Unity IoC在运行时突然出现故障

我在WebApi2中有一个控制器，并使用Unity将依赖项注入其中。这似乎很好，直到我的日志中突然出现这些异常。前两行来自控制器的成功调用（之前有数百次调用运行良好），其余的来自失败的调用（有数百次失败的异常）。当控制器在同一次运行中也正常工作时，是什么会在运行时触发这些异常？应用程序池被自动回收（从日志中的低毫秒数可以看出），控制器再次正常工作（我想直到它再次变差）。

DEBUG 2016-04-07 04:33:56,611 50302443ms EventController        Post               [eaaa3010-36ec-4cba-b9c3-22035c62430a] - Received: ...
DEBUG 2016-04-07 04:33:56,611 50302443ms EventController        Post               [eaaa3010-36ec-4cba-b9c3-22035c62430a] - Parsed ...
ERROR 2016-04-07 06:08:58,424 56004256ms LogExceptionLogger     LogAsync           [(null)] - Global exception:
System.InvalidOperationException: An error occurred when trying to create a controller of type 'EventController'. Make sure that the controller has a parameterless public constructor. ---> System.ArgumentException: Type 'KamstrupEventReceiver.Controllers.EventController' does not have a default constructor
   at System.Linq.Expressions.Expression.New(Type type)
   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.GetInstanceOrActivator(HttpRequestMessage request, Type controllerType, Func`1& activator)
   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)
   --- End of inner exception stack trace ---
   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)
   at System.Web.Http.Controllers.HttpControllerDescriptor.CreateController(HttpRequestMessage request)
   at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__1.MoveNext()
...
ERROR 2016-04-07 08:46:36,315 65462147ms LogExceptionLogger     LogAsync           [(null)] - Global exception:
System.InvalidOperationException: An error occurred when trying to create a controller of type 'EventController'. Make sure that the controller has a parameterless public constructor. ---> System.ArgumentException: Type 'KamstrupEventReceiver.Controllers.EventController' does not have a default constructor
   at System.Linq.Expressions.Expression.New(Type type)
   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.GetInstanceOrActivator(HttpRequestMessage request, Type controllerType, Func`1& activator)
   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)
   --- End of inner exception stack trace ---
   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)
   at System.Web.Http.Controllers.HttpControllerDescriptor.CreateController(HttpRequestMessage request)
   at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__1.MoveNext()
DEBUG 2016-04-07 14:33:23,606 51408ms EventController        Post               [b88bada7-d81d-4bef-84d8-ce884cc3a010] - Received: ...
DEBUG 2016-04-07 14:33:23,887 51689ms EventController        Post               [b88bada7-d81d-4bef-84d8-ce884cc3a010] - Parsed ...

我的控制器是：

[LogExceptionFilter]
public class EventController : ApiController
{
    private static readonly ILog log4 = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
    private IEventMapping _eventMapping;
    private IMessageQueue _messageQueue;
    private IMapper _mapper;
    public EventController(IEventMapping eventMapping, IMessageQueue messageQueue, IMapper mapper)
    {
        _eventMapping = eventMapping;
        _messageQueue = messageQueue;
        _mapper = mapper;
    }
    // POST: api/Event
    [HttpPost]
    public IHttpActionResult Post([FromBody]Event @event) { ... }

Unity在WebApiConfig.cs:中注册如下

        var container = new UnityContainer();
        container.RegisterType<IEventMapping, EventMapping>(new HierarchicalLifetimeManager());
        container.RegisterType<IMessageQueue, MessageQueue>(new HierarchicalLifetimeManager());
        container.RegisterInstance<IMapper>(new MapperConfiguration(cfg => {
            cfg.CreateMap<Event, EventDTO>();
        }).CreateMapper());
        config.DependencyResolver = new UnityResolver(container);
        config.Filters.Add(new LogExceptionFilterAttribute());
        config.Services.Add(typeof(IExceptionLogger), new LogExceptionLogger());

这是UnityResolver:

public class UnityResolver : IDependencyResolver
{
    protected IUnityContainer container;
    public UnityResolver(IUnityContainer container)
    {
        if (container == null)
        {
            throw new ArgumentNullException("container");
        }
        this.container = container;
    }
    public object GetService(Type serviceType)
    {
        try
        {
            return container.Resolve(serviceType);
        }
        catch (ResolutionFailedException)
        {
            return null;
        }
    }
    public IEnumerable<object> GetServices(Type serviceType)
    {
        try
        {
            return container.ResolveAll(serviceType);
        }
        catch (ResolutionFailedException)
        {
            return new List<object>();
        }
    }
    public IDependencyScope BeginScope()
    {
        var child = container.CreateChildContainer();
        return new UnityResolver(child);
    }
    public void Dispose()
    {
        container.Dispose();
    }
}

真正的异常被清除，您不可能看到真正的原因。发生这种情况的原因是：

在Web API设计中，强制IDependencyResolver.GetService在出现错误时返回null
您实现UnityResolver的方式

这个答案描述了Web API的解析管道是如何工作的，以及为什么你会得到这种愚蠢的"确保控制器有一个无参数的公共构造函数"异常消息。在关联问题的情况下，OP没有明确注册他的控制器。在您的情况下，问题有所不同，因为只有在某些情况下，您的控制器没有得到解决。我能想到为什么会发生这种情况的几个原因：

您在注入构造函数中做了太多的工作；构造函数应该很简单。这会导致它们失败，因为某些外部系统（数据库、web服务等）不可用
您的代码或Unity中存在并发错误

由于您的IDependencyResolver.GetService方法捕获所有ResolutionFailedException并返回null，因此您将丢失重要的异常信息。最好的解决方案是实际上根本不使用IDependencyResolver抽象，或者至少确保使用正确的抽象来解析控制器：IHttpControllerActivator。这里有一个例子：

public sealed class UnityControllerActivator : IHttpControllerActivator
{
    private readonly UnityContainer container;
    public UnityControllerActivator(UnityContainer container) {
        this.container = container;
    }
    public IHttpController Create(HttpRequestMessage request, 
        HttpControllerDescriptor controllerDescriptor, Type controllerType) {
        // Make sure there is an active scope before calling resolve.
        request.GetDependencyScope();
        return (IHttpController)this.container.Resolve(controllerType);
    }
}

您可以在Unity中注册此UnityControllerActivator，如下所示：

configuration.Services.Replace(typeof(IHttpControllerActivator), 
    new UnityControllerActivator());

这确保了Web API将使用此自定义UnityControllerActivator而不是其DefaultHttpControllerActivator。DefaultHttpControllerActivator调用回IDependencyResolver，但它可以返回null。相反，我们的自定义UnityControllerActivator将直接调用Unity，并且永远不会返回null。相反，Unity抛出的原始异常会被保留并记录下来。这可以让你分析真正的原因。如果这个新的异常对你来说毫无意义，你可能想回到这里，问一个关于Stackoverflow的新问题。

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