System.Web.Mvc 类型的 Unity ResolutionFailedException

Question

我的 WebApp 是 WebForms 页面和 MVC 4 页面的组合。

使用 Unity，我遇到以下异常。

异常类型：

Microsoft.Practices.Unity.ResolutionFailedException, Microsoft.Practices.Unity, Version=2.1.505.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35

消息：

Resolution of the dependency failed, type = "System.Web.Mvc.ModelMetadataProvider", name = "(none)".  Exception occurred while: while resolving.  Exception is: InvalidOperationException - The type ModelMetadataProvider does not have an accessible constructor.  At the time of the exception, the container was:
Resolving System.Web.Mvc.ModelMetadataProvider,(none)

带有Inner Exception 的：

消息：

The type ModelMetadataProvider does not have an accessible constructor.

堆栈跟踪：

at Microsoft.Practices.ObjectBuilder2.DynamicMethodConstructorStrategy.ThrowForNullExistingObje ct(IBuilderContext context)  
at BuildUp_System.Web.Mvc.ModelMetadataProvider(IBuilderContext )  
at Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlan.BuildUp(IBuilderContext context)  
at Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context)  
at Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context)  
at Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable`1 resolverOverrides)  

对于以下类型，我始终得到相同的分辨率异常：

System.Web.Mvc.ITempDataProvide  
System.Web.Mvc.Async.IAsyncActionInvoker  
System.Web.Mvc.IActionInvoker  
System.Web.Mvc.ModelMetadataProvider  
System.Web.Mvc.IViewPageActivator

在 Application_Start 中，我正在设置 IoC：

DependencyResolver.SetResolver(new UnityDependencyResolver(UnityConfigurator.GetContainer()));

...其中GetContainer返回所有注册类型，UnityDependencyResolver是-a IDependencyResolver

任何想法是什么导致了这个问题？

谢谢，
--埃德

Answer 1

我不确定UnityConfigurator.GetContainer() 返回的容器实例的行为如何，但我认为它会在需要时尝试单独解决ModelMetadataProvider。这个ModelMetadataProvider 有一些你的容器不知道的依赖关系，只有初始 MVC 解析器可以解析。 “初始 MVC 解析器”是指在您调用 DependencyResolver.SetResolver(...) 之前 DependencyResolver.Current 的结果。 因此，您需要在自己的容器上实现代理，并将初始 MVC 解析器用作后备。

代理的实现应该是什么样子的一个简单示例：

public class MvcDependencyResolver:System.Web.Mvc.IDependencyResolver
{
    private readonly IDependencyResolver _resolver;

    private readonly System.Web.Mvc.IDependencyResolver _fallbackResolver;

    public MvcDependencyResolver(IDependencyResolver resolver, System.Web.Mvc.IDependencyResolver fallbackResolver)
    {
        this._resolver = resolver;
        this._fallbackResolver = fallbackResolver;
    }

    public object GetService(Type serviceType)
    {
        if ((this._resolver.IsRegistered(serviceType)) || (serviceType.IsClass))
        {
            try
            {
                return this._resolver.GetService(serviceType);
            }
            catch (ResolutionFailedException)
            {
            }
        }
        return this._fallbackResolver.GetService(serviceType);
    }

然后，在 Application_Start() 中，您使用此代理的实例覆盖 MVC 解析器，如下所示：

var bootstrapper = new Bootstrapper();
var resolver = bootstrapper.Run();
DependencyResolver.SetResolver(new MvcDependencyResolver(resolver, DependencyResolver.Current));