V tej vadnici bomo izvedeli več o predprocesorskih direktivah, razpoložljivih direktivah v jeziku C # in o tem, kdaj, zakaj in kako se uporabljajo.
Kot že ime upravičuje, so direktive predprocesorja blokov stavkov, ki se obdelajo pred dejanskim prevajanjem. Direktive C # predprocesorja so ukazi za prevajalnik, ki vplivajo na postopek prevajanja.
Ti ukazi določajo, katere odseke kode je treba zbrati ali kako ravnati s posebnimi napakami in opozorili.
Direktiva C # za predprocesor se začne s # (hash)
simbolom in vse direktive za predprocesor trajajo eno vrstico. Predprocesorske direktive new line
namesto ne semicolon
.
Navodila za predprocesor, ki so na voljo v C #, so:
Predprocesorske smernice v jeziku C #Direktiva o predprocesorju | Opis | Sintaksa |
---|---|---|
#if | Preveri, ali je izraz predprocesorja resničen ali ne |
#if predprocesorska-izrazna koda za prevajanje #endif |
#elif | Uporablja se skupaj z #if za preverjanje več izrazov predprocesorja |
#if pretprocesor-expression-1 koda za prevajanje #elif preprocessor-expression-2 koda za prevajanje #endif |
#else | Uporablja se skupaj z #if za ustvarjanje sestavljene pogojne direktive. |
#if predprocesorska-izrazna koda za prevajanje #elif koda za prevajanje #endif |
#endif | Uporablja se skupaj z #if za označevanje konca pogojne direktive |
#if predprocesorska-izrazna koda za prevajanje #endif |
#define | Uporablja se za definiranje simbola |
#define SYMBOL |
#undef | Uporablja se za nedoločitev simbola |
#undef SIMBOL |
#warning | Omogoča, da iz naše kode ustvarimo opozorilo prve stopnje |
# opozorilno opozorilno sporočilo |
#error | Omogoča generiranje napake iz naše kode |
#error error-message |
#line | Omogoča, da spremenimo številko vrstice in ime datoteke prevajalnika, da prikažemo napake in opozorila |
# vrstica-številka vrstice-ime datoteke |
#region | Omogoča, da ustvarimo regijo, ki jo lahko razširimo ali strnemo z uporabo urejevalnika kod Visual Studio |
#region-opisne kode regije #endregion |
#endregion | Označuje konec regije |
#region-opisne kode regije #endregion |
#pragma | Prevajalniku daje posebna navodila za prevajanje datoteke, v kateri je prikazana. |
#pragma pragma-ime pragma-argumenti |
#define direktiva
#define
Direktiva nam omogoča, da določi znak.- Simboli, ki so definirani ob uporabi skupaj z
#if
direktivo, bodo ocenjeni kot resnični. - S temi simboli lahko določite pogoje za sestavljanje.
- Sintaksa:
#define SYMBOL
- Na primer:
#define TESTING
Tu je TESTIRANJE simbol.
Direktiva #undef
#undef
Direktiva nam omogoča, da undefine simbol.- 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-ime pragma-argumenti
- Na primer:
#pragma opozorilo onemogočeno
Primer 7: Kako uporabiti direktivo #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 ) ) )
Ko zaženemo program, bo rezultat:
Program.cs (12,22): opozorilo CS1030: #warning: 'To je opozorilo 2' (/home/myuser/csharp/directive-project/directive-project.csproj)
Vidimo, da je na izhodnem zaslonu prikazano samo drugo opozorilo .
To je zato, ker smo sprva onemogočili vsa opozorila pred prvim opozorilom in jih obnovili šele pred drugim opozorilom. To je razlog, zakaj je bilo prvo opozorilo skrito.
Namesto vseh opozoril lahko tudi onemogočimo določeno opozorilo.
Če želite izvedeti več o tem #pragma
, obiščite #pragma (referenca C #).