How to use #define inside of #if?

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I needed to define a preprocessor directive, that abstracts some conditions.

I want to detect platform File I/O support.

So i want to define a preprocessor directive like FILE_IO to detect does this platforms supports or not.

I needed a preprocessor directive because i can't use File api on unsupported platforms, so i need to stop using them at compile time.

Take a look the below script.

How can i achieve this?

Any help is appreciated.

What I have tried:

C#

// It won't work
#define FILE_IO = !SAMSUNG_TV && !WEBGL

// OR

#if !SAMSUNG_TV && !WEBGL

// It get defined anyway
#define FILE_IO

#endif

Posted 24-Jul-17 2:40am

EmpireWorld

Updated 24-Jul-17 3:02am

v2

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[no name] 24-Jul-17 8:57am

a.) The first one is wrong. See e.g. #if (C# Reference) | Microsoft Docs[^],

Unlike C and C++, you cannot assign a numeric value to a symbol; the #if statement in C# is Boolean and only tests whether the symbol has been defined or not

b.) The second one Looks pretty ok. Maybe you should first make sure that it is undefined:
#undef FILE_IO
#if !SAMSUNG_TC && !WEBGL ....

EmpireWorld 24-Jul-17 8:59am

Can i assign boolean value to #define?

Can i use #define inside of #if?

Dave Kreskowiak 24-Jul-17 9:07am

#define directives have no value at all. You can only check if they are defined or not.

[no name] 24-Jul-17 9:10am

In c# you control it whether a Symbol exists or not. There is no Need to assign a bool or any other value.

What I know you can use #define inside an #if.

In your case I guess, that FILE_IO is allready defined, maybe not by you. Therefore make sure it is undefined before you make your tests:

#undef FILE_IO
#if !SAMSUNG_TV && !WEBGL
#define FILE_IO
#endif

... should do it

EmpireWorld 24-Jul-17 9:13am

FILE_IO isn't defined, i want to define it in my own code.

Let me try the #define inside #if.

[no name] 24-Jul-17 9:20am

You wrote "It get defined anyway". If it is not by your "#if !SAMSUNG_TV && !WEBGL" then it most probably it is defined by another part of code you are using...

EmpireWorld 24-Jul-17 9:28am

I meant the #define defines the directive anyway, when it is inside a #if is statement or not.

[no name] 24-Jul-17 9:32am

I don't understand. Anyway, does the second Version works or not (maybe with an #undef as first)? From my Point of view it should.

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Satya Prakash Swain · Answer 1 · 2017-07-24T03:02:00

Solution 3

#define SAMSUNG_TC
#define WEBGL
#if !SAMSUNG_TC && !WEBGL
#define FILE_IO
FILE_IO = !SAMSUNG_TC && !WEBGL;
#endif

Posted 24-Jul-17 3:02am

Satya Prakash Swain

Comments

EmpireWorld 24-Jul-17 9:15am

The line FILE_IO = !SAMSUNG_TC && !WEBGL; does not works.

[no name] 24-Jul-17 9:27am

Yes it does not work. Again a link to the documentation of MS which is worth to read it: #if (C# Reference) | Microsoft Docs[^]