ASP.NET 2.0 : Best Practice for writing Error Page

Question

In asp.net 2.0 web site, what is the best way of writing Error page. I have seen following section at following location:

Web.Config

Answer 1

BigBlondeViking's response worked great for me except that I found it did not process 403's (which ASP generates when you try to directly access the /Scripts/ or /Content/ directories.) It appears that this is not propegated as an exception and thus not trappable in the Application_Error handling. (this was identified as a "security vulnerability" by an external firm - don't get me started on that!)

protected void Application_PostRequestHandlerExecute(object sender, EventArgs e)
{
    if (!Context.Items.Contains("HasHandledAnError")) // have we alread processed?
    {
        if (Response.StatusCode > 400 &&  // any error
            Response.StatusCode != 401)   // raised when login is required
        {
            Exception exception = Server.GetLastError();    // this is null if an ASP error
            if (exception == null)
            {
                exception = new HttpException((int)Response.StatusCode, HttpWorkerRequest.GetStatusDescription(Response.StatusCode));
            }
            HandleRequestError(exception); // code shared with Application_Error
        }
    }
}

I also made a minor change in my common Error Handling. As we are using ASP.NET MVC, I wanted to explicitly call into a controller, as well as pass the exception object. This allowed me to access the exception itself so that I can log / send detailed e-mail depending on the code;

public ActionResult ServerError(Exception exception)
{
    HttpException httpException = exception as HttpException;
    if(httpException != null)
    {
        switch (httpException.GetHttpCode())
        {
            case 403:
            case 404:
                Response.StatusCode = 404;
                break;
        }
        // no email...
        return View("HttpError", httpException);
    }
    SendExceptionMail(exception);
    Response.StatusCode = 500;
    return View("ServerError", exception);
}

In order to pass the exception OBJECT (not just the message and code) I explicitly invoked the controller:

protected void HandleRequestError(Exception exception)
{
    if (Context.Items.Contains("HasHandledAnError"))
    {
        // already processed
        return;
    }
    // mark as processed.
    this.Context.Items.Add("HasHandledAnError", true);

    CustomErrorsSection customErrorsSection = WebConfigurationManager.GetWebApplicationSection("system.web/customErrors") as CustomErrorsSection;

    // Do not show the custom errors if
    // a) CustomErrors mode == "off" or not set.
    // b) Mode == RemoteOnly and we are on our local development machine.
    if (customErrorsSection == null || !Context.IsCustomErrorEnabled ||
        (customErrorsSection.Mode == CustomErrorsMode.RemoteOnly && Request.IsLocal))
    {
        return;
    }

    int httpStatusCode = 500;   // by default.
    HttpException httpException = exception as HttpException;
    if (httpException != null)
    {
        httpStatusCode = httpException.GetHttpCode();
    }

    string viewPath = customErrorsSection.DefaultRedirect;
    if (customErrorsSection.Errors != null)
    {
        CustomError customError = customErrorsSection.Errors[((int)httpStatusCode).ToString()];
        if (customError != null && string.IsNullOrEmpty(customError.Redirect))
        {
            viewPath = customError.Redirect;
        }
    }

    if (string.IsNullOrEmpty(viewPath))
    {
        return;
    }

    Response.Clear();
    Server.ClearError();

    var httpContextMock = new HttpContextWrapper(Context);
    httpContextMock.RewritePath(viewPath);
    RouteData routeData = RouteTable.Routes.GetRouteData(httpContextMock);
    if (routeData == null)
    {
        throw new InvalidOperationException(String.Format("Did not find custom view with the name '{0}'", viewPath));
    }
    string controllerName = routeData.Values["controller"] as string;
    if (String.IsNullOrEmpty(controllerName))
    {
        throw new InvalidOperationException(String.Format("No Controller was found for route '{0}'", viewPath));
    }
    routeData.Values["exception"] = exception;

    Response.TrySkipIisCustomErrors = true;
    RequestContext requestContext = new RequestContext(httpContextMock, routeData);
    IControllerFactory factory = ControllerBuilder.Current.GetControllerFactory();
    IController errorsController = factory.CreateController(requestContext, controllerName);
    errorsController.Execute(requestContext);
}