Preprocessor directives in C#

Question

In C#, if preprocessor directives are instructions pre-processed before actual compilation then why is it not executed first in this program?

static void Main(string[] args) { Program1.display(); Program2 p2 = new Program2(); p2.show(); #if DEBUG Console.WriteLine("DEBUG from preprocessor directive is working!"); #endif }

Expected Output:

DEBUG from preprocessor directive is working! .......(from display()) .......(from show())

But Actual Output:

.......(from display()) .......(from show()) DEBUG from preprocessor directive is working!

Preprocessor directives are instructions to the compiler about how to compile the program. They do not have any bearing on code execution order. — Glorin Oakenfoot
– Glorin Oakenfoot, Commented Oct 20, 2016 at 16:47

Shreevardhan · Accepted Answer · 2016-10-20 17:04:47Z

The output you are expecting is wrong.

Code processed (to be compiled) in DEBUG mode/configuration

static void Main(string[] args) { Program1.display(); Program2 p2 = new Program2(); p2.show(); Console.WriteLine("DEBUG from preprocessor directive is working!"); }

Code processed (to be compiled) in non-DEBUG mode/configuration

static void Main(string[] args) { Program1.display(); Program2 p2 = new Program2(); p2.show(); }

Hope this clears your confusion that preprocessors don't decide execution order.

Hamid Pourjam · Accepted Answer · 2016-10-20 16:49:20Z

C# Language Specification, Section2.5

The pre-processing directives provide the ability to conditionally skip sections of source files, to report error and warning conditions, and to delineate distinct regions of source code. The term “pre-processing directives” is used only for consistency with the C and C++ programming languages. In C#, there is no separate pre-processing step; pre-processing directives are processed as part of the lexical analysis phase

Pre-processing directives are not tokens and are not part of the syntactic grammar of C#. However, pre-processing directives can be used to include or exclude sequences of tokens and can in that way affect the meaning of a C# program

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