Get last-modified header from response in ASP.NET

May 21, 2009

I’ve been working lately with some ASP.NET performance optimization automation HTTP modules. In one of them I needed to know if the last-modified header had been set through the Response.Cache.SetLastModified(DateTime) method. For some reason, there is no API available anywhere within the BCL to retrieve the last modified date of a response – you can only set it.

Since the module wouldn’t work without a way to read the last modified date of the response, I had to use Reflector to figure out how to pull the information out using reflection. The result became a simple little method to retrieve the date. It looks like this:

private static DateTime ReadLastModifiedFromResponse(HttpCachePolicy cache)
{
  BindingFlags flags = BindingFlags.NonPublic | BindingFlags.GetField | BindingFlags.Instance;
  return (DateTime)cache.GetType().GetField("_utcLastModified", flags).GetValue(cache);
}

And you can use it like this:

DateTime responseModified = ReadLastModifiedFromResponse(Response.Cache);
 
if (responseModified > DateTime.MinValue)
{
  // Last-modified is set. Do something...
}

If you know of another way to retrieving the last-modified date, please let me know.

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Comments (10) -

gOODiDEA

5/22/2009 6:56:03 PM #

try

    public static bool IsClientCached( HttpContext context, DateTime contentModified )
    {
      string clientCached = context.Request.Headers ["If-Modified-Since"];
      if ( clientCached != null )
      {
        DateTime isModifiedSince;
        if ( DateTime.TryParse ( clientCached, out isModifiedSince ) )
        {
          return isModifiedSince > contentModified;
        }
      }
      return false;
    }

Mads Kristensen

5/23/2009 4:32:08 PM #

Your method looks in the browser's cache instead of the actual response. It's good for conditional GETs, but it doesn't examine the actual response to retrieve the last-modified date.

Melayu Boleh

5/26/2009 12:58:34 AM #

i want to test your method. It look interesting. i will share here again after i test it.

codeclue

5/29/2009 4:43:08 AM #

just tried its and its working,, i dont know if there is other ways but i think there is

9yr

6/1/2009 6:44:23 PM #

i have tried the method but its not working, i dont know what the problem is

Sam

6/2/2009 3:12:12 AM #

Have you looked at HttpWorkerRequest? Sometimes you can use this for low-level stuff. It has a GetKnownRequestHeader method. It also does some stuff with the response. I use it for getting and setting headers a lot.

tummy tuck surgery

6/6/2009 5:43:31 AM #

Sam, HttpWorkerRequest is a bit more complicated stuff. I like more a variant of M. Kristensen. Cute and goodworking.

Chris Eargle

6/7/2009 8:10:34 PM #

I don't see a better way to get it. I guess it's a good thing that reflection is available to break encapsulation when necessary.

AB Web Design, LLC

7/23/2009 10:23:12 PM #

Thanks Mad,

I was actually looking for a way to accomplish this a while back...never did find an easy way to do it. I will try this method out.

Chris