asp.net mvc 框架 5.2.7 依赖注入 FilterProviderCollection.GetFilters 错误

Question

我一直在尝试使用 Microsoft.Extensions.DependencyInjection 为 asp.net mvc 框架 5.2.7 设置 DI，它似乎有点工作，因为我可以在调试时看到我自己的服务正在为控制器解析。 但是之后它会抛出一个“NullReferenceException”并打印以下堆栈跟踪：

[NullReferenceException: Object reference not set to an instance of an object.]
   System.Web.Mvc.FilterProviderCollection.GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor) +127
   System.Web.Mvc.ControllerActionInvoker.GetFilters(ControllerContext controllerContext, ActionDescriptor actionDescriptor) +37
   System.Web.Mvc.Async.AsyncControllerActionInvoker.BeginInvokeAction(ControllerContext controllerContext, String actionName, AsyncCallback callback, Object state) +193
   System.Web.Mvc.<>c.<BeginExecuteCore>b__152_0(AsyncCallback asyncCallback, Object asyncState, ExecuteCoreState innerState) +27
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) +30
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +128
   System.Web.Mvc.Controller.BeginExecuteCore(AsyncCallback callback, Object state) +494
   System.Web.Mvc.<>c.<BeginExecute>b__151_1(AsyncCallback asyncCallback, Object callbackState, Controller controller) +16
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) +20
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +128
   System.Web.Mvc.Controller.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +403
   System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +16
   System.Web.Mvc.<>c.<BeginProcessRequest>b__20_0(AsyncCallback asyncCallback, Object asyncState, ProcessRequestState innerState) +54
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) +30
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +128
   System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) +427
   System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) +48
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) +16
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +105
   System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +50
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +163

Global.asax.cs：

public class Global : System.Web.HttpApplication
    {

        protected void Application_Start(object sender, EventArgs e)
        {
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            DependencyInjectionConfig.SetDependencyResolver();
        }
    }

设置解析器：

public class DependencyInjectionConfig
    {
        public static void SetDependencyResolver()
        {
            var services = new ServiceCollection();

            services.AddTransient<ITestService, TestService>();

            AddControllersAsServices(services, typeof(DependencyInjectionConfig).Assembly.GetExportedTypes()
               .Where(t => !t.IsAbstract && !t.IsGenericTypeDefinition)
               .Where(t => typeof(IController).IsAssignableFrom(t)
                  || t.Name.EndsWith("Controller", StringComparison.OrdinalIgnoreCase)));

            var serviceProvider = services.BuildServiceProvider();

            var dependencyResolver = new Logic.DependencyInjection.DependencyResolver(serviceProvider);

            System.Web.Mvc.DependencyResolver.SetResolver(dependencyResolver);
        }

        public static IServiceCollection AddControllersAsServices(IServiceCollection services, IEnumerable<Type> controllerTypes)
        {
            foreach (var type in controllerTypes)
            {
                services.AddTransient(type);
            }

            return services;
        }
    }

我的依赖解析器：

public class DependencyResolver : IDependencyResolver
    {
        private readonly IServiceProvider serviceProvider;

        public DependencyResolver(IServiceProvider serviceProvider)
        {
            this.serviceProvider = serviceProvider;
        }

        public object GetService(Type serviceType)
        {
            var service = this.serviceProvider.GetService(serviceType);
            return service;
        }

        public IEnumerable<object> GetServices(Type serviceType)
        {
            return new List<object> { GetService(serviceType) };
        }
    }

Answer 1

问题可能出在您的GetServices实现中：

        public IEnumerable<object> GetServices(Type serviceType)
        {
            return new List<object> { GetService(serviceType) };
        }

这不符合约定，因为GetServices永远不应该返回包含元素为null的集合，而您总是返回一个元素可能为null单元素集合。

相反，实现应该是这样的：

        public IEnumerable<object> GetServices(Type serviceType)
        {
            return this.serviceProvider.GetServices(serviceType);
        }