The Vala preprocessor is a particular part of Vala that acts at syntax level only, allowing you to conditionally write pieces of your software depending upon certain compile-time conditions. Preprocessor directives will never be generated in the resulting code.
All preprocessor directives start with a hash (#), except for the first line of a file starting with #! (used for Vala scripts).
vala-code: [ any vala code ] [ pp-condition ] [ any vala code ] pp-condition: #if pp-expression vala-code [ pp-elif ] [ pp-else ] #endif pp-elif: #elif pp-expression vala-code [ pp-elif ] pp-else: #else vala-code pp-expression: pp-or-expression pp-or-expression: pp-and-expression [ || pp-and-expression ] pp-and-expression: pp-binary-expression [ && pp-binary-expression ] pp-binary-expression: pp-equality-expression pp-inequality-expression pp-equality-expression: pp-unary-expression [ == pp-unary-expression ] pp-inequality-expression: pp-unary-expression [ != pp-unary-expression ] pp-unary-expression: pp-negation-expression pp-primary-expression pp-negation-expression: ! pp-unary-expression pp-primary-expression: pp-symbol ( pp-expression ) true false pp-symbol: identifier
The semantics of the preprocessor are very simple: if the condition is true then the Vala code surrounded by the preprocessor will be parsed, otherwise it will be ignored. A symbol evaluates to true if it is defined at compile-time. If a symbol in a preprocessor directive is not defined, it evaluates to false.
It's not possible to define a preprocessor symbol inside the Vala code (like with C). The only way to define a symbol is to feed it through the valac option -D. 
| Name | Description | 
| POSIX | Set if the profile is posix | 
| GOBJECT | Set if the profile is gobject | 
| DOVA | Set if the profile is dova | 
| VALA_X_Y | Set if Vala API version is equal or higher to version X.Y | 
| DBUS_GLIB | Set if using dbus-glib-1 package | 
How to conditionally compile code based on a valac option -D. 
Sample code:
// Vala preprocessor example public class Preprocessor : Object { public Preprocessor () { } /* public instance method */ public void run () { #if PREPROCESSOR_DEBUG // Use "-D PREPROCESSOR_DEBUG" to run this code path stdout.printf ("debug version \n"); #else // Normally, we run this code path stdout.printf ("production version \n"); #endif } /* application entry point */ public static int main (string[] args) { var sample = new Preprocessor (); sample.run (); return 0; } }
Normal build/run:
$ valac -o preprocessor Preprocessor.vala $ ./preprocessor
Debug build/run:
$ valac -D PREPROCESSOR_DEBUG -o preprocessor-debug Preprocessor.vala $ ./preprocessor-debug