Question

In our Asp.Net Application we have an IHttpHandler handling requests for Images.
The Handler is called with a special id which identifies an image in our Image Cache.
Once an image is put into the cash it never changes.
My question is:

Is it okay to always return the http status code 304 for requests with a “If-Modified-Since” header without actually checking the given date?
The reasoning is that the browser must already have a copy of the image (since it provided the modified-since header).

This would make life easier, because we do not (yet) track the image creation date.

Here is the actual code (Update: I included the if-modified header now in the server response as recommended by Aristos):

public void ProcessRequest(HttpContext context)
{
    if (!String.IsNullOrEmpty(context.Request.Headers["If-Modified-Since"])) {
        //Is this okay?
        context.Response.StatusCode = 304;
        context.Response.StatusDescription = "Not Modified";
        return;
    }
    var thumbnailId = context.Request.QueryString["thumbnail"];
    using (var thumbnailCache = new CThumbnailCache()) {
        var imageBytes = thumbnailCache.GetImageById(thumbnailId);

        context.Response.ContentType = "image/png";
        var outputStream = context.Response.OutputStream;
        outputStream.Write(imageBytes, 0, imageBytes.Count());
        context.Response.Cache.SetCacheability(HttpCacheability.Public);
        context.Response.Cache.SetLastModified(DateTime.UtcNow);
        // added after Aristos post
        context.Response.AddHeader("If-Modified-Since", DateTime.UtcNow.ToString("r"));

        const int maxAge = 86400 * 14; // 14 Tage
        context.Response.Cache.SetExpires(DateTime.Now.AddSeconds(maxAge));
        context.Response.Cache.SetMaxAge(new TimeSpan(0, 0, maxAge));
        context.Response.CacheControl = "private";
        context.Response.Cache.SetValidUntilExpires(true);

    }
}

Answer 1

its ok, from the moment you get the If-Modified-Since the resource is on browser and you decide if you let it be as it is.

I like here to say you two thinks. You do not seems that you set this header so you probably not get it by the browser.

To make this actual works you need to add this line

context.Response.AddHeader("If-Modified-Since", LastModifledOfImage.ToString()); 

or as you do send the current date-time:

context.Response.AddHeader("If-Modified-Since", DateTime.UtcNow.ToString());

when you send the image.
I see that you use the SetLastModified but this is set the Last-Modified header not the one you check.

Consider to make actually static this images if possible because even this If-Modified-Since make one call to the server, but the actually static resource gets the image direct from the browser cache with out asking the server.