There are generally 3 places that need to be set (otherwise the popup value cannot be null when you create a new item name: Path1. See also: http://www.cnblogs.com/fang8206/p/5020942.html)
1.tools->options Select Cross Platform, set up the JDK and Android SDK
2.tools->options select Xamarin->, set up JDK and Android SDK
3. Add a key named "Adbpath" under "Hkcu\software\microsoft\visualstudioemulator\android" in the registry, and the value is Android The Platform-tools Absolute Path (D:\Android\android-sdk\platform-tools) of the SDK. For details, see:
Visual Studio 2015 development Android app startup debugging always fails to complete application deployment solution
Otherwise the VS comes with Android emulator is already running, but the project build APK cannot be installed to the emulator. The reason is that the VS simulator searches the path of the above registry for Adb.exe, and then installs the APK by calling the ADB command.
Note: JDK requires more than 1.8
VS. Android Environment settings