Proper error handling in ASP.NET Web Forms

I want to show users two custom pages:

1) Error500.aspx -- for uncaught exceptions.

2) Error404.aspx -- for non-existing page.

The trick is that ASP.NET handles requests to *.aspx pages differently than to other types of pages.

Here's how I do it.

In Web.Config:

<system.web>

        <customErrors mode="RemoteOnly" defaultRedirect="~/Error.aspx" redirectMode="ResponseRewrite">

               <error statusCode="404" redirect="~/Error404.aspx"/>

               <error statusCode="500" redirect="~/Error500.aspx"/>

        customErrors>

system.web>

<system.webServer>

        <modules runAllManagedModulesForAllRequests="true">

               <add name=" PostJobFreeModule" type="PostJobFree.PostJobFreeModule" preCondition="managedHandler"/>

        modules>

        <httpErrors existingResponse="PassThrough" />

system.webServer>

In PostJobFreeModule.cs I use:

void application_EndRequest(object sender, EventArgs e)

{

        PageNotFoundHelper.ShowError404IfNeeded();

}

public static class PageNotFoundHelper

{

        public static void Set404StatusCode()

        {

                HttpContext.Current.Response.StatusCode = (int)System.Net.HttpStatusCode.NotFound;

               HttpContext.Current.Items["Set404StatusCode"] = true;

        }

        public static void ShowError404IfNeeded()

        {

               if (!HttpContext.Current.IsCustomErrorEnabled) return;

               if (HttpContext.Current.Response.StatusCode == 404)

               {

                       if (HttpContext.Current.Items["Set404StatusCode"] == null)

                       {

                               HttpContext.Current.Server.Transfer("~/Error404.aspx");

                       }

               }

        }

}