#if __IOS__ seems not working

I used this code in my app, but when it was running, it didn't print the text(both in simulator and my device).

#if __IOS__
            System.Diagnostics.Debug.WriteLine ("I'm in iOS");
#endif

The Define Symbols of my project is DEBUG;MOBILE;IOS;.

The same issue occurs in all the solution I created.
I found that the run menu is different between my solution and MobileCRM, just as the following images shows:

• My solution:
• MobileCRM: