一个从asp.net mvc 3升级到asp.net mvc 4的项目发生了如下错误：

[A]System.Web.WebPages.Razor.Configuration.HostSection cannot be cast to [B]System.Web.WebPages.Razor.Configuration.HostSection. Type A originates from 'System.Web.WebPages.Razor, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context 'Default' at location 'C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_1.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll'. Type B originates from 'System.Web.WebPages.Razor, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' in the context 'Default' at location 'C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\7c2060f6\6145b34e\assembly\dl3\9036b7c7\fc8023c0_73fecb01\System.Web.WebPages.Razor.DLL'.

这 是因为本地程序集和GAC的程序集冲突了， 是因为在Web Pages 1.0中他在web.config内新增了一个配置节点。Web Pages 1.0在Web.config中目前我已发现到新增了一个配置节点，路径在~/Views/web.config.而不是web.config.

<configuration>

<system.web.webPages.razor /> <!--注意他于常用的system.web同级—>

<system.web />

...

</configuration>

修改~/Views/web.config成如下：

<?xml version="1.0"?>

<configuration>

  <configSections>

    <sectionGroup name="system.web.webPages.razor" type="System.Web.WebPages.Razor.Configuration.RazorWebSectionGroup, System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">

      <section name="host" type="System.Web.WebPages.Razor.Configuration.HostSection, System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" />

      <section name="pages" type="System.Web.WebPages.Razor.Configuration.RazorPagesSection, System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" />

    </sectionGroup>

  </configSections>

  <system.web.webPages.razor>

    <host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />

    <pages pageBaseType="System.Web.Mvc.WebViewPage">

      <namespaces>

        <add namespace="System.Web.Mvc" />

        <add namespace="System.Web.Mvc.Ajax" />

        <add namespace="System.Web.Mvc.Html" />

        <add namespace="System.Web.Optimization"/>

        <add namespace="System.Web.Routing" />

      </namespaces>

    </pages>

  </system.web.webPages.razor>

  <appSettings>

    <add key="webpages:Enabled" value="false" />

  </appSettings>

  <system.web>

    <httpHandlers>

      <add path="*" verb="*" type="System.Web.HttpNotFoundHandler"/>

    </httpHandlers>

    <!--

        Enabling request validation in view pages would cause validation to occur

        after the input has already been processed by the controller. By default

        MVC performs request validation before a controller processes the input.

        To change this behavior apply the ValidateInputAttribute to a

        controller or action.

    -->

    <pages

        validateRequest="false"

        pageParserFilterType="System.Web.Mvc.ViewTypeParserFilter, System.Web.Mvc, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"

        pageBaseType="System.Web.Mvc.ViewPage, System.Web.Mvc, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"

        userControlBaseType="System.Web.Mvc.ViewUserControl, System.Web.Mvc, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">

      <controls>

        <add assembly="System.Web.Mvc, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" namespace="System.Web.Mvc" tagPrefix="mvc" />

      </controls>

    </pages>

  </system.web>

  <system.webServer>

    <validation validateIntegratedModeConfiguration="false" />

    <handlers>

      <remove name="BlockViewHandler"/>

      <add name="BlockViewHandler" path="*" verb="*" preCondition="integratedMode" type="System.Web.HttpNotFoundHandler" />

    </handlers>

  </system.webServer>

</configuration>

接着修改~/web.config文件

<system.web>
  <compilation debug="true" targetFramework="4.0">

<assemblies>

<add assembly="System.Web.Abstractions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />

<add assembly="System.Web.Helpers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />

<add assembly="System.Web.Routing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />

<add assembly="System.Web.Mvc, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />

<add assembly="System.Web.WebPages, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />

</assemblies>

</compilation>

<authentication mode="Forms"></authentication>

<pages>

<namespaces>

<add namespace="System.Web.Helpers" />

<add namespace="System.Web.Mvc" />

<add namespace="System.Web.Mvc.Ajax" />

<add namespace="System.Web.Mvc.Html" />

<add namespace="System.Web.Routing" />

<add namespace="System.Web.WebPages" />

</namespaces>

</pages>

</system.web>

具体也可以参照 ASP.NET MVC3 系列教程 - 如何使项目Debug进MVC3源代码。