C # Preprocessor direktiver

I denne veiledningen lærer vi om Preprocessor Directives, tilgjengelige direktiver i C #, og når, hvorfor og hvordan hvorfor de brukes.

Som navnet rettferdiggjør, er forprosessordirektivene en blokk med utsagn som blir behandlet før selve kompileringen starter. C # preprosessor-direktiver er kommandoene for kompilatoren som påvirker kompileringsprosessen.

Disse kommandoene spesifiserer hvilke deler av koden du skal kompilere eller hvordan du skal håndtere spesifikke feil og advarsler.

C # preprosessor-direktiv begynner med et # (hash)symbol, og alle preprosessor-direktiver varer i en linje. Preprocessor-direktiver avsluttes av new lineheller enn semicolon.

Forprosessordirektivene som er tilgjengelige i C # er:

Forprosessordirektiv i C #
Forprosessordirektivet Beskrivelse Syntaks
#if Sjekker om et preprosessoruttrykk er sant eller ikke 
 #if kode for preprosessoruttrykk for å kompilere #endif
#elif Brukes sammen med for #ifå sjekke flere preprosessoruttrykk 
 #if preprocessor-expression-1 code to compile #elif preprocessor-expression-2 code to compile #endif
#else Brukes sammen med for #ifå lage sammensatt betinget direktiv. 
 #if forprosessor-uttrykkekode for å kompilere #elif-kode for å kompilere #endif
#endif Brukes sammen med for #ifå indikere slutten på et betinget direktiv 
 #if kode for preprosessoruttrykk for å kompilere #endif
#define Brukes til å definere et symbol 
 #definer SYMBOL
#undef Brukes til å definere et symbol 
 #undef SYMBOL
#warning Tillater oss å generere nivå 1 advarsel fra koden vår 
 #advarsel-advarsel
#error Tillater oss å generere feil fra koden vår 
 #error feilmelding
#line Tillater oss å endre kompilatorens linjenummer og filnavn for å vise feil og advarsler 
 #linje linjenummer filnavn
#region Tillater oss å opprette en region som kan utvides eller skjules når vi bruker en Visual Studio Code Editor 
 # region region-description codes #endregion
#endregion Indikerer slutten på en region 
 # region region-description codes #endregion
#pragma Gir kompilatoren spesielle instruksjoner for kompilering av filen der den vises. 
 #pragma pragma-name pragma-argumenter

#definer direktivet

  • Den #defineDirektivet tillater oss å definere et symbol.
  • Symboler som er definert når de brukes sammen med #ifdirektivet, vil evalueres til sant.
  • Disse symbolene kan brukes til å spesifisere betingelser for kompilering.
  • Syntaks: 
     #definer SYMBOL
  • For eksempel: 
     #definer TESTING
    Her er TESTING et symbol.

#undef-direktivet

  • Den #undefDirektivet tillater oss å unde et symbol.
  • Udefinerte symboler når de brukes sammen med #ifdirektivet, blir evaluert til falske.
  • Syntaks: 
     #undef SYMBOL
  • For eksempel: 
     #undef TESTING
    Her er TESTING et symbol.

#hvis direktiv

  • Den #ifdirektivet brukes til å teste forprosessoren uttrykk.
  • Et preprosessoruttrykk kan bare bestå av et symbol eller en kombinasjon av symboler sammen med operatorer som &&(AND), ||(OR), !(NOT).
  • #ifdirektiv blir fulgt av et #endifdirektiv.
  • Kodene i #ifdirektivet er bare samlet hvis uttrykket som er testet med, #ifevaluerer til sant.
  • Syntaks: 
     #if kode forprosessoruttrykk for å kompilere <#endif
  • For eksempel: 
    #if TESTING Console.WriteLine ("Testes for øyeblikket"); #slutt om

Eksempel 1: Hvordan bruker du # if directive?

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

Når vi kjører programmet, vil utdataene 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-name pragma-argumenter
  • For eksempel: 
     #pragma advarsel deaktivere

Eksempel 7: Hvordan bruker du #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 kjører programmet, vil utdataene 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 bare den andre advarselen vises på utgangsskjermen.

Dette skyldes at vi opprinnelig deaktiverte alle advarsler før den første advarselen og gjenopprettet dem bare før den andre advarselen. Dette er grunnen til at den første advarselen ble skjult.

Vi kan også deaktivere spesifikk advarsel i stedet for all advarsel.

For å lære mer om #pragma, besøk #pragma (C # referanse).

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