C# uses the preprocessor directives to control the compilation.
The preprocessor directives begin with # and it must appear on its own line.
The preprocessor directives conditional compilation is #if, #else and, #endif, and #elif.
The #if directive tells the compiler to ignore a section of code unless a specified symbol has been defined.
To define a symbol we use the #define or set it in the compilation.
#define directives must be at top of file.
Symbol names are uppercase by convention.
#define TESTMODE
using System;
class Program
{
static void Main()
{
#if TESTMODE
Console.WriteLine("in test mode!"); // OUTPUT: in test mode!
#endif
}
}
The output:
in test mode!
To define a symbol assembly-wide, specify the /define switch when compiling:
csc Program.cs /define:TESTMODE,ANOTHER
Preprocessor directives C# statements
#if if
#else else
#elif or #else #if else if
The ||, &&, and ! operators can be used to perform or, and, and not operations.
Logical Operators Supported by the #if..#endif Directive
Operator Example Description
== #if winXP == true Evaluates to true if the symbol winXP is defined. Equivalent to #if winXP.
!= #if winXP != true Evaluates to true if the symbol winXP is not defined. Equivalent to #if !winXP.
&& #if winXP && release Evaluates to true only if the symbols winXP and release are defined.
|| #if winXP || release Evaluates to true if either of the symbols winXP or release are defined.
() #if (winXP || win7) && release Parentheses allow you to group expressions. Evaluates to true if the symbols winXP or win7 are defined and the symbol release is defined.
The following code uses #define, #undef, #elif and #else to do the conditional compilation.
#define win7
#define release
#undef win2000
using System;
using System.Diagnostics;
class MainClass
{
[Conditional("DEBUG")]
public static void DumpState()
{
Console.WriteLine("Dump some state...");
}
public static void Main()
{
string platformName;
#if winXP // Compiling for Windows XP
platformName = "Microsoft Windows XP";
#elif win2000 // Compiling for Windows 2000
platformName = "Microsoft Windows 2000";
#elif win7 // Compiling for Windows 7
platformName = "Microsoft Windows 7";
#else // Unknown platform specified
platformName = "Unknown";
#endif
Console.WriteLine(platformName);
}
}
To build the example and define the symbols winXP, use the command csc /define:winXP; Example.cs.