c# - ASP.Net MVC - "Cannot Create Instance of an Interface"

Question

我已经尝试解决这个问题一段时间了，但我已经走到了死胡同，所以也许你们可以提供帮助。请注意，这与此问题无关。

我在 MVC3 中使用 Entity-Code First。我一定对我的一个对象做了一些狡猾的事情，因为当它第一次尝试创建数据库并从我的 Seed 函数填充它时，会发生此错误(Seed 函数从未实际调用过，但数据库和表被创建了)。

系统深处抛出异常:

>   mscorlib.dll!System.RuntimeType.CreateInstanceSlow(bool publicOnly, bool skipCheckThis, bool fillCache = true) + 0x63 bytes 
    mscorlib.dll!System.Activator.CreateInstance(System.Type type, bool nonPublic) + 0x46 bytes 
    System.Web.Mvc.dll!System.Web.Mvc.DependencyResolver.DefaultDependencyResolver.GetService(System.Type serviceType) + 0x25 bytes 
    System.Web.Mvc.dll!System.Web.Mvc.DependencyResolverExtensions.GetService<System.Web.Mvc.IControllerFactory>(System.Web.Mvc.IDependencyResolver resolver) + 0x3d bytes  
    System.Web.Mvc.dll!System.Web.Mvc.SingleServiceResolver<System.Web.Mvc.IControllerFactory>.Current.get() + 0x7e bytes   
    System.Web.Mvc.dll!System.Web.Mvc.MvcRouteHandler.GetSessionStateBehavior(System.Web.Routing.RequestContext requestContext = {System.Web.Routing.RequestContext}) + 0x72 bytes  
    System.Web.Mvc.dll!System.Web.Mvc.MvcRouteHandler.GetHttpHandler(System.Web.Routing.RequestContext requestContext = {System.Web.Routing.RequestContext}) + 0x2a bytes   
    System.Web.Mvc.dll!System.Web.Mvc.MvcRouteHandler.System.Web.Routing.IRouteHandler.GetHttpHandler(System.Web.Routing.RequestContext requestContext) + 0xb bytes 
    System.Web.dll!System.Web.Routing.UrlRoutingModule.PostResolveRequestCache(System.Web.HttpContextBase context = {System.Web.HttpContextWrapper}) + 0x108 bytes  
    System.Web.dll!System.Web.Routing.UrlRoutingModule.OnApplicationPostResolveRequestCache(object sender, System.EventArgs e) + 0x57 bytes 
    System.Web.dll!System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() + 0x95 bytes   
    System.Web.dll!System.Web.HttpApplication.ExecuteStep(System.Web.HttpApplication.IExecutionStep step = {System.Web.HttpApplication.SyncEventExecutionStep}, ref bool completedSynchronously = true) + 0x4c bytes    
    System.Web.dll!System.Web.HttpApplication.ApplicationStepManager.ResumeSteps(System.Exception error) + 0x13e bytes  
    System.Web.dll!System.Web.HttpApplication.System.Web.IHttpAsyncHandler.BeginProcessRequest(System.Web.HttpContext context, System.AsyncCallback cb, object extraData) + 0xad bytes  
    System.Web.dll!System.Web.HttpRuntime.ProcessRequestInternal(System.Web.HttpWorkerRequest wr = {Microsoft.VisualStudio.WebHost.Request}) + 0x1a2 bytes  
    System.Web.dll!System.Web.HttpRuntime.ProcessRequestNoDemand(System.Web.HttpWorkerRequest wr) + 0x7d bytes  
    System.Web.dll!System.Web.HttpRuntime.ProcessRequest(System.Web.HttpWorkerRequest wr) + 0x47 bytes  
    WebDev.WebHost40.dll!Microsoft.VisualStudio.WebHost.Request.Process() + 0x17b bytes 
    WebDev.WebHost40.dll!Microsoft.VisualStudio.WebHost.Host.ProcessRequest(Microsoft.VisualStudio.WebHost.Connection conn = {System.Runtime.Remoting.Proxies.__TransparentProxy}) + 0x6c bytes 
    [Appdomain Transition]  
    WebDev.WebHost40.dll!Microsoft.VisualStudio.WebHost.Server.OnSocketAccept(object acceptedSocket) + 0x83 bytes   
    mscorlib.dll!System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(object state) + 0x2d bytes 
    mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool ignoreSyncCtx) + 0xb0 bytes    
    mscorlib.dll!System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem() + 0x5a bytes 
    mscorlib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch() + 0x147 bytes  
    mscorlib.dll!System.Threading._ThreadPoolWaitCallback.PerformWaitCallback() + 0x2d bytes    
    [Native to Managed Transition]  

目前我只有一个对象。请注意，正如我之前所说，它是在数据库中创建的:

// class Category
public class Category
{
    [Key]
    public int CategoryID { get; set; }

    [Required]
    [StringLength(64)]
    public string Name { get; set; }

    public int? ParentCategoryID { get; set; }
    [ForeignKey("ParentCategoryID")]
    public Category ParentCategory { get; set; }

    [Required]
    public int ListOrder { get; set; }

    // left/right
    public int TreeLeft { get; set; }
    public int TreeRight { get; set; }
}   // eo class Category

这是我的 DbContext 派生类:

// class ModelContext
public class ModelContext : DbContext
{
    public DbSet<Models.CMS.Category> ContentCategories { get; set; }

    // property to get the object context
    public ObjectContext ObjectContext
    {
        get
        {
            return ((IObjectContextAdapter)this).ObjectContext;
        }
    }


    // OnModelCreating
    protected override void OnModelCreating(System.Data.Entity.ModelConfiguration.ModelBuilder modelBuilder)
    {

        base.OnModelCreating(modelBuilder);
    }   // eo OnModelCreating

}   // eo class ModelContext

那里没有太多进展。最后，我的初始化程序。 Seed 函数从未被调用:

// class ModelInitializer
public class ModelInitializer : DropCreateDatabaseIfModelChanges<ModelContext>
{
    // seed
    protected override void Seed(ModelContext context)
    {
        var catRepo = Models.CMS.CategoryRepository.Instance;

        // Root category node
        context.ContentCategories.Add(catRepo.CreateRoot());

        // Test data
        Category home = catRepo.Add(new Category { Name = "Home", ListOrder = 10 });
        Category news = catRepo.Add(new Category { Name = "News", ListOrder = 20 });

        catRepo.Add(new Category { Name = "Current News", ListOrder = 10 }, news);
        catRepo.Add(new Category { Name = "Older News", ListOrder = 20 }, news);
    }   // eo Seed

}   // eo class ModelInitializer

您可能需要或不需要更多信息，我会明确提供。我不知道从哪里开始看。该错误甚至可能与我的模型无关，但无论如何。我注意到它发生在 MvcRouteHandler 中，我在这里没有做任何特别的事情，如果我忽略异常，我最终会进入 HomeController 的操作处理程序。我在这里收到一个错误，因为数据库没有播种(因为从未调用过 Seed)。

哦，如果有帮助的话，global.asax.cs 中的初始化程序:

    protected void Application_Start()
    {
        DbDatabase.SetInitializer<ModelContext>(new ModelInitializer());

        AreaRegistration.RegisterAllAreas();

        RegisterGlobalFilters(GlobalFilters.Filters);
        RegisterRoutes(RouteTable.Routes);
    }

如有任何帮助或指点，我们将不胜感激!

编辑: Controller 看起来像这样:

    public ActionResult Index()
    {
        Models.CMS.CategoryRepository repo = Models.CMS.CategoryRepository.Instance;
        List<Models.CMS.Category> cats = repo.GetAll();
        return View();
    }

但是，错误发生在它到达此处理程序之前。该 GetAll() 调用中的某些代码随后会出错，因为数据库未填充(从未调用 Initializer)。

Answer 1

我下载了项目。首先，为了在没有任何模型问题的情况下重新开始，我将 ModelInitializer 设置为从 DropCreateDatabaseAlways 继承，因为我不希望每次调试时数据库中都有任何陈旧数据。

当我运行该应用程序时，我在 CategoryRepository 的 Add() 方法中遇到一个空异常。我认为您可能对存储库模式有误解。目前，您使用的是静态存储库，IMO 不适合这样做。您还在几个地方创建了一个新的模型上下文，您应该通过将其作为参数传递到存储库来避免这种情况。理想情况下，类别存储库的界面应如下所示:

public interface ICategoryRepository
{
    void Add(Category category);
    void Remove(Category category);
    List<Category> GetAll(int parentId = 0);
    Category GetRoot();
    Category Get(int id);
}

然后在您的存储库实现中将是这样的:

public class CategoryRepository : ICategoryRepository
{
    private readonly ModelContext context;

    // Here we pass in the context so that it can be used by methods.
    public CategoryRepository(ModelContext context)
    {
        this.context = context;
    }

    #region ICategoryRepository Members

    public void Add(Category category, Category parent = null)
    {

        if (parent == null)
        {
            parent = this.GetRoot();
        }
        // Snipped the stuff here.

        // Finally add to the current context.
        this.context.ContentCategories.Add(category);
    }

    // And all other methods...
}

我对项目进行了一些重大更改，因此我希望它们对存储库模式有意义。您可以找到 updated project here .在我改变这个之后，我得到了一个关于关系的异常(exception) - 所以我认为你最好的办法是解释你如何尝试建模类别(某种形式的树结构......)我可以从那里帮助你。另外，一定要清理你的项目并确保数据库是空的。干杯!