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I just saw some examples about implementing the if-modified-since header. I really don't understand how it works very well. I have seen this example and it seems that if I set a date value to the header "Last-Modified", then I will get a value for the header "If-Modified-Since" if I check it later, am I right?

Also, It seems that, even though he provides a helper, I have to use it in every action?

I also looked into this post. He created a filter to handle the "If-Modified-Since" header. However, I don't understand much what he is trying to do in there, but as I can see, if the "Last-Modified" and "If-Modified-Since" headers are null then it will read it as dateModified = true and then it won't do anything inside the filter. Therefore, I think it depends on setting those header values in every action?

FInally,I'm not so sure about how should I use it, I think that I should send a new "Last-Modified" date value whenever I make an update to the view, is this the right aproach?

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Morgan Soren
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2 Answers2

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I came into your question and you actually helped me resolving the same problem you were having.

I'm going to explain you what happens with the header and what I've done to resolve it in C#:

  • You specify the Last-Modified as a server response.

    In C# this is done by overriding the OnActionExecuting function on the controller you want. As an example, this is how I've done it:

        protected override void OnActionExecuting(ActionExecutingContext filterContext)
{
    filterContext.RequestContext.HttpContext.Response.AppendHeader("Last-Modified", "Wed, 01 Sep 2004 13:24:52 GMT");
    base.OnActionExecuting(filterContext);
}

  • Like you see I've specified this into the controller, so it will have effect for every action into the controller

  • Now, the first time you visit the page, you will get a response header like Last-Modified:Wed, 01 Sep 2004 13:24:52 GMT. Next time you visit the page, you should see in your request header this If-Modified-Since:Wed, 01 Sep 2004 13:24:52 GMT

Hope this helps

lpaloub
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  • god, i asked this question a million years ago. It took me a while to understand what i was asking myself. Anyway, I understand your answer, it seems simpler now than then. – Morgan Soren May 25 '13 at 04:56
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According RFC 2616 "The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last modified." For exemple: If your view is about an post blog, you can send the date of last edit or modification. Often it is used for cache.

About If-Modified-Since, this header allows that webserver sends 304 status instead 200 with body. It's means: "This webpage has not changed, no need to download again.".

More Info

Jonathan Simon Prates
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  • so that means I should set the Last-Modified header whenever the entity related to this view or bounded has changed? for example, lets say a new post appears in that blog, that means the quantity of posts in that blog have changed; therefore, should I send a new date for the last-modified header? I also heard that not all browsers support the If-Modified-Since header, but I dont know if thats true. – Morgan Soren Aug 14 '12 at 23:44
  • About your exemple, yes. The page changed, so you must send a new "Last-Modified" value. If-Modified-Since is a request-header and all major browsers support and send it. – Jonathan Simon Prates Aug 15 '12 at 02:30
  • @MorganSoren please let me know if answer was useful. – Jonathan Simon Prates Aug 15 '12 at 17:48
  • My biggest concern is in how I should implement it. For the posts given in my question, it seems that I can programatically add the If-Modified-Since response header in every action. However, I don't know if that is the best approach, I guess I could mark an answer which tells me that I must send those headers in most of my action methods (if I want to use it)or recommend me a workaround on that. – Morgan Soren Aug 16 '12 at 17:28