Encountered an unexpected error when attempting to resolve tag helper directive '@addTagHelper' with value '"*, Microsoft.AspNet.Mvc.TagHelpers"'

project.json 配置：

{
  "version": "1.0.0-*",
  "compilationOptions": {
    "emitEntryPoint": true
  },

  "dependencies": {
    "Microsoft.AspNet.Diagnostics": "1.0.0-rc2-16303",
    "Microsoft.AspNet.IISPlatformHandler": "1.0.0-rc2-15994",
    "Microsoft.AspNet.Mvc": "6.0.0-rc2-16614",
    "Microsoft.AspNet.Mvc.TagHelpers": "6.0.0-rc2-16614",
    "Microsoft.AspNet.Server.Kestrel": "1.0.0-rc2-16156",
    "Microsoft.AspNet.StaticFiles": "1.0.0-rc2-16036",
    "Microsoft.AspNet.Tooling.Razor": "1.0.0-rc2-15994",
    "Microsoft.Extensions.Configuration.FileProviderExtensions": "1.0.0-rc2-15905",
    "Microsoft.Extensions.Configuration.Json": "1.0.0-rc2-15905",
    "Microsoft.Extensions.Logging": "1.0.0-rc2-15907",
    "Microsoft.Extensions.Logging.Console": "1.0.0-rc2-15907",
    "Microsoft.Extensions.Logging.Debug": "1.0.0-rc2-15907",
    "Microsoft.VisualStudio.Web.BrowserLink.Loader": "14.0.0-rc2-16142"
  },

  "commands": {
    "web": "Microsoft.AspNet.Server.Kestrel"
  },

  "frameworks": {
    "dnx451": { }
  },

  "exclude": [
    "wwwroot",
    "node_modules"
  ],
  "publishExclude": [
    "**.user",
    "**.vspscc"
  ],
  "scripts": {
    "prepublish": [ "npm install", "bower install", "gulp clean", "gulp min" ]
  }
}

注意 dependencies 中的程序包都是 rc2 版本（截止目前的最新版本），这时候运行将会抱下面的异常：

fail: Microsoft.AspNet.Diagnostics.ExceptionHandlerMiddleware[0]
An exception was thrown attempting to execute the error handler.
System.InvalidOperationException: Unable to resolve service for type 'Microsoft.Extensions.CompilationAbstractions.ILibraryExporter' while attempting to activate 'Microsoft.AspNet.Mvc.Razor.Compilation.RoslynCompilationService'.
at Microsoft.Extensions.DependencyInjection.ServiceLookup.Service.PopulateCallSites(ServiceProvider provider, ISet`1 callSiteChain, ParameterInfo[] parameters, Boolean throwIfCallSiteNotFound)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.Service.CreateCallSite(ServiceProvider provider, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetResolveCallSite(IService service, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetServiceCallSite(Type serviceType, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.Service.PopulateCallSites(ServiceProvider provider, ISet`1 callSiteChain, ParameterInfo[] parameters, Boolean throwIfCallSiteNotFound)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.Service.CreateCallSite(ServiceProvider provider, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetResolveCallSite(IService service, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetServiceCallSite(Type serviceType, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.Service.PopulateCallSites(ServiceProvider provider, ISet`1 callSiteChain, ParameterInfo[] parameters, Boolean throwIfCallSiteNotFound)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.Service.CreateCallSite(ServiceProvider provider, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetResolveCallSite(IService service, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetServiceCallSite(Type serviceType, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.Service.PopulateCallSites(ServiceProvider provider, ISet`1 callSiteChain, ParameterInfo[] parameters, Boolean throwIfCallSiteNotFound)
at Microsoft.Extensions.DependencyInjection.ServiceLookup.Service.CreateCallSite(ServiceProvider provider, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetResolveCallSite(IService service, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetServiceCallSite(Type serviceType, ISet`1 callSiteChain)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.CreateServiceAccessor(Type serviceType, ServiceProvider serviceProvider)
at System.Collections.Concurrent.ConcurrentDictionaryExtensions.GetOrAdd[TKey,TValue,TArg](ConcurrentDictionary`2 dictionary, TKey key, Func`3 valueFactory, TArg arg)
at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(Type serviceType)
at Microsoft.Extensions.DependencyInjection.ServiceProviderExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
at Microsoft.Extensions.DependencyInjection.ServiceProviderExtensions.GetRequiredService[T](IServiceProvider provider)
at Microsoft.AspNet.Mvc.Razor.Internal.MvcRazorMvcViewOptionsSetup.ConfigureMvc(IServiceProvider serviceProvider, MvcViewOptions options)
at Microsoft.AspNet.Mvc.Razor.Internal.MvcRazorMvcViewOptionsSetup.<>c__DisplayClass0_0.<.ctor>b__0(MvcViewOptions options)
at Microsoft.Extensions.OptionsModel.ConfigureOptions`1.Configure(TOptions options)
at Microsoft.Extensions.OptionsModel.OptionsManager`1.<>c.<get_Value>b__4_0(TOptions options, IConfigureOptions`1 setup)
at System.Linq.Enumerable.Aggregate[TSource,TAccumulate](IEnumerable`1 source, TAccumulate seed, Func`3 func)
at Microsoft.Extensions.OptionsModel.OptionsManager`1.get_Value()
at Microsoft.AspNet.Mvc.ViewEngines.CompositeViewEngine..ctor(IOptions`1 optionsAccessor)
--- End of inner exception stack trace ---

解决方案：将 dnx 的版本升级到 rc2，命令行输入 dnvm upgrade -u，升级之后：

如果 VS 2015 调试运行，需要在 Web 属性中选择 dnx 的版本：

重新运行，又出现下面的异常：

fail: Microsoft.AspNet.Server.Kestrel[13]
      An unhandled exception was thrown by the application.
      Microsoft.AspNet.Mvc.Razor.Compilation.CompilationFailedException: One or more compilation failures occurred:
      Error @ (38:1,14)(36) - [Encountered an unexpected error when attempting to resolve tag helper directive '@addTagHelper' with value '"*, Microsoft.AspNet.Mvc.TagHelpers"'. Error: The given assembly name or codebase was invalid. (Exception from HRESULT: 0x80131047)] (1,14) Encountered an unexpected error when attempting to resolve tag helper directive '@addTagHelper' with value '"*, Microsoft.AspNet.Mvc.TagHelpers"'. Error: The given assembly name or codebase was invalid. (Exception from HRESULT: 0x80131047)
         at Microsoft.AspNet.Mvc.Razor.Compilation.CompilationResult.EnsureSuccessful()
         at Microsoft.AspNet.Mvc.Razor.Compilation.CompilerCache.CreateCacheEntry(String normalizedPath, Func`2 compile)
         at Microsoft.AspNet.Mvc.Razor.Compilation.CompilerCache.GetOrAdd(String relativePath, Func`2 compile)
         at Microsoft.AspNet.Mvc.Razor.DefaultRazorPageFactoryProvider.CreateFactory(String relativePath)
         at Microsoft.AspNet.Mvc.Razor.RazorViewEngine.CreateCacheResult(HashSet`1 expirationTokens, String relativePath, Boolean isMainPage)
         at Microsoft.AspNet.Mvc.Razor.RazorViewEngine.OnCacheMiss(ViewLocationExpanderContext expanderContext, ViewLocationCacheKey cacheKey)
         at Microsoft.AspNet.Mvc.Razor.RazorViewEngine.LocatePageFromViewLocations(ActionContext actionContext, String pageName, Boolean isMainPage)
         at Microsoft.AspNet.Mvc.Razor.RazorViewEngine.FindView(ActionContext context, String viewName, Boolean isMainPage)
         at Microsoft.AspNet.Mvc.ViewEngines.CompositeViewEngine.FindView(ActionContext context, String viewName, Boolean isMainPage)
         at Microsoft.AspNet.Mvc.ViewFeatures.ViewResultExecutor.FindView(ActionContext actionContext, ViewResult viewResult)
         at Microsoft.AspNet.Mvc.ViewResult.<ExecuteResultAsync>d__24.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at Microsoft.AspNet.Mvc.Controllers.FilterActionInvoker.<InvokeResultAsync>d__45.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
         at Microsoft.AspNet.Mvc.Controllers.FilterActionInvoker.<InvokeResultFilterAsync>d__44.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at Microsoft.AspNet.Mvc.Controllers.FilterActionInvoker.<InvokeAllResultFiltersAsync>d__43.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
         at Microsoft.AspNet.Mvc.Controllers.FilterActionInvoker.<InvokeResourceFilterAsync>d__38.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at Microsoft.AspNet.Mvc.Controllers.FilterActionInvoker.<InvokeAsync>d__33.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at Microsoft.AspNet.Mvc.Infrastructure.MvcRouteHandler.<InvokeActionAsync>d__8.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at Microsoft.AspNet.Builder.RouterMiddleware.<Invoke>d__4.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at Microsoft.AspNet.IISPlatformHandler.IISPlatformHandlerMiddleware.<Invoke>d__4.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at Microsoft.AspNet.Diagnostics.ExceptionHandlerMiddleware.<Invoke>d__6.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at Microsoft.AspNet.Diagnostics.ExceptionHandlerMiddleware.<Invoke>d__6.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at Microsoft.AspNet.Hosting.Internal.RequestServicesContainerMiddleware.<Invoke>d__3.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at Microsoft.AspNet.Hosting.Internal.HostingApplication.<ProcessRequestAsync>d__7.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---
         at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
         at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
         at Microsoft.AspNet.Server.Kestrel.Http.Frame`1.<RequestProcessingAsync>d__3.MoveNext()

解决方案：将 Views/_ViewImports.cshtml 中的 @addTagHelper "*, Microsoft.AspNet.Mvc.TagHelpers" 修改如下：

@using WebApplication1
@addTagHelper *, Microsoft.AspNet.Mvc.TagHelpers

修改之后，VS 2015 会提示错误，不用管它，dnx web 重新运行 OK。