Directives de préprocesseur C #

Dans ce didacticiel, nous découvrirons les directives de préprocesseur, les directives disponibles en C # et quand, pourquoi et comment elles sont utilisées.

Comme son nom le justifie, les directives du préprocesseur sont un bloc d'instructions qui est traité avant le début de la compilation. Les directives de préprocesseur C # sont les commandes du compilateur qui affectent le processus de compilation.

Ces commandes spécifient les sections du code à compiler ou comment gérer des erreurs et des avertissements spécifiques.

La directive de préprocesseur C # commence par un # (hash)symbole et toutes les directives de préprocesseur durent une ligne. Les directives du préprocesseur se terminent par new lineplutôt que par semicolon.

Les directives de préprocesseur disponibles en C # sont:

Directives de préprocesseur en C #
Directive du préprocesseur La description Syntaxe
#if Vérifie si une expression de préprocesseur est vraie ou non 
 #if code d'expression de préprocesseur à compiler #endif
#elif Utilisé avec #ifpour vérifier plusieurs expressions de préprocesseur 
 #if code preprocessor-expression-1 pour compiler #elif preprocessor-expression-2 code pour compiler #endif
#else Utilisé avec #ifpour créer une directive conditionnelle composée. 
 #if code d'expression de préprocesseur pour compiler le code #elif à compiler #endif
#endif Utilisé avec #ifpour indiquer la fin d'une directive conditionnelle 
 #if code d'expression de préprocesseur à compiler #endif
#define Utilisé pour définir un symbole 
 #define SYMBOLE
#undef Utilisé pour définir un symbole 
 #undef SYMBOLE
#warning Nous permet de générer un avertissement de niveau 1 à partir de notre code 
 #warning warning-message
#error Nous permet de générer des erreurs à partir de notre code 
 #error error-message
#line Nous permet de modifier le numéro de ligne et le nom de fichier du compilateur pour afficher les erreurs et les avertissements 
 #line numéro de ligne nom-fichier
#region Nous permet de créer une région qui peut être développée ou réduite lors de l'utilisation d'un éditeur de code Visual Studio 
 #region region-description codes #endregion
#endregion Indique la fin d'une région 
 #region region-description codes #endregion
#pragma Donne au compilateur des instructions spéciales pour la compilation du fichier dans lequel il apparaît. 
 #pragma pragma-name pragma-arguments

#define directive

  • La #definedirective nous permet de définir un symbole.
  • Les symboles définis lorsqu'ils sont utilisés avec la #ifdirective seront évalués à vrai.
  • Ces symboles peuvent être utilisés pour spécifier les conditions de compilation.
  • Syntaxe: 
     #define SYMBOLE
  • Par exemple: 
     #define TESTING
    Ici, TESTING est un symbole.

#undef directive

  • La #undefdirective nous permet de définir un symbole.
  • Undefined symbols when used along with #if directive will evaluate to false.
  • Syntax: 
     #undef SYMBOL
  • For example: 
     #undef TESTING
    Here, TESTING is a symbol.

#if directive

  • The #if directive are used to test the preprocessor expression.
  • A preprocessor expression may consists of a symbol only or combination of symbols along with operators like && (AND), || (OR), ! (NOT).
  • #if directive is followed by an #endif directive.
  • The codes inside the #if directive is compiled only if the expression tested with #if evaluates to true.
  • Syntax: 
     #if preprocessor-expression code to compile< #endif
  • For example: 
     #if TESTING Console.WriteLine("Currently Testing"); #endif

Example 1: How to use #if directive?

 #define CSHARP using System; namespace Directive ( class ConditionalDirective ( public static void Main(string() args) ( #if (CSHARP) Console.WriteLine("CSHARP is defined"); #endif ) ) )

When we run the program, the output will be:

 CSHARP is defined

In the above program, CSHARP symbol is defined using the #define directive at the beginning of program. Inside the Main() method, #if directive is used to test whether CSHARP is true or not. The block of code inside #if directive is compiled only if CSHARP is defined.

#elif directive

  • The #elif directive is used along with #if directive that lets us create a compound conditional directive.
  • It is used when testing multiple preprocessor expression.
  • The codes inside the #elif directive is compiled only if the expression tested with that #elif evaluates to true.
  • Syntax: 
     #if preprocessor-expression-1 code to compile #elif preprocessor-expression-2 code-to-compile #endif
  • For example: 
     #if TESTING Console.WriteLine("Currently Testing"); #elif TRAINING Console.WriteLine("Currently Training"); #endif

#else directive

  • The #else directive is used along with #if directive.
  • If none of the expression in the preceding #if and #elif (if present) directives are true, the codes inside the #else directive will be compiled.
  • Syntax: 
     #if preprocessor-expression-1 code to compile #elif preprocessor-expression-2 code-to-compile #else code-to-compile #endif
  • For example: 
     #if TESTING Console.WriteLine("Currently Testing"); #elif TRAINING Console.WriteLine("Currently Training"); #else Console.WriteLine("Neither Testing nor Training"); #endif

#endif directive

  • The #endif directive is used along with #if directive to indicate the end of #if directive.
  • Syntax: 
     #if preprocessor-expression-1 code to compile #endif
  • For example: 
     #if TESTING Console.WriteLine("Currently Testing"); #endif

Example 2: How to use conditional directive (if, elif, else, endif) ?

 #define CSHARP #undef PYTHON using System; namespace Directive ( class ConditionalDirective ( static void Main(string() args) ( #if (CSHARP && PYTHON) Console.WriteLine("CSHARP and PYTHON are defined"); #elif (CSHARP && !PYTHON) Console.WriteLine("CSHARP is defined, PYTHON is undefined"); #elif (!CSHARP && PYTHON) Console.WriteLine("PYTHON is defined, CSHARP is undefined"); #else Console.WriteLine("CSHARP and PYTHON are undefined"); #endif ) ) )

When we run the program, the output will be:

 CSHARP is defined, PYTHON is undefined

In this example, we can see the use of #elif and #else directive. These directive are used when there are multiple conditions to be tested. Also, symbols can be combined using logical operators to form a preprocessor expression.

#warning directive

  • The #warning directive allows us to generate a user-defined level one warning from our code.
  • Syntax: 
     #warning warning-message
  • For example: 
     #warning This is a warning message

Example 3: How to use #warning directive?

 using System; namespace Directives ( class WarningDirective ( public static void Main(string() args) ( #if (!CSHARP) #warning CSHARP is undefined #endif Console.WriteLine("#warning directive example"); ) ) )

When we run the program, the output will be:

 Program.cs(10,26): warning CS1030: #warning: 'CSHARP is undefined' (/home/myuser/csharp/directives-project/directives-project.csproj) #warning directive example

After running the above program, we will see the output as above. The text represents a warning message. Here, we are generating a user-defined warning message using the #warning directive.

Note that the statements after the #warning directive are also executed. It means that the #warning directive does not terminate the program but just throws a warning.

#error directive

  • The #error directive allows us to generate a user-defined error from our code.
  • Syntax: 
     #error error-message
  • For example: 
     #error This is an error message

Example 4: How to use #error directive?

 using System; namespace Directive ( class Error ( public static void Main(string() args) ( #if (!CSHARP) #error CSHARP is undefined #endif Console.WriteLine("#error directive example"); ) ) )

When we run the program, the output will be:

 Program.cs(10,24): error CS1029: #error: 'CSHARP is undefined' (/home/myuser/csharp/directives-project/directives-project.csproj) The build failed. Please fix the build errors and run again.

We will see some errors, probably like above. Here we are generating a user-defined error.

Another thing to note here is the program will be terminated and the line #error directive example won't be printed as it was in the #warning directive.

#line directive

  • The #line directive allows us to modify the line number and the filename for errors and warnings.
  • Syntax: 
     #line line-number file-name
  • For example: 
     #line 50 "fakeprogram.cs"

Example 5: How to use #line directive?

 using System; namespace Directive ( class Error ( public static void Main(string() args) ( #line 200 "AnotherProgram.cs" #warning Actual Warning generated by Program.cs on line 10 ) ) )

When we run the program, the output will be:

 AnotherProgram.cs(200,22): warning CS1030: #warning: 'Actual Warning generated by Program.cs on line 10' (/home/myuser/csh arp/directive-project/directive-project.csproj)

We have saved the above example as Program.cs. The warning was actually generated at line 10 by Program.cs. Using the #line directive, we have changed the line number to 200 and the filename to AnotherProgram.cs that generated the error.

#region and #endregion directive

  • The #region directive allows us to create a region that can be expanded or collapsed when using a Visual Studio Code Editor.
  • This directive is simply used to organize the code.
  • The #region block can not overlap with a #if block. However, a #region block can be included within a #if block and a #if block can overlap with a #region block.
  • #endregion directive indicates the end of a #region block.
  • Syntax: 
     #region region-description codes #endregion

Example 6: How to use #region directive?

 using System; namespace Directive ( class Region ( public static void Main(string() args) ( #region Hello Console.WriteLine("Hello"); Console.WriteLine("Hello"); Console.WriteLine("Hello"); Console.WriteLine("Hello"); Console.WriteLine("Hello"); #endregion ) ) )

When we run the program, the output will be:

 Hello Hello Hello Hello Hello

#pragma directive

  • The #pragma directive is used to give the compiler some special instructions for the compilation of the file in which it appears.
  • The instruction may include disabling or enabling some warnings.
  • C# supports two #pragma instructions:
    • #pragma warning: Used for disabling or enabling warnings
    • #pragma checksum: It generates checksums for source files which will be used for debugging.
  • Syntax: 
     #pragma pragma-name pragma-arguments
  • Par exemple: 
     #pragma avertissement désactiver

Exemple 7: Comment utiliser la directive #pragma?

 using System; namespace Directive ( class Error ( public static void Main(string() args) ( #pragma warning disable #warning This is a warning 1 #pragma warning restore #warning This is a warning 2 ) ) )

Lorsque nous exécutons le programme, la sortie sera:

 Program.cs (12,22): avertissement CS1030: #warning: 'Ceci est un avertissement 2' (/home/myuser/csharp/directive-project/directive-project.csproj)

Nous pouvons voir que seul le deuxième avertissement est affiché sur l'écran de sortie.

En effet, nous avons initialement désactivé tous les avertissements avant le premier avertissement et les avons restaurés uniquement avant le deuxième avertissement. C'est la raison pour laquelle le premier avertissement a été masqué.

Nous pouvons également désactiver les avertissements spécifiques au lieu de tous les avertissements.

Pour en savoir plus #pragma, visitez #pragma (référence C #).

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