ASP.NET MVC 5 中的空引用异常答案

【问题标题】：Null Reference exception in ASP.NET MVC 5ASP.NET MVC 5 中的空引用异常
【发布时间】：2014-11-18 08:16:28
【问题描述】：

我正在从 Pro Asp.Net MVC5 学习 ASP，当我向解决方案添加第二个数据库和类时，每当我在加载客户或产品页面时在 EFDbContext 中添加 DbSet 时都会遇到错误。

注意：产品页面工作正常，一旦我从 EFDbContext 中删除/移动 DbSet 并检查了What is a NullReferenceException and how do I fix it

EFDbContext.cs

using SportsStore.Domain.Entities;
namespace SportsStore.Domain.Concrete 
{
   public class EFDbContext : DbContext
   {
      public DbSet<Product> Products { get; set; }

      **public DbSet<Customer> Customers{ get; set; }**
   }
}

EFProductRepostory.cs

namespace SportsStore.Domain.Concrete 
{
   public class EFProductRepository : IProductRepository 
   {
       private EFDbContext context = new EFDbContext();

       public IEnumerable<Product> Products 
       {
          get { return context.Products; }
       }

       public IEnumerable<Customer> Customers
       {
           get { return context.Customers; }
       }
}

NinjectDependencyResolver.cs

private void AddBindings()
{
    kernel.Bind<IProductRepository>().To<EFProductRepository>();
}

Web.config：

<connectionStrings>
    <add name="EFDbContext" connectionString="Data Source=(localdb)\v11.0;Initial
        Catalog=SportsStore;Integrated Security=True"
        providerName="System.Data.SqlClient"/>
</connectionStrings>

错误：

System.NullReferenceException was unhandled by user code
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=System.Web
  StackTrace:
       at System.Web.UI.ParseChildrenAttribute.GetHashCode()
       at System.Collections.Generic.ObjectEqualityComparer`1.GetHashCode(T obj)
       at System.Collections.Generic.HashSet`1.InternalGetHashCode(T item)
       at System.Collections.Generic.HashSet`1.AddIfNotPresent(T value)
       at System.Collections.Generic.HashSet`1.UnionWith(IEnumerable`1 other)
       at System.Collections.Generic.HashSet`1..ctor(IEnumerable`1 collection, IEqualityComparer`1 comparer)
       at System.Collections.Generic.HashSet`1..ctor(IEnumerable`1 collection)
       at System.Data.Entity.ModelConfiguration.Utilities.AttributeProvider.GetAttributes(Type type)
       at System.Data.Entity.ModelConfiguration.Utilities.AttributeProvider.<>c__DisplayClass4.<GetAttributes>b__3(PropertyInfo pi)
       at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
       at System.Data.Entity.ModelConfiguration.Utilities.AttributeProvider.GetAttributes(PropertyInfo propertyInfo)
       at System.Data.Entity.ModelConfiguration.Conventions.PropertyAttributeConfigurationConvention`1.<.ctor>b__0(ConventionTypeConfiguration ec)
       at System.Data.Entity.ModelConfiguration.Conventions.TypeConvention.ApplyCore(Type memberInfo, ModelConfiguration modelConfiguration)
       at System.Data.Entity.ModelConfiguration.Conventions.TypeConventionBase.Apply(Type memberInfo, ModelConfiguration modelConfiguration)
       at System.Data.Entity.ModelConfiguration.Configuration.ConventionsConfiguration.ApplyModelConfiguration(Type type, ModelConfiguration modelConfiguration)
       at System.Data.Entity.ModelConfiguration.Conventions.Convention.ApplyModelConfiguration(Type type, ModelConfiguration modelConfiguration)
       at System.Data.Entity.ModelConfiguration.Configuration.ConventionsConfiguration.ApplyModelConfiguration(Type type, ModelConfiguration modelConfiguration)
       at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.MapComplexType(Type type, Boolean discoverNested)
       at System.Data.Entity.ModelConfiguration.Mappers.PropertyMapper.MapPrimitiveOrComplexOrEnumProperty(PropertyInfo propertyInfo, Func`1 structuralTypeConfiguration, Boolean discoverComplexTypes)
       at System.Data.Entity.ModelConfiguration.Mappers.PropertyMapper.MapIfNotNavigationProperty(PropertyInfo propertyInfo, EntityType entityType, Func`1 entityTypeConfiguration)
       at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.<>c__DisplayClass14.<MapEntityType>b__e(PropertyMapper m, PropertyInfo p)
       at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.MapStructuralElements[TStructuralTypeConfiguration](Type type, ICollection`1 annotations, Action`2 propertyMappingAction, Boolean mapDeclaredPropertiesOnly, Func`1 structuralTypeConfiguration)
       at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.MapEntityType(Type type)
       at System.Data.Entity.ModelConfiguration.Mappers.NavigationPropertyMapper.Map(PropertyInfo propertyInfo, EntityType entityType, Func`1 entityTypeConfiguration)
       at System.Data.Entity.ModelConfiguration.Mappers.TypeMapper.MapEntityType(Type type)
       at System.Data.Entity.DbModelBuilder.<>c__DisplayClassd.<MapTypes>b__7(Type type)
       at System.Linq.Enumerable.WhereListIterator`1.MoveNext()
       at System.Data.Entity.Utilities.IEnumerableExtensions.Each[T](IEnumerable`1 ts, Action`1 action)
       at System.Data.Entity.DbModelBuilder.MapTypes(EdmModel model)
       at System.Data.Entity.DbModelBuilder.Build(DbProviderManifest providerManifest, DbProviderInfo providerInfo)
       at System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection)
       at System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext)
       at System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input)
       at System.Data.Entity.Internal.LazyInternalContext.InitializeContext()
       at System.Data.Entity.Internal.InternalContext.Initialize()
       at System.Data.Entity.Internal.InternalContext.GetEntitySetAndBaseTypeForType(Type entityType)
       at System.Data.Entity.Internal.Linq.InternalSet`1.Initialize()
       at System.Data.Entity.Internal.Linq.InternalSet`1.GetEnumerator()
       at System.Data.Entity.Infrastructure.DbQuery`1.System.Collections.Generic.IEnumerable<TResult>.GetEnumerator()
       at ASP._Page_Views_admin_Admin_cshtml.Execute() in c:\Visual Studio 2013\Projects\Store\SportsStore.WebUI\Views\Admin\Index.cshtml:line 46
       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)

更新： 最后一件事要解释。当我移动公共 DbSet 客户{ get;放; } 进入另一个类，管理页面正常加载，所以 repository.Products 不返回 null，我不知道为什么会发生这种情况

Source Error:
Line 44:             </tr>
Line 45: 
Line 46:             @foreach (var item in Model)
Line 47:             {
Line 48:                 <tr>

管理员/index.cshtml：

@foreach (var item in Model)
{
    <tr>
        <td>@Html.DisplayFor(modelItem => item.Name)</td>
        <td>@Html.DisplayFor(modelItem => item.Description)</td>
        <td>@Html.DisplayFor(modelItem => item.Price)</td>
        <td>@Html.DisplayFor(modelItem => item.Category)</td>
        <td>
            @Html.ActionLink("Edit", "Edit", new { id=item.ProductID }) |
            @Html.ActionLink("Details", "Details", new { id=item.ProductID }) |
            @Html.ActionLink("Delete", "Delete", new { id=item.ProductID })
        </td>
    </tr>
}

AdminController.cs

namespace SportsStore.WebUI.Controllers
{
    public class AdminController : Controller
    {
        private IProductRepository repository;

        public AdminController(IProductRepository repo)
        {
            repository = repo;
        }

        public ViewResult Index()
        {
            return View(repository.Products);
        }
    }
}

【问题讨论】：

欢迎来到 SO。请显示触发异常的代码行。
您没有显示控制器中的代码，但我猜您没有将模型从控制器传递给视图。
我确实通过了模型，如果我删除了 DbSet Customers{ get; 这个页面可以正常工作放; } 但是当我添加它时 repository.Products 返回 null，
关闭者注意：这不是规范 NullReferenceException 问题的重复。在最坏的情况下，这可能是一个新案例，应该添加到该问题的答案中。
这可能是一个愚蠢的问题，但是您的数据库中有客户表吗？当您单步执行代码时，是控制器中的 IProductRepository 是空引用，还是 DbSet 导致错误？

标签： c# asp.net-mvc asp.net-mvc-5

【解决方案1】：

错误是因为有一个属性为空值

------------------------------------
| FNAME | LNAME | ........| AVATAR |
------------------------------------
| xxxxx | xxxxx | xxxxxxx |  NULL  |
------------------------------------

用值填充 Avatar 似乎可以解决它，我从来没有怀疑过，因为该字段的其余部分已经有一些数据

【讨论】：