Welcome to Windows app development! Here's a breakdown of compatibility for different Windows versions and some guidance on how to approach development.

App Compatibility Across Windows Versions

Microsoft has had different development frameworks for its platforms, and compatibility depends on which one you are using:

1. Windows Phone 8.0 Apps on Other Versions

• ✅ Windows Phone 7.8 – Not fully compatible. While some APIs may work, Windows Phone 8 introduced new capabilities (e.g., native code support, Direct3D) that are not backward compatible. You would need to target Windows Phone 7.x separately.

• ❌ Windows RT – Not compatible. Windows RT (for ARM-based tablets) runs apps built for Windows Store (Metro/Modern UI apps), not Windows Phone.

• ❌ Windows 8 / 8.1 – Not directly compatible. Windows 8 and 8.1 use WinRT (Windows Runtime), which is different from Windows Phone 8 SDK. However, you can share code if you create a Universal App for both platforms.

2. Windows 8 / 8.1 and Windows RT Apps on Other Versions

• ✅ Windows RT apps work on Windows 8/8.1 but not on Windows Phone without modification.

• ❌ Windows Phone apps won’t run on Windows 8/8.1 or RT without adaptation.

What You Need to Do

If you want your app to run across multiple platforms, consider these approaches:

1. Use Windows Phone 7.x SDK (if targeting WP7.8)

   • If your app must support Windows Phone 7.8, use the Windows Phone SDK 7.1 (not 8.0).

   • However, WP7.8 is very outdated, and it’s better to focus on newer versions.

2. Develop a Universal Windows App (for Windows 8.1 and Windows Phone 8.1)

   • If you want to support both Windows Phone 8.1 and Windows 8.1, use the Universal Windows App framework.

   • This lets you share a common codebase while keeping platform-specific optimizations.

3. Target UWP (Universal Windows Platform) for Future-Proofing

   • If possible, migrate to UWP (Windows 10 and later), which supports desktops, tablets, and phones under one framework.