Conditional compilation

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

The ||, &&, and ! operators can be used to perform or, and, and not operations.

Logical Operators Supported by the #if..#endif Directive

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.