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What is the compatibility between a WinRT class library and Windows Phone 8?
I’m designing a windows store app for desktop/tablets and WP8. The app interface will be the same for desktop/tablets, but different for WP8. Would you suggest I separate this into two projects or do everything in a single windows store app project?
You need to separate. A Windows store app don’t work on windows phone 8. You can share parts of code between win8 & wp8 using portable class libraries and winrt components, but need to do a project for windows store app and other for windows phone app.