browser caching

In regards to forcing a browser to download a new copy of a file that it would typically cache, would adding a parameter to the url have the same effectiveness as changing the filename itself? ie: file.css?v=123 vs file123.css
nzinsliAsked:
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Dave BaldwinFixer of ProblemsCommented:
That works if it constantly changes.  The problem of course is getting the browser to start downloading that version in the first place.  As long as the original pages are still cached with the original file name, the browser will still keep using those versions until they expire from the local cache.
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nzinsliAuthor Commented:
The browser shouldn't use the cached file though if the html is calling for a new filename in the code tho, correct? I was just curious as to if one method was preferred over another.
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Dave BaldwinFixer of ProblemsCommented:
The problem usually is that the HTML file is cached and the new file name for the CSS doesn't get picked up until the HTML file in the cache expires.
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nzinsliAuthor Commented:
While that may be true, I can't say I've ever had a problem with the html being cached or not showing the new version when loading. Wouldn't the browser look at the last modified date on the server version before rendering the cached html? Include files on the other hand...is where my question lies. If you suppose that it is not loading a cached html, is there any difference for caching if I call an included css file with file.css?v=123 or file-123.css?
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Dave BaldwinFixer of ProblemsCommented:
"file.css?v=123 or file-123.css"  No difference.  If the link is "file.css?v=124" next time, then there is.  It has appeared that some times the browser only looks at the last-modified date of the HTML file and if it hasn't changed, then it loads CSS and javascript from cache.  I think that maybe that has improved in the last year or two now that you mention it because it hasn't been as much of a problem for me recently.  But I've also upgraded my web server twice in the last year.
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nzinsliAuthor Commented:
"If the link is "file.css?v=124" next time, then there is." ...If the version # changes, then there is a difference??

I'm glad you brought up modified date. The reason this all came about, is I'm going to be dynamically building the file include list via literals in vb.net. The version # is going to be read from a xml file that gets updated when one of these files is updated, so the user is more assured to get the latest code w/o refreshing. But, since the actual html file isnt being edited & saved, the modified date wont change. I wonder if this will cause an issue...hmmm...
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Dave BaldwinFixer of ProblemsCommented:
The browser cache is done by the complete URL including the query string.  Adding the current time is a standard technique in AJAX to prevent the browser from just returning info from it's cache.

Look here for info on preventing caching in IIS: http://www.iis.net/ConfigReference/system.webServer/staticContent/clientCache
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