There are times when you want to build an app targeting different platforms in Windows using the conditional compilation with pre-processor directives.
Cross-Platform Conditional Compilation Symbol for Windows Phone
If you are targeting the Xbox platform, one may use the pre-processor directive XBOX, similarly if you are targeting the Windows Phone platform , you can use the WINDOWS_PHONE pre-processor directive as shown below.
#if WINDOWS_PHONE
#endif
More details about the Conditional compilation with pre-processor directives in Windows Phone 8 and Windows 8 can be found here