I've been spending some time recently working on my very first Windows Phone application. As part of the application, I decided to use the WebBrowser control to display content. This worked well until I had image links. I had read that for them to show, they had to be in isolated storage and so did the HTMLID_CAP_NETWORKING capability!" rel="footnote">1, so I spent some time coding that and it appeared to work. However, I didn't want all the images to be a part of my app, I wanted to download them on demand and then store them for later.
I wrote some great code based on the many examples out there that use BitmapImage, HttpWebRequest or WebClient to grab the images, but to no avail. No amount of searching seemed to solve my problem. Every single time I would get a WebException telling me the resource was "Not Found". Not one Bing, Google or divining rod search got me an answer and I was near ready to give up. The Internet and my own abilities had failed me.
Just as I was about to go to bed I had an epiphany; CAPABILITIES! I quickly opened my WMAppManifest.xml and checked the information on MSDN to confirm my suspicion. I had not added the ID_CAP_NETWORKING capability which meant my app was not allowed to download data. A quick change and suddenly, everything worked.
I am amazed that not one search provided me with this answer. Every search around image problems for Windows Phone showed up incorrect advice about ClientAccessPolicy.xml (not necessary for Silverlight on Windows Phone) or terrible code examples that completely misuse disposable items and extension methods. In a future post, I'd like to expand on this topic to provide a more rounded set of samples for downloading images, but for now, I just want to get something out there that helps someone else when they discover this problem.
I highly recommend removing all capabilities from your manifest and then adding them back in as you discover which ones you need – after all, an app that wants less access is more desirable (at least for me, anyway) – however, I now realise that the act of discovering what capabilities you should have can be a bit of a pain in some circumstances.
This advice is not entirely true. If you want your HTML to reference images that are in isolated storage, your HTML also needs to be loaded from isolated storage. If the images are online somewhere, your HTML can come from anywhere, just make sure you've added the [crayon-5a1e98e1a730a889795657-i/] capability! ↩