[A]System.Web.WebPages.Razor.Configuration.HostSection 无法强制转换为

“/”应用程序中的服务器错误。

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[A]System.Web.WebPages.Razor.Configuration.HostSection 无法强制转换为 [B]System.Web.WebPages.Razor.Configuration.HostSection。类型 A 源自“System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35”(在上下文“Default”中的“C:\windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_2.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll”位置处)。类型 B 源自“System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35”(在上下文“Default”中的“C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\7bbdc962\4cf2d8fd\assembly\dl3\8daee47e\90137f27_720bd201\System.Web.WebPages.Razor.dll”位置处)。

说明: 执行当前 Web 请求期间，出现未经处理的异常。请检查堆栈跟踪信息，以了解有关该错误以及代码中导致错误的出处的详细信息。 

异常详细信息: System.InvalidCastException: [A]System.Web.WebPages.Razor.Configuration.HostSection 无法强制转换为 [B]System.Web.WebPages.Razor.Configuration.HostSection。类型 A 源自“System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35”(在上下文“Default”中的“C:\windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_2.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll”位置处)。类型 B 源自“System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35”(在上下文“Default”中的“C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\7bbdc962\4cf2d8fd\assembly\dl3\8daee47e\90137f27_720bd201\System.Web.WebPages.Razor.dll”位置处)。

源错误: 

执行当前 Web 请求期间生成了未经处理的异常。可以使用下面的异常堆栈跟踪信息确定有关异常原因和发生位置的信息。

堆栈跟踪: 

[InvalidCastException: [A]System.Web.WebPages.Razor.Configuration.HostSection 无法强制转换为 [B]System.Web.WebPages.Razor.Configuration.HostSection。类型 A 源自“System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35”(在上下文“Default”中的“C:\windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_2.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll”位置处)。类型 B 源自“System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35”(在上下文“Default”中的“C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\7bbdc962\4cf2d8fd\assembly\dl3\8daee47e\90137f27_720bd201\System.Web.WebPages.Razor.dll”位置处)。]
   System.Web.WebPages.Razor.WebRazorHostFactory.GetRazorSection(String virtualPath) +151
   System.Web.WebPages.Razor.WebRazorHostFactory.CreateHostFromConfig(String virtualPath, String physicalPath) +210
   System.Web.WebPages.Razor.WebRazorHostFactory.CreateHostFromConfig(String virtualPath) +38
   System.Web.WebPages.Razor.RazorBuildProvider.GetHostFromConfig() +51
   System.Web.WebPages.Razor.RazorBuildProvider.CreateHost() +57
   System.Web.WebPages.Razor.RazorBuildProvider.get_Host() +56
   System.Web.WebPages.Razor.RazorBuildProvider.EnsureGeneratedCode() +77
   System.Web.WebPages.Razor.RazorBuildProvider.get_CodeCompilerType() +54
   System.Web.Compilation.BuildProvider.GetCompilerTypeFromBuildProvider(BuildProvider buildProvider) +59
   System.Web.Compilation.BuildProvidersCompiler.ProcessBuildProviders() +209
   System.Web.Compilation.BuildProvidersCompiler.PerformBuild() +30
   System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath) +9973453
   System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) +299
   System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) +103
   System.Web.Compilation.BuildManager.GetVirtualPathObjectFactory(VirtualPath virtualPath, HttpContext context, Boolean allowCrossApp, Boolean throwIfNotFound) +165
   System.Web.Compilation.BuildManager.GetCompiledType(VirtualPath virtualPath) +10
   System.Web.Compilation.BuildManager.GetCompiledType(String virtualPath) +28
   System.Web.Mvc.BuildManagerWrapper.System.Web.Mvc.IBuildManager.GetCompiledType(String virtualPath) +38
   System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +155
   System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +431
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +39
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) +116
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilterRecursive(IList`1 filters, Int32 filterIndex, ResultExecutingContext preContext, ControllerContext controllerContext, ActionResult actionResult) +529
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +106
   System.Web.Mvc.Async.<>c__DisplayClass28.<BeginInvokeAction>b__19() +321
   System.Web.Mvc.Async.<>c__DisplayClass1e.<BeginInvokeAction>b__1b(IAsyncResult asyncResult) +185
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +42
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +133
   System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +56
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +40
   System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +34
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +70
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +139
   System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59
   System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40
   System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +44
   System.Web.Mvc.Controller.<BeginExecute>b__15(IAsyncResult asyncResult, Controller controller) +39
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +62

错误：

解决方法：web.config

把原来的web.config排除掉，重新加载就行