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Preprocessor directives

Preprocessor directives are instructions that tell the compiler how to process the source code before actual compilation begins. They do not produce executable code but instead control how the compiler treats parts of the code.

Think of them as the control-flow of the compilation and not of the actual code.

Directive Description
#define Defines a symbol (not a variable).
#undef Undefines a previously defined symbol.
#if, #elif, #else, #endif Conditionally includes/excludes code.
#region, #endregion Organizes code into collapsible sections.
#error Forces a compilation error with a custom message.
#warning Issues a compiler warning with a custom message.
#pragma Provides special compiler instructions (e.g., disabling warnings).
#line Modifies compiler line numbers for debugging.

Example of usage: 

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class Program
{
    #region Initialization
    static void Initialize()
    {
        Console.WriteLine("Initializing...");
    }
    #endregion

    #region Cleanup
    static void Cleanup()
    {
        Console.WriteLine("Cleaning up...");
    }
    #endregion
}