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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.