[asp.net-mvc] "An exception occurred while processing your request. Additionally, another exception occurred while executing the custom error page..."

I'm trying to publish an MVC website as an Azure webrole.

When I run it locally, everything works fine.

But once I publish it to Azure and surf to some MVC action, I get this error:

Server Error in '/' Application.

Runtime Error

Description: An exception occurred while processing your request. Additionally, another exception occurred while executing the custom error page for the first exception. The request has been terminated.

I don't understand how the error handler can encounter an exception, because errors are handled in the default way:

public class FilterConfig
{
    public static void RegisterGlobalFilters(GlobalFilterCollection filters)
    {
        filters.Add(new HandleErrorAttribute());
    }
}

This is my web.config:

<?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.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>

    <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>

This is Error.cshtml:

@model System.Web.Mvc.HandleErrorInfo

@{
    ViewBag.Title = "Error";
}

<h2>
    Sorry, an error occurred while processing your request.
</h2>

What can cause this exception, and why can't I reproduce it locally?

If you add this to your web.config transformation file, you can also set certain publish options to have debugging enabled or disabled:

<system.web>
    <customErrors mode="Off" defaultRedirect="~/Error.aspx" xdt:Transform="Replace"/>
</system.web>

You can use Oracle.ManagedDataAccess.dll instead (download from Oracle), include that dll in you project bin dir, add reference to that dll in the project. In code, "using Oracle.MangedDataAccess.Client". Deploy project to server as usual. No need install Oracle Client on server. No need to add assembly info in web.config.

I wasn't using Azure, but I got the same error locally. Using <customErrors mode="Off" /> seemed to have no effect, but checking the Application logs in Event Viewer revealed a warning from ASP.NET which contained all the detail I needed to resolve the issue.

I had this problem with only with redirectMode="ResponseRewrite" (redirectMode="ResponseRedirect" worked fine) and none of the above solutions helped my resolve the issue. However, once I changed the server's application pool's "Managed Pipeline Mode" from "Classic" to "Integrated" the custom error page appeared as expected.

When publishing to IIS, by Web Deploy, I just checked the File Publish Options and executed. Now it works! After this deploy the checkboxes do not need to be checked. I don't think this can be a solutions for everybody, but it is the only thing I needed to do to solve my problem. Good luck.