ASP.NET MVC 3 RTM項目中的這個錯誤是什麼？

我創建新項目ASP.NET MVC 3 RTM和使用ASP.NET MVC的更換參考3 的源代碼ASP.NET MVC 3 RTM項目中的這個錯誤是什麼？

，但是當我再次運行項目，得到此異常：

[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\2505c7ee\369315c5\assembly\dl3\f7bae428\d03eeed8_85bbcb01\System.Web.WebPages.Razor.DLL'.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

源錯誤：

Line 106:  internal static RazorWebSectionGroup GetRazorSection(string virtualPath) { 
Line 107:   // Get the individual sections (we can only use GetSection in medium trust) and then reconstruct the section group 
Line 108:   return new RazorWebSectionGroup() { 
Line 109:    Host = (HostSection)WebConfigurationManager.GetSection(HostSection.SectionName, virtualPath), 
Line 110:    Pages = (RazorPagesSection)WebConfigurationManager.GetSection(RazorPagesSection.SectionName, virtualPath)