Question

Will applications working on wp7.5 work on wp8 also?

Answer 1

Yes, WP 7, WP 7.5 apps will continue to work on WP8, including apps using XNA framework. Although WP8 apps won’t work on older platforms (which is expected).

Any of the existing WP devices will NOT receive the WP8 update. They will get the WP 7.8 update, which include the new start screen and ‘user experience’ but nothing else.

If you write an app targeting WP7.5, it will work on all devices.

UPDATE from BUILD 2012:

• Windows Phone 8 SDK is now publicly available.
• All your existing apps targeting WP 7 or WP 7.5 will ‘just work’
• You can update your apps to WP8 platform the same way you’ve used to upgrade WP 7 to WP 7.5, although this will make your app only work with WP8 devices. (AFAIK. can’t confirm this.)

If you mark your app as a WP8 app, it will not work on WP7 or WP 7.5. Although you can mark the app as WP7.5 app, then you can use reflection to find the APIs you want, and call them (and degrade properly on a WP7.5 device where these APIs won’t be available.)

In my opinion, reflection is messy, but you still have an option to support both using the same code, until WP 7.5 devices fade away. (You may have to take extra care when using reflection, because AFAIK there are some limitations to reflection on WinRT to avoid security and stability issues, which may be still true for WP8+)