C# Preprocessor directives

In this post we will see what are C# preprocessor directives, and what is their purpose.

C# preprocessor directives are a salient feature that allows you to conditionally include, exclude, or organize code before compilation.

Preprocessor directives are instructions processed by the C# compiler before actual compilation. They all begin with the # character. Their key characteristic is that they have no effect at runtime; they simply determine which code gets compiled and which does not.

Main points of Preprocessor directives:

• They provide instruction code to compiler before compilation process.
• Based on the instruction/directive, a piece of code can be excluded or included for compilation
• All directives begin with the # character.
• They have no effect at runtime; they simply determine which code gets compiled and which does not.

There are different C# preprocessor directives which are used for different purposes. Some of them are tabulated below:

C# Preprocessor Directives List

Directive	Meaning
#define	Defines a compilation symbol that can be used in conditional compilation.
#undef	Undefines a previously defined symbol.
#if	Compiles code only if a specified symbol or expression evaluates to true.
#elif	Specifies an alternative condition after an #if or another #elif.
#else	Specifies code to compile when preceding #if or #elif conditions are false.
#endif	Marks the end of an #if, #elif, or #else block.
#warning	Generates a custom compiler warning message.
#error	Generates a custom compiler error and stops compilation.
#line	Changes the compiler's line-number and file-name information for error reporting and debugging.
#region	Defines a collapsible code region in IDEs such as Visual Studio.
#endregion	Marks the end of a #region block.
#pragma	Controls compiler behavior, such as enabling or disabling specific warnings.
#nullable	Controls nullable reference type annotations and warnings.

#define and #undef — Defining symbols

These preprocessor directives define or remove a symbol. These symbols are used anywhere in the code to check conditions. A directives symbol does not have a value; it simply either exists or it doesn't.

EXAMPLE 1

#define DEBUG_MODE
#define PREMIUM_USER
class Program
{
    static void Main()
    {
#if DEBUG_MODE
        Console.WriteLine("Debug: Application starting...");
#endif

#undef PREMIUM_USER   // symbol removed

#if PREMIUM_USER
        Console.WriteLine("Premium features"); // it will not execute
#endif
    }
}

Preprocessor directives (#define and #undef) are always written at the top of the file, before any using statements. In Real-world,

DIRECTIVE	USAGE
#if DEBUG	debug-only logging
#if WINDOWS	platform-specific code
#pragma warning disable	legacy code warnings
#nullable enable	to write modern null-safe code.