I'm planning to implement a trial mode for my app, but just realised that the installation date has to be saved in the isolated storage and compared to the time limit I have set for my trial App. However the user can uninstall the trial App, obviously it removes the isolated storage too and it means they can reinstall the trial app and use it again !
What's the best way to store installation date or any kind of unique id to the phone somewhere other than isolated storage !
Using time trial locally isn't a good idea. One way to do a time trial is to have a Service (in the cloud?) called from the app. But you need to store a unique id for the user, the due date, the...
It can be a pain to maintain.
You should try to use the Trial Mode like in the link Joel provided and activate your (paid) features only when the user has purchased your app.
As the documentation says:
Do not rely on usage time limited trials to protect your application’s value.
Typically, it is best to protect the value of your full mode application by limiting trial access to key code paths. A user may uninstall and retry an application without restriction so a trial design that offers full mode behavior for a limited time provides only inconvenience as a barrier to reuse.
You could try implementing time trial mode using your own server which would store userId with real trial start date, but it is not easy and it would make internet connection necessary, at least when app is started for the first time after installation.
