Question

I want to be able to generate IIS’s standard 404 response, not a custom 404 page, in response to an error condition in a code-behind class. My Page_Load method looks like this:

protected void Page_Load(object sender, System.EventArgs e)
{
    this.a = ...
    ...
    if (this.a == null) ... // *** generate a 404
}

So if a is not null the .aspx file renders, etc, but if a is null I want the standard “The resource cannot be found” page to be shown with a 404 response code.

If I use:

if (a == null) Response.StatusCode = 404;

(which is what seems to be the correct method according to what I can find) the page continues to render (and gives a NullReferenceException with a response code of 500 when it tries to use a).

If I use:

if (a == null) throw new HttpException(404, "Not found");

The response code is 404 but the page content is ASP.NET’s unhandled exception page showing the HttpException (which is then shown as the generic “Runtime Error” when customErrors is On).

I want to show IIS’s 404 page because the users will understand that, but they probably won’t check the response code of the page if it shows a server error.

EDIT: It looks like I can’t do this exactly. Given @Smudge202’s answer I let my code-behind throw the HttpException and added code to Global.Application_Error() to handle those exceptions:

void Application_Error(object sender, EventArgs e)
{
var exception = Server.GetLastError();

    if (exception is HttpException)
    {
        var httpException = (HttpException)exception;
        if (httpException.GetHttpCode() == 404)
        {
            var context = HttpContext.Current;
            context.Response.Clear();
            context.Response.StatusCode = 404;
            context.Response.StatusDescription = "Not Found";
            context.Response.Write("<h1>404 Not Found</h1>");
            context.Server.ClearError();
        }
    }
    // ...
}

This lets me set my own response to the exception. This approach has two drawbacks:

• What I really wanted to do was revert control back to IIS and let it display its default 404 response, I can’t find a way to do that
• The new response (written by context.Response.Write()) is shown for every 404, not just ones generated in code

So it looks like I will need to either:

• From the code-behind, redirect to a bespoke page explaining that the ‘thing’ can’t be found (which also sets the 404 response code), or
• Set up a custom global 404 page

Answer 1

I doubt this is a clean solution, but it might work…

If you combine this post (and several similar I’m sure). It seems from global asax writing a 404 out to the response stream bypasses the custom errors.

With that in mind, you could intentionally raise a custom exception in your code behind. Check for that custom exception in Application_Error, and respond with 404 from there? That will prevent the page from rendering, and everything should work as you’ve asked?

EDIT:

If this is shown for every 404 exception and you don’t want it to be, create a custom exception for your original code behind to fire. Only catch that particular exception in Application_Error, allow the others to pass through as normal?

I also found this thread where the OP was trying, like you, to raise the actual 404 page, and not a mock-up of it. There are a couple suggestions on there on how to do it, no idea if any of them work, but probably worth trying quickly.