CODEWITHSUNDEEP
c#silverlightxamlsilverlight-4.0
StephenT
StephenT

Reputation: 819

Preventing Caching of Specific Images in Silverlight

It recently became apparent that my project is caching images. This is an issue because the user can upload a new image which does not get reflected until the browser is closed and reloaded (atleast when debugging in IE). I would like to not have to keep re-downloading images over and over again for things that have not changed, as that would very much increase the data we are sending out.

I have tried a couple solutions here and here2

The common factor seems to be that the variable that displays starts clean. But neither of those has worked for me.

I essentially am displaying images in two different ways.

1) I take a string and pass it into the source of an <Image />

2) I turn a string into a URI and turn that into a bitmap behind the scenes which then gets passed into the source of an <Image />

When the image gets updated server side the location of the user's image stays the same, only the data changes.

The coder doing server side stuff attempted a solution as well. He said he implemented some Cache preventing headers, the result was that the first time the image is requested after it has been updated it retrieves a new image and displays it. Any other places the image would be displayed do not get updated however.

I guess my ideal solution would be that once the user uploads the new image I implement something that notifies anyone that uses that particular URI to grab a new version.

Does anyone know how to selectively stop caching?

Upvotes: 1

Views: 455

Answers (2)

AnthonyWJones
AnthonyWJones

Reputation: 189555

First lets clear up the somewhat ambiguous term "caching".

We do all sorts of caching all the time. Whenever we take the result of an expensive operation and store that result for future use to avoid repeating the expensive operation we are in effect "caching". All frameworks including Silverlight will also be doing that sort of thing a lot.

However whenever the term "caching" is used in the context of a Web based application and refering to a resource fetched using HTTP the specific HTTP cache specification is what comes to mind. This is not unreasonable HTTP caches obviously play a major role and getting the response header settings on server right is important for correct operation.

An often missed aspect though of HTTP resource caching is that the reponsiblity to honor cache headers only lies with the HTTP stack itself, it does not lie with application using HTTP to even know anything about caching.

If then the application chooses to maintain its own "cache" of URIs to resources requested from the HTTP stack, it is not required to implement HTTP compliant caching algorithms. If such a "cache" is asked to provide a specific application object matching a specified Uri it is entirely free to do so without reference to HTTP.

If the HTTP caching were the only cache to worry about then assuming your "server coder" has actually got the cache headers set correctly then all should be well. However there may still be an application layer cache involved as well.

Ulitmately Robs suggestion makes sense in this case where you "version" the uri with a query string value. However its not about preventing caching, caching both at application and http level is a good thing, its about ensuring the resource referenced by the full Uri always the desired content.

Upvotes: 1

Rob
Rob

Reputation: 4753

I would try append a timestamp to the Uri of the image you are requesting, this should help stop the browser (or any proxies) caching

e.g. http://www.example.com/myimage.jpg?ts=2011081107333454

Upvotes: 2

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