C # Preprocessor direktiver

I denne vejledning lærer vi om præprocessordirektiver, tilgængelige direktiver i C #, og hvornår, hvorfor og hvordan hvorfor de bruges.

Som navnet retfærdiggør, er forprocessordirektiver en blok med udsagn, der behandles, inden den faktiske kompilering starter. C # preprocessor-direktiver er kommandoer til compileren, der påvirker kompileringsprocessen.

Disse kommandoer specificerer, hvilke sektioner af koden der skal kompileres, eller hvordan man håndterer specifikke fejl og advarsler.

C # preprocessor-direktivet begynder med et # (hash)symbol, og alle preprocessor-direktiver varer i en linje. Preprocessor-direktiver opsiges af new linesnarere end semicolon.

De forprocessordirektiver, der findes i C #, er:

Forbehandlingsdirektiver i C #
Preprocessor-direktivet Beskrivelse Syntaks
#if Kontrollerer, om et præprocessorudtryk er sandt eller ej 
 #if preprocessor-expression-kode til at kompilere #endif
#elif Bruges sammen med #iftil at kontrollere flere præprocessorudtryk 
 #if preprocessor-expression-1 kode for at kompilere #elif preprocessor-expression-2 kode for at kompilere #endif
#else Brugt sammen med #iftil at skabe sammensatte betingede direktiv. 
 #if forprocessor-ekspressionskode for at kompilere #elif-kode for at kompilere #endif
#endif Bruges sammen med for #ifat indikere afslutningen på et betinget direktiv 
 #if preprocessor-expression-kode til at kompilere #endif
#define Bruges til at definere et symbol 
 #definer SYMBOL
#undef Bruges til at definere et symbol 
 #undef SYMBOL
#warning Tillader os at generere niveau 1 advarsel ud fra vores kode 
 # advarselsmeddelelse
#error Tillader os at generere fejl fra vores kode 
 #fejl-fejlmeddelelse
#line Tillader os at ændre kompilatorens linjenummer og filnavn for at vise fejl og advarsler 
 #linie-nummer filnavn
#region Tillader os at oprette en region, der kan udvides eller skjules, når vi bruger en Visual Studio Code Editor 
 #region region-beskrivelse koder #endregion
#endregion Angiver slutningen af ​​en region 
 #region region-beskrivelse koder #endregion
#pragma Giver kompilatoren specielle instruktioner til kompilering af den fil, hvor den vises. 
 #pragma pragma-navn pragma-argumenter

#definer direktiv

  • Den #defineDirektivet giver os mulighed for at definere et symbol.
  • Symboler, der defineres, når de bruges sammen med #ifdirektivet, evalueres til sandt.
  • Disse symboler kan bruges til at specificere betingelser for kompilering.
  • Syntaks: 
     #definer SYMBOL
  • For eksempel: 
     #definer TESTING
    Her er TESTING et symbol.

#undef-direktivet

  • Den #undefDirektivet giver os mulighed for at undefine et symbol.
  • Udefinerede symboler, når de bruges sammen med #ifdirektivet, evalueres til falske.
  • Syntaks: 
     #undef SYMBOL
  • For eksempel: 
     #undef TESTING
    Her er TESTING et symbol.

# hvis direktiv

  • Den #ifDirektivet anvendes til at teste præprocessoren udtryk.
  • Et præprocessorudtryk kan kun bestå af et symbol eller en kombination af symboler sammen med operatorer som &&(AND), ||(OR), !(NOT).
  • #ifefterfølges af et #endifdirektiv.
  • Koderne inde i #ifdirektivet kompileres kun, hvis det testede udtryk #ifvurderes til sandt.
  • Syntaks: 
     #if preprocessor-expression-kode til at kompilere <#endif
  • For eksempel: 
    #if TESTING Console.WriteLine ("Testes i øjeblikket"); #Afslut Hvis

Eksempel 1: Hvordan bruges # if direktivet?

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

Når vi kører programmet, vil output være:

 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-navn pragma-argumenter
  • For eksempel: 
     #pragma advarsel deaktiveret

Eksempel 7: Hvordan bruges #pragma-direktivet?

 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 ) ) )

Når vi kører programmet, vil output være:

 Program.cs (12,22): advarsel CS1030: #advarsel: 'Dette er en advarsel 2' (/home/myuser/csharp/directive-project/directive-project.csproj)

Vi kan se, at kun den anden advarsel vises på outputskærmen.

Dette skyldes, at vi oprindeligt deaktiverede alle advarsler før den første advarsel og kun gendannede dem før den anden advarsel. Dette er grunden til, at den første advarsel blev skjult.

Vi kan også deaktivere specifik advarsel i stedet for al advarsel.

For at lære mere om #pragma, besøg #pragma (C # reference).

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