Get last-modified header from response in ASP.NET

Published 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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