetRolesForUser(username); 
      var role = (roles != null) ? roles.FirstOrDefault() : null; 

      // Retrieve the Ad Company (with logic if it is null) 
      var adCompany = (user.AD_COMPANIES != null) 
       ? user.AD_COMPANIES.ad_company_name ?? "Not an Advertiser" 
       : "Not an Advertiser"; 

      var empName = (user.EMPLOYEE != null) 
       ? user.EMPLOYEE.emp_name ?? "Not an Employee" 
       : "Not an Employee"; 

      yield return new UserSummary 
       { 
        UserName  = username, 
        Email  = email, 
        Role   = role, 
        AdCompany = adCompany, 
        EmployeeName = empName, 
       }; 
     } 
    } 
} 

Answer 2

替换：

Roles.GetRolesForUser(u.UserName) 

通过：

Roles.Provider.GetRolesForUser(u.UserName); 

CFR：http://www.lhotka.net/weblog/CallingRolesGetRolesForUserInAWCFService.aspx