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Asked: 2026-05-24T03:34:28+00:00

I make an Ajax request in which I set the response cacheability and last

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I make an Ajax request in which I set the response cacheability and last modified headers:

if (!String.IsNullOrEmpty(HttpContext.Current.Request.Headers["If-Modified-Since"]))
{
    HttpContext.Current.Response.StatusCode = 304;
    HttpContext.Current.Response.StatusDescription = "Not Modified";
    return null;
}
HttpContext.Current.Response.Cache.SetCacheability(HttpCacheability.Public);
HttpContext.Current.Response.Cache.SetLastModified(DateTime.UtcNow);

This works as expected. The first time I make the Ajax request, I get 200 OK. The second time I get 304 Not Modified.

When I hard refresh in Chrome (Ctrl+F5), I get 200 OK – fantastic!

When I hard refresh in Internet Explorer/Firefox, I get 304 Not Modified. However, every other resource (JS/CSS/HTML/PNG) returns 200 OK.

The reason is because the “If-Not-Modified” header is sent for XMLHttpRequest’s regardless of hard refresh in those browsers. I believe Steve Souders documents it here.

I have tried setting an ETag and conditioning on “If-None-Match” to no avail (it was mentioned in the comments on Steve Souders page).

Has anyone got any gems of wisdom here?

Thanks,
Ben

Update

I could check the “If-Modified-Since” against a stored last modified date. However, hopefully this question will help other SO users who find the header to be set incorrectly.

Update 2

Whilst the request is sent with the “If-Modified-Since” header each time. Internet Explorer won’t even make the request if an expiry isn’t set or is set to a future date. Useless!

Update 3

This might as well be a live blog now. Internet Explorer doesn’t bother making the second request when localhost. Using a real IP or the loopback will work.

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1 Answer

  1. Editorial Team

    Prior to IE10, IE does not apply the Refresh Flags (see http://blogs.msdn.com/b/ieinternals/archive/2010/07/08/technical-information-about-conditional-http-requests-and-the-refresh-button.aspx) to requests that are not made as a part of loading of the document.

    If you want, you can adjust the target URL to contain a nonce to prevent the cached copy from satisfying a future request. Alternatively, you can send max-age=0 to force IE to conditionally revalidate the resource before each reuse.

    As for why the browser reuses a cached resource that didn’t specify a lifetime, please see http://blogs.msdn.com/b/ie/archive/2010/07/14/caching-improvements-in-internet-explorer-9.aspx

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